This document combines several documents related to tree trenches and tree boxes. Individual documents can be viewed by clicking on the appropriate link below. Fact sheets are not included in this combined document

Contents

Design guidelines for tree quality and planting

Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.
Caution: Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.
Information: James Urban recently published a blog discussing why trees fail to establish. Many of the points raised in the article are related to the following design guidelines for tree quality and planting

Purchasing trees

schematic of bare root tree
Schematic of a bare root tree. Illustration is from Planting & Care at arborday.org and provided courtesy of the Arbor Day Foundation.
schematic of containerized tree
Schematic of a containerized tree. Illustration is from Planting & Care at arborday.org and provided courtesy of the Arbor Day Foundation.
schematic of balled and burlapped tree
Schematic of a balled and burlapped tree. Illustration is from Planting & Care at arborday.org and provided courtesy of the Arbor Day Foundation.

Trees can be purchased in many different forms. Gillman and Johnson (1999) describe some of the various forms as follows.

  • "Bare Root. Bare root plants are dug from nursery fields in the fall or spring. Soil is removed from the roots, and plants are held in humidity- and temperature-controlled storage over winter. They must be planted in early spring before growth begins. Because many roots are cut during field digging, bare root plants suffer severely from transplanting shock. Bare root stock is normally the least expensive, but if handled improperly, can have the highest mortality. When handling or transporting bare root stock, keep the roots moist and protected from sun and wind at all times.
  • Packaged. Packaged trees and shrubs are bare root plants with their roots packed in moist material such as peat moss or shingle tow. Plant them in early spring before growth starts. Keep packing materials moist, and the package cool and shaded until planted. These plants should be treated as bare root plants.
  • Field-Potted. Field-potted nursery stock are field-grown plants dug with a ball of field soil intact which is then placed as is, in a container. These plants should be sold and planted during the spring, as field soil will not provide good plant growth in a container. It is important that the root ball be disturbed as little as possible during the digging and planting process.
  • Containerized. Containerized trees and shrubs are dug from the nursery in the spring or fall as bare root stock, placed in a container with a special growing medium, and sold in the container. If containerized in early spring, most plants will be sufficiently established in the container and can be transplanted in late spring, summer, or fall. Roots must be established in the container and hold the media together before transplanting. Do not completely break up the root ball at planting time, but do cut any circling roots prior to planting. The tighter the root ball, the more the root system should be disturbed.
  • Container Grown. Container grown stock has been growing in a container throughout most of its production. Because the roots of these plants are not disturbed at the time of planting, container grown plants suffer little transplant shock and may be planted at any time during the growing season. Plants that have outgrown their containers may have deformed root systems, which can result in girdling roots. Large plants may be root bound in the container. The root ball of these plants must be torn or cut open [or box cut] to eliminate subsequent circling or girdling roots.
  • Balled and Burlapped (B & B). Balled and burlapped trees and shrubs are dug with a firm ball of soil around the roots, and held securely in place with burlap, twine, and sometimes a wire basket. A broken, damaged, or dry soil ball can result in serious damage to the roots. The stem should not wobble in the soil ball. Because of the weight of the soil ball, B & B trees can be difficult to transport and plant without special equipment. B & B stock is often the most expensive, but if handled and planted properly, is as reliable as container grown stock. Always lift B & B plants from beneath the ball, never by the stem. B & B stock can be planted in spring, summer, and fall.
  • Tree Spade. Larger plants are often moved with a tree spade—a machine that digs a mass of soil including the plant and some of its roots. The plant and root ball may stay in the machine until it is planted into a pre-dug matching hole, or it may be placed in a wire basket lined with burlap. The size of the root ball is critical and species dependent. An experienced machine operator can make the difference between success and failure. Matching soils from the digging site to the planting site is also important, as is compaction within the planting hole. Roughing up the sides of the hole can offset some of this compaction. Plants can be moved in most seasons with a spade, although plants dug in summer and early fall should have an oversized ball and receive special attention relative to species, condition, handling, and irrigation."
  • Gravel Bed Method to market bare root trees. The Gravel Bed Method is a method of handling bare root nursery stock in which dormant plants are placed in an irrigated bed of gravel in the spring with their roots submerged in gravel. These trees are held for up to a year before planting bare root (in full leaf) in the landscape. It is a lower cost method to grow trees that extends the planting season of bare root stock. Root growth in gravel is very extensive and point fibrous. Unlike with bark mulch, sawdust or sand, few roots are damaged when plants are removed from the gravel. The gravel bed method also greatly minimizes the risk of girdling roots compared to containerized and B&B trees. For more information on gravel beds, see Busiahn and Peterson (2013).

It is recommended the Owner or Project Landscape Architect inspect trees prior to digging at the nursery.

Many tree material and planting specifications have already been developed by others. Many municipalities and jurisdictions, for example, have developed their own tree material and planting specifications (see [1] [2] [3] [4]). Many university extension services and tree organizations also provide tree planting guidelines (see [5]). The tree material and planting guidelines below were developed based on experience and research by the contracted and technical teams for this project, combined with the resources listed in the references section. Additional guidance regarding tree planting is available in the references section below, as well as at the following 2 websites:

Tree material and planting guidelines

Caution: The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification. However, they are NOT a finished specification

The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification. However, they are NOT a finished specification. Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Terminology and requirements in the final specifications must be consistent with the terminology in other parts of the construction documents including plans and detail nomenclature.

tree planting detail schematic
Schematic showing tree planting detail

See the File:Trees-4-Detail.pdf which supplements the tree planting guidelines. Additional drawings regarding tree planting are available in the references section below, as well as at the following 2 websites:

Additional information on tree planting guidelines or specifications can be found at the following:

General guidelines

Submittals

  • Product Data: Submit manufacturer product data and literature describing all products required by this section to the Owner for approval.
  • Plant Growers Certificates: Submit Plant Growers certificates for all plants indicating that each plant meets the requirements, including the requirements for root system quality, to the Owner for approval.
  • Samples: Submit samples of each product and material where required to the Owner for approval. Label samples to indicate product, characteristics, and locations in the Work. Samples will be reviewed for appearance only. Compliance with all other requirements is the exclusive responsibility of the contractor.

Quality Assurance

  • Plant Acceptance
    • The Owner will inspect all work for Plant Acceptance upon written request of the Contractor.
    • Plant Acceptance by the Owner shall be for general conformance to specified size, character and quality and not relieve the Contractor of responsibility for full conformance to the contract documents, including correct species.
    • Any plant that is deemed defective as defined under the warranty provisions below shall not be accepted.
    • The Contractor is responsible for the condition and quality of work and materials during construction, and until Plant Acceptance. Contractor shall bear the total cost of replacing any and all plants until this time.
  • Warranty
    • The contractor agrees to replace defective work and plants as defined below.
    • Plants warranty shall begin on the date of Plant Acceptance and continue for one year.
    • All plants shall be warranted to be healthy, reasonably free of defects, in flourishing condition, and shall bear foliage of normal density, size, and color for the species at the end of the warranty period.
    • Defective Plants: Plants shall be deemed defective that are dead, diseased, insect infested, or not in a vigorous, thriving condition, during or at the end of the warranty period. The following conditions shall be deemed as indicating a defective plant.
      • Any plant that has a canopy or root system with 25 percent or more of its volume dead, diseased, insect infested, or not in a vigorous, thriving condition.
      • Evidence of damage to plants, which diminishes the aesthetic character and form or structural integrity of the plant or group of plants.
      • Plants that have had more than 25 percent of the canopy reduced by removed limbs that were not removed under the direction of the Owner.
      • Plants that do not meet the requirements for stem girdling and kinked roots and proper depth of the root crown.
      • Plants packaged with non-biodegradable fabrics or twine that have not been removed during the planting process.
      • Any tree that has open wounds (not completely healed) that penetrates the cambium into the wood on trunks or major limbs, the removal of which would result in the loss of 25 percent or more of the structure and form of the tree.
      • Properly-made pruning wounds that are not yet fully healed over can be considered as satisfactory if callus tissue has formed around the entire circumference of the wound.
      • The Owner shall make the final determination that plants are defective.
    • Plants determined to be defective shall be replaced without cost to the Owner, as soon as weather conditions permit and within the specified planting period.
    • Any work required by the Quality Assurance document or the Owner during the progress of the work to remediate plant defects, including the removal of roots or branches, or planting plants that have been bare rooted during installation to inspect for or correct root defect, shall not be considered as grounds to void any conditions of the warranty. In the event the contractor feels that such remediation work may compromise the future health of the plant, the plant or plants in question shall be rejected and replaced with plants that do not contain defects that require remediation.
    • The Contractor is exempt from replacing plants, after Plant Acceptance and during the warranty period, that are removed by others or lost or damaged due to occupancy of project, by a third party, through vandalism, or as a result of any natural disaster.
    • The warranty of all replacement plants shall extend for an additional one-year period from the date of their acceptance after replacement. In the event that a replacement plant is not acceptable during or at the end of the said extended warranty period, the Owner may elect one more replacement items or credit for each item. These tertiary replacement items are not protected under a warranty period.
    • During and by the end of the warranty period, remove all tree wrap, ties, and guying unless agreed to by the Owner to remain in place. All trees that have leaned shall be straightened.
  • Plant Final Acceptance: At the end of the warranty period, the Owner shall inspect all warranted work, upon written request of the Contractor. The request shall be received at least 10 calendar days before the anticipated date of final inspection.

Selection and inspection of plants

  • Purchasing trees from the growing nursery is preferred over re-wholesale suppliers. When re-wholesale suppliers are utilized, the contractor shall submit the name and location of the growing nursery from where the trees were obtained by the re-wholesale seller. The re-wholesale nursery shall be responsible for any required plant quality certifications.
  • The contractor shall require the grower or re-wholesale supplier to permit the Owner to inspect the root system of all plants including random removal of soil around the base of the plant. Inspections may be as frequent and as extensive as needed to verify that plants conform to the grower’s root quality certifications. For field grown plants, viewing of plants by the Owner may be at the growing nursery prior to the harvesting of the plant.
  • The Owner may choose to attach their seal to each plant, or a representative sample. Viewing and/or sealing of plants by the Owner at the nursery does not preclude the Owner’s right to reject material while on site.
  • Where requested by the Owner, submit photographs of plants or representative samples of plants. Photographs shall be legible and clearly depict the plant specimen. Each submitted image shall contain a height reference, such as a measuring stick. The approval of plants by the Owner via photograph does not preclude the Owner's right to reject material while on site.
  • Unless approved by the landscape architect, plants shall have been grown at a latitude not more than 325 km (200 miles) north or south of the latitude of the project unless the provenance of the plant can be documented to be compatible with the latitude and cold hardiness zone of the planting location. Many tree species are sensitive to the photoperiod of their native provenance. For example, red maple stock from native southern stock will not harden off in time for northern winters.

Plant substitutions for plants not available

  • Submit all requests for substitutions of plant species, or size to the Owner, for approval, prior to purchasing the proposed substitution.

Site conditions

  • It is the responsibility of the Contractor to be aware of all surface and sub-surface conditions, and to notify the Owner, in writing, of any circumstances that would negatively impact the health of plantings. Do not proceed with work until unsatisfactory conditions have been corrected.
  • Do not install plants into saturated or frozen soils. Do not install plants during inclement weather, such as heavy rain or snow or during extremely hot, cold or windy conditions.

Planting around utilities

  • Contractor shall carefully examine the civil, record, and survey drawings to become familiar with the existing underground conditions before digging.
  • Determine location of underground utilities and perform work in a manner that will avoid possible damage. Hand excavate, as required. Maintain grade stakes set by others until parties concerned mutually agree upon removal.
  • Notification of Local Utility Locator Service, is required 72 hours prior to digging. The Contractor is responsible for knowing the location of and avoiding utilities that are not covered by the Local Utility Locator Service.

Product guidelines

Plants: General

  • Standards and measurement: Provide plants of quantity, size, genus, species, and variety or Cultivars as shown and scheduled in contract documents.
    • Tree stock shall conform to Mn/DOT specification 3861.2, ANSI Z60.1, American Standard for Nursery Stock, and all state requirements for nursery stock except where they are modified by this specification. Where there is a conflict between this specification and the above specifications, this specification will apply.
    • Plants larger than specified may be used if acceptable to the Owner. Use of such plants shall not increase the contract price. If larger plants are accepted the root ball size shall be increased in proportion to the size of the plant. Larger plants may not be acceptable if the resulting root ball cannot be fit into the required planting space.
  • Plant Quality
    • General: Provide healthy, vigorous stock, grown in a recognized nursery and reasonably free of disease, insects, eggs, bores, and larvae. At the time of planting all plants shall have root system, stem, and branch form that will not restrict normal growth, stability and vigor for the expected life of the plant.
    • Plant quality above the soil line:
      • Plants shall be of exceptional quality with the color, shape, size and distribution of trunk, stems, branches, buds and leaves normal to the plant type specified.
      • There should be one dominant leader to the top of the tree with the largest branches spaced at least 6 inches apart. All trees are assumed to be single leader plants unless a different form is specified in the plant list or drawings.
      • Tree shall have no significant branch unions with included bark between stems.
      • Tree trunks shall be reasonably straight with lateral limbs reasonably symmetrical, free of large voids, and evenly distributed along the trunk. Clear trunk should be no more than 40 percent of tree height unless otherwise specified in the planting specifications.
      • Branches should be less than ½ the trunk diameter at the attachment point unless otherwise approved by Project Landscape Architect or Arborist.
      • Trees greater than 1.5 inches caliper should be able to stand erect without a supporting stake.
      • The trunk and branches shall be reasonably free of knots, scrapes, broken or split wood, fresh limb cuts, sunscald, injuries, and abrasions. All graft unions, where applicable, shall be completely healed without visible sign of graft rejection. All grafts shall be visible above the soil line.
      • Open trunk and branch wounds shall be less than 10 percent of the circumference at the wound and no more than 2 inches tall. Pruning shall not encroach on the branch collar. Properly made pruning cuts are not considered open trunk wounds. Pruning cuts in accordance with ANSI standards are considered properly made pruning cuts.
    • Plant quality at or below the soil line
      • The roots shall be reasonably free of scrapes, broken or split wood.
      • A minimum of three structural roots reasonably distributed around the trunk shall be found in each plant.
      • Plants with structural roots on only one side of the trunk (J roots) shall be rejected.
      • The root crown must not be more than 2 inches below the soil line.
      • The root system shall be reasonably free of stem girdling roots above the root collar, vertical roots and or kinked roots from nursery production practices. Stem girdling roots, vertical and kinked roots include roots on the interior of the root ball. There shall be no roots greater than 1/10 the diameter of the trunk circling more than one-third the way around in the top half of the root ball. Roots larger than this may be cut provided they are smaller than one-third the trunk diameter. There shall be no kinked roots greater than 1/5 the trunk diameter. Roots larger than this can be cut provided they are less than one-third the trunk diameter.
      • Trees may be rejected if the extent of root cutting required to remedy girdling, kinked, and vertical roots renders the tree unlikely to thrive by the end of the warranty period.
      • The final plant grower shall be responsible for determining that the plants have been root pruned at each step in the plant production process to remove stem girdling roots and kinked roots, or practices that produce a root system throughout the root ball that meets these requirements. Regardless of the work of previous growers, the plant’s root system shall be modified at the final production stage to produce the required plant root quality. The final grower shall certify in writing that all plants are reasonably free of stem girdling and kinked roots.
      • Except for bare root trees, all trees should be rooted into the root ball so that soil or media remains intact and trunk and root ball move as one when lifted, but not root bound. The trunk should bend when gently pushed and should not be loose so it pivots at or below the soil line.
    • Submittals: Submit for approval the required plant quality certifications from the grower where plants are to be purchased, for each plant type. The certification must state that each plant meets all the above plant quality requirements. The grower’s certification of plant quality does not prohibit the Owner from inspecting any plant or rejecting the plant if it is found to not meet the requirements.

Root ball package options

The following root ball packages are permitted. Specific Root Ball Packages shall be required where indicated on the plant list or in this document. Any type of Root Ball Package that is not specifically defined in this document shall not be permitted.

  • Balled and burlapped plants
    • All Balled and Burlapped Plants shall be field grown, and the root ball packaged in a burlapped-and-twine or burlap-and-wire basket package.
    • Plants shall be harvested with the following modifications to standard nursery practices.
      • Prior to digging any tree, using hand tools or an air spade, carefully remove the soil from the top of the root ball of each plant to locate the root crown. Care must be exercised not to damage the surface of the root crown and the top of the structural roots.
      • Balled and burlapped trees shall be dug prior to leafing out (bud break) in the spring or during the fall planting period except for plants known to be considered as fall planting hazards. Plants that are fall planting hazards shall only be dug prior to leafing out in the spring. Plants to be shipped or installed when in leaf shall be pre-dug prior to bud break and stored appropriately in protected storage yards with adequate water.
      • Twine and burlap used for wrapping the root ball package shall be natural, biodegradable material that has not been treated with preservatives to retard decomposition. If the burlap decomposes during the storage period the root ball shall be re-wrapped prior to shipping.
    • Trees greater than 5 inches in caliper shall be root-pruned a minimum of 12 months before transplanting. All root pruning shall be accomplished utilizing accepted horticultural practices for root pruning including staking and watering.
  • Spade harvested and transplanted
    • Spade Harvested and Transplanted Plants shall meet all the requirements for field grown trees. Root ball diameters shall be of similar size as the ANSI Z60.1 requirements for Balled and Burlapped plants.
    • Trees shall be harvested prior to leafing out (bud break) in the spring or during the fall planting period except for plants considered as fall planting hazards. Plants that are fall planting hazards shall only be harvested prior to leafing out in the spring.
    • Before moving, trees shall be watered thoroughly to hydrate the tree and keep the root package together during transport.
    • Trees shall be moved and planted within 48 hours of the initial harvesting and shall remain in the spade machine until planted.
  • Container grown plants
    • Container grown plants may be permitted only when indicated on the drawing, approved in this document, or approved by the Owner.
    • Provide established and well rooted plants in removable containers.
    • Container class size shall conform to ANSI Z60.1 for container plants for each size and type of plant.
    • Container-grown stock shall have been grown in a container long enough for the root system to have developed sufficiently to hold its potting medium together but not so long as to have developed Stem Girdling or matted roots circling around the edge of the container. Plants that fail to meet this requirement may be modified to correct deficiencies as describe below if approved by the Owner.
  • Containerized plants
    • Containerized plants may be permitted only when indicated on the drawing, this document or approved by the Owner.
    • Provide field grown plants in containers of similar size as the ANSI Z60.1 requirements for Balled and Burlapped plants.
    • Place the field grown plant in the container at the correct depth of the root crown as defined above.
    • Containerized plants shall not held in the container more than 12 months.
    • Remove all stem girdling above the root collar or circling roots around the edge of the container prior to planting as describe below if approved by the Owner.
  • Bare root plants
    • Provide established and well-rooted field grown plants. Harvest bare root plant while the plant is dormant and a minimum of 4 weeks prior to leaf out (bud break).
    • The root spread of the harvested plants shall conform to ANSI Z60.1 for nursery grown bare root plants for each size and type of plant.
    • Bare root stock shall be protected from drying out at all times. Roots must be covered and packed in moist straw, sawdust, or other suitable moisture-holding packing material.
    • Keep the trees in a cool dark space for storage and delivery. If daytime outside temperatures exceed 70 degrees F, utilize a refrigerated storage area with temperature between 35 and 50 degrees.
    • Where possible, plan time of planting to be before bud break. For trees to be planted after bud break, place the trees before bud break in an irrigated bed of 20 percent sand and 80 percent pea gravel.
      • The pea gravel bed shall be 18 inches deep over a sheet of plastic.
      • Space trees to allow the unbundled branches to grow without shading each other.
      • Once stored in pea gravel, allow the trees sufficient time for the new root system to flush and spring growth of leaves to fully develop before planting.
      • Pea gravel-stored trees may be kept for up to one growing season.
      • Pea gravel-stored trees shall be dipped, packaged and shipped similar to the requirements for freshly dug bare root trees.
  • In-ground fabric bag-grown (Grow bags)
    • In-ground fabric bag-grown (Grow Bags) plants may be permitted only when indicated on the drawing, this document or approved by the Owner.
    • Provide established and well rooted plants produced using Grow Bags.
    • The Grow Bag size shall conform to ANSI Z60.1 for fabric bag-grown plants for each size and type of plant.

Mulch

  • Mulch shall be as specified in MNDOT 3882, Type 6, Shredded Hardwood Mulch.
  • Submit manufacturers product data and one gallon sample for approval.

Anti-desicant

  • Anti-Desiccant shall be emulsion type, film-forming agent similar to Dowax by Dow Chemical Company, or Wilt-Pruf by Nursery Specialty Products, Inc., Croton Falls, New York, designed to permit transpiration but retard excessive loss of moisture from plants. Deliver in manufacturer’s fully identified containers and use in accordance with manufacturer’s instructions.
  • Submit manufacturers product data for approval.

Tree staking and guying material

  • Tree guying is to be flat woven polypropylene material, 3/4 inch wide, with 900 pound break strength. Product to be ArborTie, manufactured by Deep Root Partners, L.P., or approved equal.
  • Stakes shall be 2 inch by 2 inch hardwood stakes free of knots (or approved equal) and of lengths appropriate to the size plant required to adequately support the plant.
  • Dead men for Large Trees where required on the drawings shall be 4 inch by 4 inch by 4 feet long wood (or approved equal). Wood shall NOT be treated for rot protection.
  • Submit manufacturer’s product data for approval.

Watering bags

  • Watering bags shall be Treegator Irrigation Bags, sized to the appropriate model for the requirements of the plant, manufactured by Spectrum Products, Inc, Youngsville, NC 27596, or approved equal.
  • Submit manufacturers product data for approval.

Chemical or biological additives

  • Chemical or biological additives are designed to increase soil fertility. All material shall be delivered to the site in unopened containers and stored in a dry enclosed space suitable for the material and meeting all environmental regulations. Biological additives shall be protected from extreme cold and heat. All products shall be freshly manufactured and dated for the year in which the products are to be used.
    • Fertilizer for planting shall be organic fertilizer with a salt index of 25 or less. The majority of the nutrient elements are from organic sources. Fertilizer selections shall be based on the recommendations of the soil test.
    • Submit manufacturers product data for approval.

Execution guidelines

Site examination

  • Examine the surface grades and soil conditions to confirm that the soil and drainage modifications indicated on the Plans and Details have been completed. Notify the Owner in writing of any unsatisfactory conditions.

Delivery, storage and handling

  • Protect materials from deterioration during delivery and storage. Adequately protect plants from drying out, exposure of roots to sun, wind, and extremes of heat and cold temperatures.
  • Branches shall be tied with rope or twine only, in a manner that will not damage any part of the tree.
  • If planting is delayed more than 24 hours after delivery, set plants in a location protected from sun and wind.
  • Provide adequate water to the root ball package during the shipping and storage period. Using a soil moisture meter, periodically check the soil moisture in the root balls of all plants to assure that the plants are being adequately watered.
  • Do not deliver more plants to the site than can be adequately stored. Provide a suitable remote staging area for plants and other supplies.
  • The Owner shall approve the duration, method and location of storage of plants.
  • Protective covering is required over all plants during delivery.
  • Before shipping, apply 1/8 inch thick, wax sealed, corrugated cardboard trunk protection, or approved equal, around the trunk of all trees from the top of the root ball package to the first branch or up to four feet high, whichever is lower. Secure the cardboard with plastic tape.
  • If trees are moved when in full-leaf, spray with anti-desiccant per manufacturer’s recommendations at nursery no greater than 48 hours prior to digging, and again two weeks after transplanting. Spraying should take place in early morning hours with foliage at maximum turgidity.

Planting season

  • Planting shall only be performed when weather and soil conditions are suitable for planting the specified materials in accordance with locally accepted practices. Install plants during the planting time as described below unless otherwise approved in writing by the Owner. In the event that the Contractor requests planting outside the dates of the planting season, approval of the request does not change the requirements of the warranty.
    • Planting shall be completed within the following dates:
      • Coniferous trees and shrubs: between April 15 and July 15, or between September 1 and November 14
      • Deciduous trees and shrubs – for all root package options except bare root
        • Quercus (Oaks): between April 15 and May 15, during dormancy period prior to bud break
        • Populus (Poplar), Ostrya (Ironwood), Celtis (Hackberry), Malus (Crabapple), Acer (Maple), Prunus (Plum and Cherry), Sorbus (Mountain Ash), Betula (Birch), Salix (Willow), Tilia (Basswood), Cornus (Dogwood), Rhus (Sumac): between April 15 and July 15
        • All other Deciduous Trees and Shrubs: between April 15 and July 15, or between September 1 and November 14
      • Deciduous Trees and Shrubs – bare root
        • Betula (Birch), Celtis (Hackberry), Quercus (Oak), Crataegus (Hawthorn), and Ostrya (Ironwood): planting dates: Mid- to late-May due to sweating requirements.
        • All other bare root deciduous trees and shrubs: Plant only during Spring and Fall dormancy periods.

Coordination with project work

Coordinate the relocation of any irrigation lines, heads or the conduits of other utility lines that are in conflict with tree locations. Root balls shall not be altered to fit around lines. Notify the Owner of any conflicts encountered.

Layout and planting sequence

When applicable, plant trees before other plants are installed.

Soil protection during plant delivery and installation

  • Protect soil from compaction during the delivery of plants to the planting locations, digging of planting holes and installing plants.
    • Where possible deliver and plant trees requiring the use of heavy mechanized equipment prior to final soil preparation and tilling.
    • Till and restore grades to all soil that has been driven over or compacted during the installation of plants.

Installation of plants: general

  • Inspect each plant after delivery and prior to installation for damage or other characteristics that may cause rejection of the plant. Notify the Owner of any such conditions.
  • The root system of each plant, regardless of root ball package type, shall be inspected by the Contractor at the time of planting to confirm that the roots meet the requirements for tree quality. The Contractor shall undertake, at the time of planting, all modifications to the root system required by the Owner to meet these quality standards.
  • Container Root Ball Shaving: The outer surfaces of ALL plants in containers, including the top, sides and bottom of the root ball shall be shaved to remove all circling and matted roots. Shaving shall be performed using saws, knives, sharp shovels or other suitable equipment that is capable of making clean cuts on the roots. Shaving shall remove a minimum of one inch of root mat or deeper as required to remove all roots that are not growing reasonably radial to the trunk and to remove all kinked and vertical roots. For trees where shaving could potentially harm the tree because the tree would not have sufficient roots left, shaving is not required if the specifier permits not shaving the root ball.
  • Exposed Stem Tissue after Modification: The required root ball modifications may result in stem tissue that has not formed trunk bark being exposed above the soil line. If such condition occurs, wrap the exposed portion of the stem in a protective wrapping such as Dewitt Tree Wrap fabric. Secure the fabric with biodegradable tape such as 3M Scotch 234 or 232 masking tape or approved equal. DO NOT USE string, twine or any other material that may girdle the trunk if not removed.
  • Using hand tools, back hoe or mini-excavator, excavate the planting hole into the planting soil to the depth of the root ball, as measured after any root ball modification to correct root problems, and wide enough for working room around the root ball or to the size indicated on the drawing.
    • The measuring point for root ball depth shall be the average height of the outer edge of the root ball after any required root ball modification.
    • Scarify sides and bottom of planting hole.
  • For trees to be planted in prepared planting soil that is deeper than the root ball depth, compact the soil under the root ball using a mechanical tamper to assure a firm bedding for the root ball. If there is more than 12 inches of planting soil under the root ball excavate and tamp the planting soil in lifts not to exceed 12 inches.
  • Set top outer edge of the root ball 1 to 3 inches above the average elevation of the proposed finish. Set the plant plumb. The tree graft, if applicable, shall be visible above the grade. Do not place soil on top of the root ball.
  • Brace root ball by tamping planting soil around the lower portion of the root ball. Place additional planting soil around base and sides of ball in six-inch (6 inch) lifts. Lightly tamp each lift using hand tools to settle backfill and eliminate voids.
  • Where indicated on the drawings, build a 3 inch high, level saucer of planting soil around the outside of the root ball to retain water. Tamp the saucer to reduce erosion of the saucer.
  • Thoroughly water the planting soil and root ball immediately after planting.
  • Remove corrugated cardboard trunk protection after planting.
  • Follow additional requirements for the permitted root ball packages.

Permitted root ball packages and special planting requirements

The following are permitted root ball packages and special planting requirements that shall be followed during the planting process in addition to the above general planting requirements.

  • Balled and burlapped plants
    • Remove burlap or cloth wrapping and wire baskets from full depth of root ball (remove all wire and burlap except burlap and wire under root ball). Completely remove and properly dispose all strings, nails, burlap, baskets, and wrappings from the root ball and trunk before backfilling.
  • Spade harvested and transplanted trees
    • After installing the tree, loosen the soil along the seam between the root ball and the surrounding soil to a depth of 8 to 10 inches by hand digging to disturb the soil interface. Fill any gaps below this level with loose soil.
  • Container grown plants
    • Start with the assumption that most container plants likely have significant stem girdling and circling roots and that the root crown is likely too low in the root ball.
    • Remove the container.
    • Remove all roots and potting soil above the root crown and the main structural roots.
    • Remove all potting soil at the bottom of the root ball that does not contain roots.
    • Using a hose, power washer or air knife, wash out the potting mix from around the trunk and top of the remaining root ball and find and remove all stem girdling roots within the root ball above the top of the structural roots.
    • The resulting root ball may need additional staking and water after planting. The Owner may reject the plant if the root cutting process makes the tree unlikely to be vigorous at the end of the warranty period.
  • Containerized plants
    • Remove the container.
    • Cut all circling roots on the perimeter of the root ball not removed in the required root shaving such that the roots in the remaining root system are oriented approximately radial to the trunk.
    • Remove all roots, root mat and potting soil above the root crown and the main structural roots.
    • Remove all potting soil at the bottom of the root ball that does not contain roots.
  • Bare root plants
    • Soak roots in water or mud slurry for at least one hour prior to planting.
    • Dig the planting hole three times the diameter of the spread of the roots. Dig the planting hole in the center to the depth that maintains the root collar at the elevation of the surrounding finished grades.
    • Spread the roots radial to the trunk around the prepared hole.
    • Maintain the trunk plumb while backfilling soil around the roots.
    • Lightly tamp the soil around the roots to eliminate voids and reduce settlement.
  • In-ground fabric bag-grown (Grow bags)
    • Remove the fabric bag from the root ball. Cut roots at the edge of the bag as needed to extract the fabric from the roots. Make clean cuts with sharp tools; do not tear roots away from the fabric.
    • Inspect the root system after the bag is removed to confirm that the root system meets the quality standards.
    • Assure that the root crown is within 2 inches of the final soil line.

Tree staking and guying

  • Stake or guy only if necessary for the tree to be stable in unusual circumstances, for example, in strong winds and if approved by Project Landscape Architect.
  • The Owner shall have the authority to require that trees are staked or to reject staking as an alternative way to stabilize the tree.
  • Poor-quality trees with cracked, wet, or loose root balls, poorly developed trunk-to-crown ratios, or undersized root balls shall be rejected if they require staking, unless written approval to permit staking or guying as a remedial treatment is obtained from the landscape architect. Trees that settle out of plumb due to inadequate soil compaction either under or adjacent to the root ball shall be excavated and reset. In no case shall trees that have settled out of plumb be pulled upright using guy wires.
  • If a tree needs to be staked, use a method that minimizes the chance of girdling the tree. Many such systems are available on the market. Allow for some trunk movement with whatever method is used. Do not use wires or cables to guy trees.
  • Trees that are guyed shall have their guys and stakes removed after one full growing season or at other times as required by the Owner.

Straightening plants

  • Maintain all plants in a plumb position throughout the warranty period. Straighten all trees that move out of plumb including those not staked. Plants to be straightened shall be excavated and the root ball moved to a plumb position, and then re-backfilled.
  • Do not straighten plants by pulling the trunk with guys.

Installation of fertilizer and other chemical additives

  • Do not apply any fertilizer to plantings during the first year after transplanting unless soil testing demonstrates that fertilizer or other chemical additives is required. Apply chemical additives only upon the approval of the Owner.
  • Fertilizers shall be applied according to the manufacturer’s instructions and standard horticultural practices.

Pruning of trees and shrubs

  • Trees need as many leaves as possible to recover from transplant shock, so prune as little as possible at the time of planting. Prune only broken or dead branches, if present, as well as co-dominant leaders, limbs that rub against each other, and poorly angled branches if these have not been pruned out by the nursery. To minimize oak wilt spread, never prune oaks in spring or early summer.
  • Prune plants as directed by the Owner. In general, preserve the natural character of the plant and follow recommendations in An Illustrated Guide to Pruning, Third Edition (Gilman 2011).
  • All pruning shall be performed by a person experienced in landscape pruning.
  • Wherever possible and appropriate to the species, preserve or create a central leader.
  • Pruning of large trees shall be done using pole pruners or if needed, from a ladder or hydraulic man lift to gain access to the top of the tree. Do not climb in newly planted trees
  • Remove and replace excessively pruned or malformed stock resulting from improper pruning.
  • Pruning shall be done with clean, sharp tools.
  • No tree paint or sealants shall be used.

Mulching of plants

  • Apply a 2 to 3 inch deep mulch ring around tree, minimum 8 foot diameter or to the extent indicated on plans.
  • Do not install mulch on top of the root ball.

Watering

  • The Contractor shall be fully responsible to ensure that adequate water is provided to all plants from the point of installation until the date of Plant Acceptance. The Contractor shall adjust the automatic irrigation system, if available, and apply additional water using hoses as required.
  • Hand water root balls of all plants to assure that the root balls have adequate moisture. Test the moisture content in each root ball and the soil outside the root ball to determine the water content.
  • The Contractor shall install one set (two bags) of watering bags for each tree to be maintained and used for tree watering during the warranty period. Watering bags shall be removed between December 1 and March 1.

Cleanup

  • During installation, keep the site free of trash, pavements reasonably clean and the work area in an orderly condition at the end of each day.
  • Once installation is complete, wash all soil from pavements and other structures. Ensure that mulch is confined to planting beds and that all tags and flagging tape are removed from the site. The Owner seals are to remain on the trees and removed at the end of the warranty period.

Protection during construction

  • The Contractor shall protect landscape work and materials from damage due to planting operations or operations by other Contractors or trespassers. Maintain protection during installation until Plant Acceptance. Treat, repair or replace damaged planting work immediately.
  • Damage done by the Contractor, or any of their sub contractors, to plants or any other parts of the work shall be replaced by the Contractor at no expense to the Owner.
  • For information on protecting existing trees on site, see Protection of existing trees on construction sites.

Plant maintenance prior to plant acceptance

  • During the project work period and prior to Plant Acceptance, the Contractor shall maintain all plants.
  • Maintenance during the period prior to Plant Acceptance shall consist of pruning, watering, cultivating, weeding, mulching, removal of dead material, repairing and replacing of tree stakes, tightening and repairing of guys, repairing and replacing of damaged tree wrap material, resetting plants to proper grades and upright position, and furnishing and applying such sprays as are necessary to keep plantings reasonably free of insects and disease and in healthy growing condition. The threshold for applying insecticides and herbicide shall follow established Integrated Pest Management (IPM) procedures.

Maintenance during the warranty period

  • During the warranty period, provide all maintenance for all plantings to keep the plants in a healthy state and the planting areas clean and neat.
  • General requirements:
    • All chemical and fertilizer applications shall be made by licensed applicators. All work and chemical use shall comply with applicable local, provincial and federal requirements.
    • Meet with the Owner a minimum of three times a year to review the progress and discuss any changes that are needed in the maintenance program. At the end of the warranty period attend a hand over meeting to formally transfer the responsibilities of maintenance to the Owner.
  • Provide the following maintenance tasks:
    • Watering: provide all water required to keep soil within and around the root balls at optimum moisture content for plant growth.
      • Maintain all watering systems and equipment and keep them operational. Monitor soil moisture to provide sufficient water. Check soil moisture and root ball moisture with a soil moisture meter on a regular basis and record moisture readings. Do not over water.
    • Soil nutrient levels: apply fertilizers at rates recommended by soil testing.
    • Plant pruning: remove cross over branching, developing co-dominant leaders, dead wood and winter-damaged branches. Do not over prune or shear plants.
    • Restore plants: reset any plants that have settled or are leaning as soon as the condition is noticed.
    • Guying: remove tree guys and staking after the first full growing season.
    • Weed control: keep all beds reasonably free of weeds. The Owner must approve in advance the use of all chemical herbicide applications.
    • Trash removal: remove all trash and debris from all planting beds and maintain the beds in a neat and tidy appearance.
    • Disease and insect control: Provide an Integrated Plant Management (IPM) program to maintain disease and insects at acceptable and manageable levels. Manageable levels shall be defined as minimum damage to plants. Use least invasive methods to control plant disease and insect outbreaks. The Owner must approve in advance the use of all chemical pesticide applications.
    • Plant replacement: replace all plants that are defective as defined in the warranty provisions, as soon as the plant decline is obvious and in suitable weather and season for planting.
    • Mulch: refresh mulch once a year to maintain complete coverage. Do not over mulch. Do not apply mulch against the trunks or stems of any plants. Replacement mulch shall meet the requirements of the original approved material.

Other tree planting specifications

Definitions

  • Container Grown Trees: Trees that have been grown in a container at any point in the production cycle without corrective pruning of roots at each point in the production cycle, or the use of root safe containers to eliminate stem girdling and kinked roots.
  • Containerized trees: Field grown trees that are packaged into a container within 12 months of the growing season they are to be planted.
  • Field Grown Trees: Trees growing in field soil at the time of harvest. Field grown tree shall have been root pruned after each transplant cycle to remove Stem Girding and Kinked Roots or which are grown in root safe container systems during earlier parts of the production cycle.
  • Kinked Root: A root within the root package that bends 60 degrees or greater, around the trunk, or up, or down including roots deflected by the current or previous container.
  • Plant Acceptance: The date at the end of the plant installation where the Owner accepts that all work in this section is complete and the Warranty period has begun. This date may be different that the date of substantial completion for the other sections of the project.
  • Plant Final Acceptance: The date when the Owner accepts that the plants and work in this section meet all the requirements of the warranty.
  • Reasonable and Reasonably: When used in this document relative to plant quality, it is intended to mean that the conditions cited will not affect the establishment or long term stability, health or growth of the plant. This document recognizes that it is not possible to produce plants free of all defects. This document also recognizes that some decisions cannot be totally based on measured findings and that profession judgment is required. In cases of differing opinion, the Owner’s expert shall determine when conditions within the plant are judged as reasonable.
  • Root Ball: The mass of roots including any soil or potting mix that is shipped with the tree within the root ball package.
  • Root Ball Package. The material that surrounds the root ball during shipping.
  • Root Crown: The area where the majority of the main roots join the plant stem, usually at or near ground level (also known as the trunk flare). A single root emerging root perpendicular from the trunk ABOVE the Root Crown and / or the swelling of the trunk at the point of a previous graft should not be considered the root crown. For trees that develop two locations, one above the other, where three or more main roots emerge from the stem, the upper set shall be considered as the root crown.
  • Spade Harvested And Transplanted: Field grown trees that are mechanically harvested and immediately transplanted to the final growing site without being removed from the digging machine.
  • Stem: The trunk of the tree. The portion of the plant above the roots.
  • Stem Girdling Root: Any root, 1/4 inch in diameter or greater, within the root package that crosses over the top of a structural root approximately tangential to the trunk circumference. Roots shall be considered as Stem Girdling that have or are likely to have root to trunk bark contact.
  • Structural Root: Large woody root emerging from the root crown, approximately radial to the trunk and at approximately the same depth, with a diameter approximately 1/2 to 1/3 of the root crown diameter.
  • Tree: Single and multi-stemmed plants with mature height approximately greater than 5 meters.

References


Design guidelines for soil characteristics

Caution: Because tree trenches and boxes are similar to bioretention systems, it is Highly Recommended that designers be familiar with Design criteria for bioretention.
Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.
Information: James Urban recently published a blog discussing why trees fail to establish. Many of the points raised in the article are related to the following design guidelines for tree quality and planting

Soil volume guidelines

Providing adequate rootable soil volume is crucial to growing healthy trees. Several researchers have investigated minimum soil volumes needed to grow healthy trees. Results from these studies have been used to develop the following guidelines. (See summary of research)

Recommended minimum soil volume requirements for urban trees

Caution: the recommended minimum soil volume is 2 cubic feet of rootable soil volume per square foot of mature tree canopy size. See canopy size for several common tree species.

Based on research, the recommended minimum soil volume of 2 cubic feet of rootable soil volume per square foot of mature tree canopy size is considered essential to healthy growth of trees. (See canopy size for several common tree species).

Effect of soil volume on stormwater volume credits

schematic for infiltration from biofiltration with an underdrain
Schematic showing fate of water in a tree trench with a raised underdrain. Soil volume (porosity) has a direct effect on the volume of water lost below the underdrain since water stored there eventually drains into the underlying soil. Soil volume also affects stormwater volume lost to evapotranspiration, since water stored in the media above the underdrain can be captured by a tree.

Soil volume affects the stormwater volume credits for tree trenches and tree boxes. Stormwater volume credits for tree trenches and tree boxes include

  • storage of water below an underdrain,
  • interception of rain water by the tree canopy, and
  • evapotranspiration from the tree(s).

The first of these credits, storage of water below an underdrain, is a direct function of soil volume. The interception credit is a function of tree species and leaf area.

The evapotranspiration (ET) credit is a function of plant available water and is indirectly related to soil volume (e.g. available pore space). The decrease in stormwater volume available for ET is assumed to be linear with decreases in soil volume below the minimum recommended volume of 2 cubic feet of rootable soil per square foot of mature tree canopy. This approach is utilized within the Minimal Impact Design Standards (MIDS) calculator. However, in the case of rock-based structural soils, the relationship between soil volume and plant available water is not solely a function of the ratio of rock to soil. This is because the soil added to a structural soil tends to adhere to and coat the rock, thereby increasing the effective soil area in the structural soil. For a typical structural soil consisting of 80 percent rock and 20 percent loam, we recommend that 2 parts of structural soil are needed to equal 1 part of loam. This ratio can be dropped to 1.5 if the loam soil is used around the root ball.

For specific information on stormwater volume and pollutant credits for tree trenches and tree boxes, click here.

Summary of literature review on soil volume

An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above. The results of this review are presented below.

Summary of research on minimum soils volumes needed

Minimum soil volume needed to grow healthy trees has been studied several ways, including

  • field surveys investigating minimum soil volumes that grew healthy trees;
  • calculation of minimum soil volume needed based on tree water requirements; and
  • calculation of minimum soil volume needed based on tree nitrogen requirements (Kopinga 1991).

Each of these techniques indicates similar ranges of minimum soil volume needed:

  • 1 to 3 cubic feet of soil per square foot of canopy; and
  • 1000 to 2100 cubic feet of soil for a large tree (median of 1500 cubic feet and mean of 1506 cubic feet).

To put these numbers in perspective in relation to tree size and typical street tree spacing:

  • Using the above numbers, a tree with 2 cubic feet of soil per square foot of canopy would need 1413 cubic feet of soil to grow 30 feet wide.
  • Assuming 2 cubic feet of soil per square foot of canopy, 1,500 cubic feet of soil would be able to support a 31 foot wide tree.

Precedents for minimum soil volume standards

Because of the importance of providing adequate rootable soil volume to grow healthy trees, several jurisdictions have enacted minimum soil volume policies. These are summarized in the following table. Other examples can be found at this link.

Examples of jurisdictions with minimum tree soil volume requirements
Link to this table.

Jurisdiction Minimum tree soil volume
Kitchener, Ontario, Canada
  • Large stature trees (≥24” diameter at maturity): 1589 cubic feet (c.f) for single trees; 1059 c.f. for trees sharing soil volume; 530 c.f. allowable shared soil volume
  • Medium Stature trees: (≥16” diameter at maturity): 989 c.f for single trees; 653 c.f. for trees sharing soil volume; 336 c.f. allowable shared soil volume
  • Small stature trees: (≥8” diameter at maturity): 600 c.f for single trees; 389 c.f. for trees sharing soil volume; 212 c.f. allowable shared soil volume
  • For all boulevards where trees are planted, the minimal soil depth will be 450 mm (17.7 inches), and all other soil habitat zones (public/private front lawn, cul de sac, active parkland) will be 900 mm.
  • Where soil habitat zones (e.g. boulevard and front lawn) must be connected to achieve the required soil volumes, root pathways or Silva Cells will be used to provide a functional connection between the two areas.
Emeryville, CA
  • 600 cubic feet (17 cubic meters) for a small tree
  • 900 cubic feet (25 cubic meters) for a medium tree
  • 1200 cubic feet (34 cubic meters) for a large tree
  • 50 percent credit for planting areas under adjacent paving using 100% planting soil with Silva Cell or similar products.
Toronto, Ontario, Canada
  • 30 cubic meters (1059 cubic feet) of soil per tree
  • 20 cubic meters(706 cubic feet) per tree for trees with shared volume
  • Minimum 0.9 m (3’) and maximum 1.2 m (4’) depth
Markham and Oakville, Ontario, Canada; Burnaby MetroTown Development Area, British Columbia, Canada
  • 30 cubic meters (1059 cubic feet) of soil per tree
  • 15 cubic meters (530 cubic feet) per tree for trees with shared volume
North Vancouver Lonsdale Street Guideline, British Columbia, Canada 15 cubic meters (530 cubic feet) per tree for trees with shared volume
Calgary, Alberta, Canada Provide for a volume of soil suitable for a 25 year tree life span, this is approximately 14 cubic meters [494 c.f.) for a single tree. An additional 7 cubic meters [247 c.f.] of soil is required for each additional tree in interconnected plantings
Langley, British Columbia, Canada
  • Zoning Bylaw requires 10 cubic meters (353 cubic feet) of growing medium per tree planted in hard surfaced parking lots on private developments.
  • Subdivision and Development Servicing Bylaw requires 10 cubic meters (353 cubic feet) of growing medium per tree (generally street trees)
  • growing medium defined as screened, weed free, composted soil mixed according to BC Landscape Standards for the intended use and confirmed with a soil analysis report.
  • In hardscape environments, street trees are expected to be planted using structured supports such as Silva Cell to achieve the expected growing volume
Winnipeg, Manitoba, Canada, Tree Planting Details and Specifications, Downtown Area and Regional Streets
  • 8.5 cubic meters
  • (300 c.f.) to 12.75 cubic meters (450 c.f.) of soil per tree
  • 17.0 cu.m. (600 c.f.) to 25.5 cubic meters (900 c.f.) for 2 trees with shared volume
  • Optimal planting medium depth 900 mm (36in.). Minimum planting medium depth of 760 mm (30in.) will be accepted where 900 mm is not feasible.
Denver, CO Internal (city) standard: 750 c.f. of soil volume per tree
University of Florida Extension Recommendations (Urban Design for a Wind Resistant Forest)
  • Small trees (shorter than 30’) = 10’x10’x3’ = 300 c.f.
  • Medium trees (Less than 50’ height or spread) = 1,200 c.f.
  • Large trees (Greater than 50’ height or spread) = 2,700 c.f.
Minnesota B3 Guidelines
  • Small trees (e.g. serviceberry) = 400 c.f.
  • Medium trees (e.g. ironwood) = 800 c.f.
  • Large trees (e.g. hackberry) = 1,200 c.f.
If using structural soils, total soil volumes above need to be multiplied by 5 to obtain equivalent volume of soil useable by the tree.


Comparison of rock based structural soil and traditional tree soils

photo showing comparison of trees grown using different planting techniques
Overview of Bartlett’s study comparing trees grown using various planting techniques (Smiley 2013).
photo comparing trees grown with suspended pavement and in gravel/soil
Side by side comparison of suspended pavement and structural soil trees (Smiley 2013).

Research comparing rock based structural soil to loam soil indicates that significantly larger soil volumes are needed to produce the same size tree using structural soil vs. loam soils. Based on plant available water holding capacity, it appears that approximately 50 percent more Cornell University (CU) structural soil compared to a sandy loam soil to grow the same size tree (Bassuk 2010).

A pot study comparing growth of trees in CU structural soil to trees growing in loam indicates that tree growth in CU structural soil is likely limited by more than just plant available water holding capacity (Loh et al., 2003). The pot study compared the growth of fig trees in structural soil and loam soil in pots over 500 days. Tree growth was compared for the following soil types and volumes:

  • 0.4 cubic feet of loam soil (small loam soil)
  • 0.4 cubic feet of structural soil (small structural soil)
  • 2 cubic feet of loam soil (large loam soil)
  • 2 cubic feet of structural soil (large structural soil).

There were no statistically significant differences in above ground growth between trees grown in small loam soil pots and trees grown in large pots containing structural soil. This suggests greater amounts of structural soil were needed to produce the same size tree grown in the small loam soil. Only 1/5 of structural soil is composed of soil, suggesting the soil component of the structural soil is most important to the tree as a growing medium. Longer term studies are needed to confirm the exact proportion of structural soil needed to provide the value of 1 cubic foot of loam soil.

graph comparing trunk diameter over time for four planting techniques
Comparison of mean trunk diameter, in inches (y-axis), for four tree planting techniques. The studies were conducted using Bosque Elm (Smiley, 2013).
graph comparing height over time for four planting techniques
Comparison of mean tree height, in inches (y-axis), for four tree planting techniques. The studies were conducted using Bosque Elm (Smiley, 2013).

Since 2004, Bartlett Tree Research Laboratories have conducted a study comparing tree growth in natural soil under suspended pavement to growth in other media designed to prevent rooting volume compaction under pavements (stalite soil, and gravel soil (i.e. structural soil)), as well as to trees grown in compacted soil. Each tree was provided 5.7 cubic meters (200 cubic feet) of rooting space. Results to date show trees growing in loam soils in suspended pavement have greater trunk diameter and height than trees grown in rock based structural soil (Smiley et al., 2006; Smiley, 2013).

Soil quality guidelines

At this time, most Low Impact Development (LID) manuals only address trees in traditional bioretention practices (i.e. they do not provide separate soil specifications for systems that use only urban trees without herbaceous vegetation). In nature, trees typically grow with herbaceous vegetation in the same soils, but generally drier climates have fewer trees (i.e. more areas of herbaceous vegetation without trees) than wetter climates. In general tree soils need more moisture holding capacity than soils that just support herbaceous plants (depending on the tree and herbaceous species). Manufacturers of proprietary tree stormwater BMP’s such as, for example, Silva Cells tree soil systems and Cornell University (CU) structural soils, provide recommendations for soils to be used in their systems. The Puget Sound manual (Hinman and Wulkan, 2012; see Section 6.4, page 203) has a separate section on urban trees and provides some guidance on soil quality and volume for trees, but does not include a soil specification for urban trees. Many jurisdictions have their own soil specifications for urban tree planting that are not specifically targeted towards stormwater management but provide excellent guidance regarding tree needs. Books, such as Urban (2008), also address soil needs for trees.

This section provides a discussion of existing literature and research on soils designed to optimize tree growth and soils designed for optimized bioretention function. A summary of examples of soil guidelines and specifications for optimized tree growth from the literature is included at the end of this section.

Recommended soil for trees for stormwater

Bioretention Soil Mix D is recommended for bioretention with trees. Mixes B or C could also be used for bioretention with trees, but if mix B or C is used, limit the saturated hydraulic conductivity to a maximum of 4 inches per hour. Rock based structural soil may be used if the volume provided conforms to tree soil volume requirements.

Bioretention Mix D consists of sand, unscreened topsoil, and compost per the guidelines discussed below. It uses unscreened topsoil and recommends blending using a front end loader to preserve topsoil peds as much as possible. In an undisturbed soil, soil particles are clumped together into large units called peds. Peds range in size from the size of a large sand grain to several inches, so ped structure significantly increases pore space in the soil compared to a screened soil without peds. Pore spaces between soil peds improve air and water movement, water holding capacity, as well as root growth. Preserving ped structure is especially important in finer soils, because finer soils without ped structure have only very small pores and therefore have low permeability. Using an unscreened topsoil and preserving ped structure as much as possible allows for a higher clay content in Bioretention Soil Mix D compared to typical bioretention mixes because the ped structure maintains pore space and infiltration rates despite the higher clay content. The higher clay and silt content (25 to 40 percent by dry weight) is beneficial because it provides higher cation exchange capacity (for increased nutrient retention beneficial for plant growth and for increased pollutant removal) and higher water holding capacity (beneficial for plant growth).

Comparison of pros and cons of bioretention soil mixes
Link to this table.

Mix Composition in original Manual Proposed updated composition Pros Cons
A
  • 55-65% construction sand
  • 10-20% top soil
  • 25-35% organic matter2
  • 60-70% construction sand
  • 15-25% top soil
  • 15-25% organic matter2
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg (ppm) per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times
B
  • 50-70% construction sand
  • 30-50% organic matter
  • 70-85% construction sand
  • 15-30% organic matter
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
Easy to mix; least likely to clog Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly
C Not in original MN Stormwater Manual
  • 85-88 percent by volume sand and
  • 8 to 12 percent fines by volume,
  • 3 to 5 percent organic matter by volume
  • recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test
Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances1
D Not in original MN Stormwater Manual
  • All components below by dry weight:
  • 60-75% sand
  • Min. 55% total coarse and medium sand as a % of total sand
  • Less than 12% fine gravel less than 5 mm (Calculated separately from sand/silt/ clay total)
  • 2 to 5 % organic matter
  • recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test
Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances
E Not in original manual
  • 60-80% sand meeting gradation requirements of MnDOT 3126, ―Fine Aggregate for Portland Cement Concrete
  • 20-40% MnDOT 3890 Grade 2 Compost
  • 30% organic leaf compost
High infiltration rates, relatively inexpensive As compost breaks down, nutrients available for plants decreases
F Not in original manual
  • 75% loamy sand by volume:
    • Upper Limit: 85-90% sand with %Silt + 1.5 times %Clay > 15%.
    • Lower Limit: 70-85% sand with %Silt + 2 times %Clay < 30%.
    • Maximum particle size < 1-inch
  • 25% MnDOT 3890 Grade 2 Compost
Finer particles in loamy sand holds moisture for better plant growth Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing

1This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
2MnDOT Grade 2 compost is recommended.


General guidelines

Caution: The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification, however, they are NOT a finished specification.

Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Terminology and requirements in the final specifications must be consistent with the terminology in other parts of the construction documents including plans and detail nomenclature. Unless otherwise stated, the guidelines apply to Bioretention Soil Mix D.

Scheduling

Schedule all utility installations prior to beginning work.

Submittals

The following submittals are Highly Recommended.

Soil test analysis submittal

Submit soil testing results from an approved soil-testing laboratory for each soil mix. Soil suppliers that regularly prepare Bioretention Soil Mix D may submit past test results of current production runs to certify that the mix to be supplied meets the requirements, provided the test is less than 12 months prior to the submission date.

  • The testing laboratory shall be a member of the Soil Science Society of America's North American Proficiency Testing Program (NAPT), specializing in agricultural soil testing. Geotechnical engineering soil testing labs are not recommended.
  • Testing shall comply with the requirements of the Methods of Soil Analysis Part 1 and 3, published by the Soil Science Society of America, or the ASTM testing required.
  • Testing of topsoil and Bioretention Soil Mix D shall be required as defined below:
    • Physical analysis.
      • USDA particle size analysis shall include gravel, clay, silt, and coarse, medium and fine sand fractions.
      • Hydraulic Conductivity testing (Bioretention Soil Mix D only) using ASTM F1815 at 80 percent and 85 percent compaction at proctor density (ASTM D 698-91). This is a LABORATORY TEST to determine water flow at specified compaction rates.
    • Chemical analysis. Note that nutrient levels and chemical analysis shall include recommendations from the testing laboratory for ranges of each element appropriate for the types of plants to be grown in the soil mix.
      • Nutrient levels by parts per million including phosphorus, potassium, calcium, magnesium, manganese, iron, copper, zinc and calcium
      • Percent organic content
      • pH
      • Soluble salt by electrical conductivity
      • Cation Exchange Capacity (CEC). Chemical analysis shall be interpreted by the Owner based on specified plant material and testing recommendations.
Product data submittal

For each type of product, including soil cells, structural soil, coarse sand and compost, submit the manufacturer's product literature with technical data sufficient to demonstrate the product meets the requirements of the specification.

Samples for verification submittal

Submit one gallon minimum samples for the coarse sand, compost, topsoil, Bioretention Soil Mix D and structural soil. Label samples to indicate product name, source and contractor. Samples will be reviewed for appearance only. Delivered materials shall closely match the samples.

Cone penetrometer reading certification submittal

Submit for approval, a written certification that the Bioretention Soil Mix D was sufficiently compacted to fall within the required resistance ranges. The Owner may verify the certification prior to approval.

Bioretention Soil Mix D compaction testing

  • Bioretention Soil Mix D shall be tested in-situ with a cone penetrometer to the full depth of the installed soil profile or 30 inches deep, whichever is less. This is a field test to confirm correct compaction. One test shall be performed approximately every 300 square feet of Bioretention Soil Mix D surface area. The cone penetrometer shall be available at the project site at all times when the contractor is working.
  • Maintain a volumetric moisture meter on site to verify that moisture readings are within the required ranges during the installation and testing.
  • The cone penetrometer shall be “Dickey-John Soil Compaction Tester”, or “AgraTronix Soil Compaction Meter,” both distributed by Ben Meadows
  • The Contractor shall certify that penetration resistance readings meet the requirements. The Contractor’s penetrometer shall be made available to the Owner at all times, to confirm resistance readings.

Delivery, storage and handling

When warranties are required, verify with Owner's counsel that special warranties stated in this article are not less than remedies available to Owner under prevailing local laws.

  • Bulk Materials: Do not deliver or place backfill, soils and soil amendments in frozen conditions, or when the material is overly wet (defined as the material sticks to the hand when squeezed). Retain subparagraph below for bare-root stock if required; this is not an ANSI Z60.1 requirement.
  • Provide protection including tarps, plastic and or matting between all bulk materials and any finished surfaces sufficient to protect the finish material.
  • Provide erosion-control measures to prevent erosion or displacement of bulk materials and discharge of soil-bearing water runoff or airborne dust to adjacent properties, water conveyance systems, and walkways. Provide sediment control to retain excavated material, backfill, soil amendments, and planting mix within the project limits as needed.

Project conditions

Do not proceed with work when sub grade soil is frozen or is overly wet (defined as the sub grade material sticks to the hand when squeezed).

Excavation around utilities

Contractor shall carefully examine the civil, record, and survey drawings to become familiar with the existing underground conditions before digging. Notification of Local Utility Locator Service is required prior to all excavation.

Product guidelines

Specific guidelines for Mix D are discussed below.

Coarse sand

  • Coarse sand, [ASTM] C-33 Fine Aggregate, with a Fines Modulus Index of 2.8 and 3.2.
  • Sands shall be clean, sharp, natural sands free of limestone, shale and slate particles.
  • Sand pH shall be lower than 7.5
  • Provide the following particle size distribution:
Sieve size  % passing
3/8 inch 100
#4 95 to 100
#8 80 to 100
#16 50 to 85
#30 25 to 60
#50 5 to 30
#100 4 to 10
3200 2 to 4

Compost

Compost shall meet the requirements of the US Composting CouncilArchitecture/Design Specifications for Compost Use”, section Compost as a Landscape Backfill Mix Component, with the following additional requirements:

  • Compost feedstock shall be yard waste trimmings and/or source-separated municipal solid waste to produce fungi-dominated compost. Compost shall not be derived from biosolids or industrial residuals.
  • Compost physical appearance: compost shall be dark brown approximately the color of a 70 percent dark chocolate bar or darker. Particles of compost when broken shall be the same color inside as outside.
  • Compost odor: compost shall have a strong, sweet, aerobic odor indicating active biological activity. Compost with a sour anaerobic odor (indicating composting in excessive water) or an odor similar to denatured alcohol (indicating incomplete composting) shall be rejected
  • Compost testing and analysis: compost analysis shall be provided by the compost supplier. Before delivery of the compost, the supplier must provide the following documentation:

Topsoil

Topsoil texture shall be a naturally produced soil of loam, sandy loam to sandy clay loam, within the following parameters, and suitable for the germination of seeds and the support of vegetative growth.

  • Topsoil may contain up to 5 percent by volume stones, roots or masonry debris. Topsoil shall not contain metal debris, glass or other sharp objects. Topsoil shall not contain any chemicals at levels that are harmful to plants, fish or exceed EPA limitations for human contact.
  • Topsoil should not be screened or processed in a manner that breaks down soil peds. Soil peds of 2 inches in diameter or greater should be visible throughout the source pile.
  • Manufactured topsoil where sand or compost has been added to a soil material to meet the specification shall be rejected. Manufactured soil is rejected because it is screened and does not have adequate structure. Clumps or peds of soil within the sample shall be the same color and texture on the inside as the outside of the clump or ped.
  • Physical Parameters
    • Gravel: less than 10 percent by volume
    • Sand: 30 to 70 percent by volume
    • Silt: 10 to 50 percent by volume
    • Clay: 10 to 25 percent percent by volume
  • Chemical Parameters
    • Organic Matter: 2 to 8 percent by dry weight
    • pH: 5.0 to 7.3
    • Phosphorus: Sufficient to meet the maximum requirements in the Plant/Bio-retention mix once the other products are added to the mix
    • Complete submittals.

Bioretention Soil Mix D

Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:

  • silt plus clay (combined): 25 to 40 percent, by dry weight
  • total sand: 60 to 75 percent, by dry weight
  • total coarse and medium sand: minimum of 55 percent of total sand, by dry weight
  • fine gravel less than 5 millimeters: up to 12 percent by dry weight (calculated separately from sand/silt/ clay total)
  • organic matter content: 2 to 5 percent, percent loss on ignition by dry weight
  • saturated hydraulic conductivity: 1 to 4 inches per hour
  • ASTM F1815 at 85 percent compaction, Standard Proctor ASTM D698
  • phosphorus between 12 and 30 parts per million (ppm)
  • cation exchange capacity greater than 10 meq/g

Suggested mix ratio ranges by volume are

  • Coarse sand: 50 to 65 percent
  • Topsoil: 25 to 35 percent
  • Compost: 10 to 15 percent

Compost quality and particle size, coarse sand shape and variations in particle distribution, topsoil component, and silt / clay amounts within the tolerance will cause the soil blend within these suggested mix ratio ranges to drain too fast or too slow. The contractor shall adjust the final mix proportions to achieve the required drainage rate and percent organic matter. Any variation of required products, above or below the percentages listed below, needed to attain the required drainage rate or percent organic matter, shall not be grounds to change the agreed upon price of the installed material.

Lightly mix the Bioretention Soil Mix D using a front end loader to preserve topsoil peds as much as possible. Topsoil peds 2 inches in diameter or larger should be visible in the finished stockpile. Do not over mix or screen the material.

Submittals shall be completed and interpreted by the Owner based on specified plant material and testing recommendations.

Structural soil

  • A mixture of stone and soil formulated to be compacted to 95 percent of maximum dry density, Standard Proctor and to support tree roots.
  • Structural soil shall be “CU Soil” as manufactured by Amereq, Inc. New York City, NY, or approved equal.
  • Complete submittals.

Soil cells

Pre-engineered modular structures designed to hold up pavement and to be filled with soil to support tree roots and treat storm water, with the goal of protecting soil within the cells from compaction from the loads on the overlying pavement, shall be capable of supporting loads up to and including AASHTO H-20, when used in conjunction with approved pavement profiles.

Soil Cells shall meet the following requirements:

  • The structure design shall permit an uninterrupted mass of soil throughout the structure. All openings between different cell units shall have a minimum area of 36 square inches
    • to permit a continuous mass of soil that allows for capillary transfer of water;
    • to foster the growth of large tree roots; and
    • to permit the structure to be built around, over, under and through existing and proposed utilities.
  • The structure shall permit damp soils with large soil peds to be installed and for the soil to be installed without vibrating the structure. The structural openings must allow all of the soil to be checked for compaction and complete filling of all cell areas.
  • The Soil cell top surface shall be perforated to allow the free flow of water through the deck. Soil cell installation shall include all accessories, geotextiles, geogrids, and aggregate layers required by the Soil cell manufacture.
  • Complete submittals.

Execution guidelines

Site examination

Examine the surface grades and soil conditions for any circumstances that might be detrimental to soil drainage.

Soil preparation

  • Excavate to the proposed sub grade. Maintain all required angles of repose of the adjacent materials as shown on the drawings or as required to support adjacent materials or structures. Do not over-excavate into compacted sub grades of adjacent pavement or structures. Remove all construction debris and material.
  • Confirm that the sub grade is at the proper elevation and compacted as required. Sub grade elevations shall slope parallel to the finished grade and/or toward the subsurface drain lines as shown on the drawings.
  • Protect adjacent walls, walks and utilities from damage or staining by the soil. Use ½ inch plywood and/or plastic sheeting as directed to cover existing concrete, metal and masonry work and other items as directed during the progress of the work.
  • Clean up any soil or dirt spilled on any paved surface, including at the end of each working day.

Soil cell installation

Install soil cells in accordance with the manufacturer's requirements including all accessories, geotextile, geogrid, and aggregate layers.

Bioretention Mix D installation

  • Loosen or till the subsoil of the sub grade to a depth of 2 to 3 inches, or as required, with a backhoe or other suitable device. Where required on the drawings or other requirements, loosen the subsoil to a depth of 18 to 24 inches below the required subgrade elevation to improve infiltration into the subgrade soil. Loosen the soil using a backhoe by digging into the subgrade soil and lifting then dropping the soil in place. Do not over work the soil or break up the large clumps in the soil created by the process. Do not allow the loosened soil to become re-compacted by other work.
  • Install all required drainage aggregate and drain lines.
  • Install Bioretention Soil Mix D in approximately 12 inch lifts to the required depths. Lightly compact each lift with a maximum of two passes with a 5 horsepower vibrating plate tamper to achieve the required penetration resistance. Maintain volumetric soil moisture between 5 and 15 percent during installation and compaction.
  • Scarify the surface of each lift with the teeth of a back hoe or similar equipment prior to installing additional lifts.
  • Do not drive over delivered soil to spread or grade. Install soil in narrow bands, working out from the installed soil such that soil delivery and spreading equipment does not have to pass over previously installed soil. The width of each band of installed soil shall not exceed the reach of the delivery equipment.
  • Coordinate the installation of water harvesting distribution and drain lines within the Bioretention Soil Mix D.
  • Install soil in Soil Cells according to the cell manufacturer’s requirements.
  • Grade the finished surface of the Bioretention Soil Mix D to the grades indicated on the drawings plus extra soil for settlement as noted below.
    • The Bioretention Soil Mix D, when properly installed at the compaction levels indicated, will continue to settle. It is not the intent of these requirements to construct a soil profile that is immediately stable. Add an additional 1 inch of soil for each 10 inches of installed soil depth. The grades shown on the drawings are the grades after the anticipated settlement.
    • The tolerance for finished grades shall be plus or minus ½ inch in 10 feet.

Bioretention Soil Mix D compaction

Compact the Bioretention Soil Mix D so that the pressure reading of a cone penetrometer is between 75 and 200 pounds per square inch (psi) with the volumetric soil moisture between 5 and 15 percent.

Structural soil installation

Install and compact Structural Soil in accordance with the manufacturer's requirements.

Cleanup

Once installation is complete, remove any excess soil and trash from pavements, structures or fixtures. Remove any spilled oil or other stains on surfaces caused by the work.

Protection

Protect work and materials from damage including compaction, contamination, and erosion due to operations by other contractors or by trespassers. Repair all damage and loosen compaction prior to acceptance.

Information: Excessive soil compaction is considered the most important factor affecting tree health since it affects soil water availability, soil aeration, and soil drainage. Shanstrom (2014) provides a discussion of compaction in The Most Important Factor for Growing Healthy Trees. The manual page on Alleviating compaction from construction activities, while written for active construction sites, provides some guidance that can be applied to alleviating compaction

Repair of settled Bioretention Soil Mix D

Twelve months after the date of substantial completion of the Bioretention Soil Mix D installation work, inspect the site and restore any areas where the grades have settled beyond the elevations shown on the drawings by an amount greater than 5 percent of the soil depth.

Literature review of soils optimized for tree growth

Examples of soil guidelines and specifications for optimized tree growth are discussed below.

  • Toronto Street Trees Guide to Standard Planting Options. 2010. City of Toronto Urban Design Streetscape Manual. In collaboration with Parks, Forestry & Recreation.
    • High-quality soil shall consist of a minimum 0.9 meter and maximum 1.2 meter depth, over and above any required drainage system and/or granular material, be uncompacted, and be sandy loam with the following composition:
      • Sand: 50 to 60 percent
      • Silt: 20 to 40 percent
      • Clay: 6 to 10 percent
      • Organic: 2 to 5 percent
      • pH = 7.5 or less
  • DTAH et al 2013 tree manual for Toronto recommends:
    • pH 6.0 to 7.8
    • Drainage rates of between 12 millimeter and 75 millimeter per hour. Drainage rates must be measured in the field; however - soil can be tested in the lab if it is compacted to a prescribed density.
    • Soil organic matter should be between 3 and 5 percent dry weight in the upper layer of the profile at the time the soil is installed. Organic matter in the lower soil level should be between 1.5 and 3 percent dry weight. This difference can be accomplished by making two different soils or tilling additional compost into the surface of the soil.
    • See pages 56 to 62 of the manual for detailed conceptual explanation of other tree soil requirements, including the importance of soil structure and obtaining unscreened soil with soil ped structure intact, recommendations for soil mixes and proportion of coarse to medium sands; difference between natural soil organic matter and compost.
  • Urban (Up by Roots: Healthy Soils and Trees in the built environment, 2008) contains extensive explanations of tree soil needs. Some relevant conclusions and recommendations include the following.
    • Develop a soil that has good drainage capability, while providing adequate moisture and nutrient holding capacity for the plants.
    • Ensure the final coarse to medium size sand content exceeds 50 percent, because at lower amounts of sand, drainage rates are not increased.
    • As the sand and aggregate content increases above 55 percent, the water and nutrient holding capacity goes down and the drainage rate goes up.
    • High sand content soils are reliably drained, are more tolerant of a wide range of compaction rates, and are are less likely to become over compacted, but require a continuous supply of water and nutrients, which increases maintenance. Note that directing stormwater to these soils lessens this problem because it increases water and nutrient supply, though the trees will likely still be exposed to periodic droughts.
    • As the sand is reduced in the mix, close to the 55 percent threshold, compaction rates become critical. With too much compaction in finer-grained soil, drainage rates become unacceptable. With too little compaction in finer grained soil soil settlement can become unacceptable. The window between too much and too little compaction is hard to manage during construction.
    • Soil mix design is a multi-step process and does not end with the publication of specifications. The specifications should set performance standards and the types of mix components to be used. At the time of writing the specifications, the designer cannot know the actual source of the soil or organic amendment. Minor variations in these materials will change the mix proportions.
    • Testing of the soil to be used in the mix must include the measuring of the difference sizes of sand particles, known as sand fractions.
    • A good measure to evaluate the performance of a soil mix is its infiltration rate when compacted to a known level. A developing standard is to test infiltration at 80 and 85 percent of maximum dry density as measured by the Standard Proctor test.

Literature review of soils optimized for bioretention

Using trees as stormwater BMP’s adds the following soil requirements to those of traditional street trees that are not planted for stormwater management.

  • As more stormwater is directed to the trees, there is more danger of soils clogging, so limiting the fines content of the soil becomes more crucial. However, research shows that while the hydraulic conductivity of a bioretention practice typically decreases initially, hydraulic conductivity goes back up as plants and microbes improve soil structure over time and thus the infiltration rate does not decrease significantly long term (Hatt et al., 2009; Jenkins et al., 2010; Li & Davis, 2008b; Barrett et al., 2011).
  • Where nutrient reduction in stormwater runoff is a goal, limiting the nutrient content of the soil is required to minimize nutrient leaching from the soil.

A representative sampling of recent comprehensive literature on bioretention media guidelines is summarized below.

  • North Carolina Bioretention Soil Specification (referenced e.g. in North Carolina Department of Environment and Natural Resources. 2009. Stormwater BMP Manual Chapter 12: Bioretention. Revised 07-24-09.). This is one of the most widely used bioretention media specifications in the US (i.e. several other states have also adopted it). Some important conclusions and recommendations include the following.
    • Bioretention soils may clog more easily in northern climates where de-icers are used (Bannerman 2013, personal communication). Additional research is needed to determine if flushing salts out of the soil in spring with water eliminates SAR (sodium adsorption ratio) effects).
    • A minimum of one tree, three shrubs, and three herbaceous species should be incorporated in the bioretention planting plan unless it is a grassed cell.
    • Although there is some concern among tree specialists that the North Carolina media mix (Mix C) may have too little organic matter to grow healthy trees, this soil has been used extensively to grow trees with success in North Carolina (Winston and Hunt, 2013, personal communication). North Carolina does receive more annual precipitation than Minnesota, so moisture retention is more crucial in Minnesota than in North Carolina and trees may not perform as well in Minnesota as in North Carolina in a soil with low moisture content. Research by Fassman et al. (2013), however, indicates that sand based bioretention media can have as much plant available water as horticultural soils with less sand and more organic matter.
  • Australian research on using trees for bioretention: Breen 2004; Denman 2006; Denman 2011 (all describing same experiment)
    • Trees grew equally well in soils of a wide range of saturated hydraulic conductivities, and grew better when irrigated with stormwater versus tapwater. “Trees were grown outdoors in experimental biofiltration systems, constructed with 240 millimeter (9.45 inch) diameter columns, cut into 600 millimeter (23.6 inch) lengths. The constructed soil profiles were 500 millimeters (20 inches) deep with 10 percent (v:v) composted green waste added to the surface 200 millimeters (8 inches). The three soils used were sands with saturated hydraulic conductivities (SHC) of 4, 95 and 170 millimeters per hour (0.16, 3.75 and 6.7 inches per hour) and the soils are referred to as low, medium and high SHC soil, respectively."
    • Tree growth was similar in the three soils studied (Denman 2006).
    • Trees grew taller and had greater root density when irrigated with stormwater compared to tapwater.
    • The low SHC soil was more effective in reducing nitrogen losses, particularly the inorganic forms(Denman 2006).
    • Averaged across all species, planting resulted in an increase in infiltration rate compared to the unplanted control (Breen et al 2004).
  • Fassman et al., 2013, (Media Specification for Stormwater Bioretention Devices) produced a 115 page report that includes a literature review of bioretention media, a summary of trends in how bioretention media specifications are changing over time based on recent research and experience, and research to determine best bioretention media for New Zealand. The following conclusions were reported.
    • Common reasons for hydraulic failure of bioretention media include:
      • incorrect media specification, where the media has incorrect physical/chemical properties for removing targeted pollutants;
      • incorrect media specification, where the media may have high clay content or extremely fine particles, and vulnerability to compaction which cause inadequate drainage and over-extended ponding;
      • incorrect compaction, often resulting from poor compaction specifications or using media vulnerable to compaction. The media is either under-compacted and too loose resulting in low contact time, or over-compacted and too dense (resulting in inadequate drainage and over-extended ponding); and
      • clogging, where excessive sediment loads restrict the pores of the media, hindering infiltration and causing inadequate drainage and over-extended ponding. Clogging most commonly occurs at the surface as crusting, capping, or sealing. Sediment from unstable catchments or from catchments with active construction, or fine particles within the filter media may contribute to clogging.
    • Cites that Warynski and Hunt (2011) “performed an inspection of 20 bioretention cells throughout the state. They found 82 percent of bioretention cell filter media having incorrect particle size distributions (PSD), and 44 percent of bioretention cells having incorrect permeabilities. Furthermore, 50 percent of bioretention cells were undersized.”
    • Summarizes recommended typical bioretention media composition specifications from worldwide sources (see table below).

Summary of recommended bioretention filter media mixes from worldwide sources
Link to this table.

Guideline Aggregate Organic Note
Auckland Regional Council (2003), Waitakere City Council (2004) Sandy loam, loamy sand, loam, loam/sand mix (35 - 60% v/v sand) Not specified Clay content < 25% v/v1
Prince George’s County, Maryland (2007) 50 - 60% v/v sand 20 - 30% v/v well aged leaf compost, 20 - 30% v/v topsoil2 Clay content < 5% v/v
The SUDS manual (Woods-Ballard et al. 2007) 35 - 60% v/v sand, 30 - 50% v/v silt 0 - 4% v/v organic matter 10 - 25% v/v clay content
Facility for Advanced Water Biofiltration (FAWB, 2009a) Washed, well graded sand with specified PSD band 3% w/w organic material Clay content < 3% w/w, top 100 mm to be ameliorated with organic matter and fertilizer
Seattle Public Utilities (2008) 60 - 65% v/v mineral aggregate, PSD limit (“clean sand” with 2 - 5% passing #200 sieve), U3 ≥ 4 35 - 40% v/v fine compost which has > 40% w/w organic matter content
Puget Sound Partnership (2009) 40% v/v compost, or 8 - 10% w/w organic matter
North Carolina Cooperative Extension Service (Hunt & Lord 2006) 85 - 88% v/v washed medium sand4 3 - 5% v/v organic matter 8 - 12% v/v silt and clay
City of Austin (2011) 70 - 80% v/v concrete sand5 20 - 30% v/v screened bulk topsoil2 70 - 90% sand content, 3 - 10% clay content, silt and clay content < 27% w/w. Warning not to use sandy loam (“red death”).6

1 % v/v is percent by volume; % w/w is percent by weight (mass)
2“Topsoil” is a non-technical term for the upper or outmost layer of soil, however there is no technical standard for topsoil.
3U, Coefficient of Uniformity = D60/D10, where D60 is particle diameter at 60% passing and D10 is particle diameter at 10% passing.
4A specific definition for “medium sand” was not identified. ASTM D2487-10 classifies coarse-grained sandsas those with > 50% retained on the (USA) No. 200 sieve (75 m) and > 50% of coarse fraction passing the No. 4 sieve (4.76 mm). Clean sands contain < 5% fines. Fine-grained soils are silts and clays whereby > 50% passes the No. 200 sieve.
5Concrete sand is described by ASTMD2487-10 as coarse sand that is retained by a (USA) No. 10 sieve (2.00mm)
6“Red death” is commercially available fill material in Austin marketed as sandy loam.


    • While hydraulic conductivity initially declines as the filter media is compacted, FAWB (2009a) and Barret et al. (2011) found it often recovers back to the design value over time as increased plant root growth counters the effects of compaction and clogging.
    • Summarizes worldwide sampling of typical bioretention media hydraulic conductivity specifications (see table below).

Summary of recommended hydraulic conductivities of bioretention filter media
Link to this table.

Publication Hydraulic conductivity
Auckland Council Rain Garden Construction Guide (2011) 12.5 mm hr-1 (minimum)
California Bioretention TC-32 (CASQA, 2003) 12.5 mm hr-1 (minimum)
City of Austin (2011) 50.8 mm hr-1 (minimum)
USEPA (2004) 12.7 mm hr-1 (minimum)
FAWB (2009b) 100 - 300 mm hr-1 (temperate climates) 100 - 500 mm hr-1 (tropical climates)
Prince George’s County, Maryland (2007) 12.7 mm hr-1 (minimum)
The SUDS manual (Woods-Ballard et al. 2007) 12.6 mm hr-1
North Carolina Cooperative Extension Service (Hunt and Lord 2006) 25.4 mm hr-1 (for nitrogen removal) 50.8 mm hr-1 (for phosphorus, metal and other pollutant removal)
Puget Sound Partnership (2009) Seattle Public Utilities (2011) 25.4 - 305 mm hr-1


    • Particle size distribution (PSD) is used as a gauge of the hydraulic performance of a filter media in several international guidelines.
    • PSD may be a useful gauge of the potential hydraulic performance of a filter media, but it should not be used to replace hydraulic conductivity testing.
    • In addition to meeting gradation limits, media should be well-graded over the entire range to avoid structural collapse due to particle migration (FAWB, 2009a).
    • Summarized a world wide sampling of specifications for compaction during construction (see table below).

Summary of recommended installation methods
Link to this table.

Jurisdiction Guideline on lifts Guideline on compaction
Prince George’s County, (2007) 200 to 300 mm lifts Natural compaction with light watering
ARC TP10 (2003) 300 to 400 mm lifts Loose compaction by light tamping with backhoe bucket
North Shore City Council (2009) 300 mm lifts Natural compaction with wetting of soil
Melbourne (FAWB 2009b) Two lifts if depth is over 500 mm Light compaction; single pass with vibrating plate for small systems; single pass with roller for large systems
Seattle Public Utilities (2008) Loose lifts Compact to 85 to 90% of modified maximum dry density
California Stormwater (CASQA 2003) 460 mm or greater lifts Light compaction


    • If specific compaction details (such as water content) are not designed for, filter media may easily be over or under compacted, leading to an undesirable hydraulic conductivity and potential failure of the bioretention cell. An advantage of materials that are relatively insensitive to compaction is a greater certainty of achieving design conductivity range.
    • The density of predominantly sand based media are less susceptible to the effects of compaction and water content.
    • Mixes that met the author’s desired hydraulic conductivity range did not meet international PSD guidelines. The three mixes that best fit their desired hydraulic conductivity range had higher fines than the recommended guidelines, while two out of three are poorly graded. The authors conclude that “With this result, it is clear the PSD-based guidelines should not be used as a substitute to hydraulic conductivity testing.”
    • A coarse sand (all passing 2 millimeters, with U 3) or high silt/clay component (considered to be greater than 20 percent, as might be found in natural soils) will be susceptible to the field installation compaction method, and hence so will be the hydraulic conductivity. In practice, if these materials are used, careful installation procedures and post-installation testing of infiltration and/or bulk density would be strongly recommended.
    • Mixes that satisfied aggregate PSDs tended to produce extremely high hydraulic conductivities, even when mixed with relatively high proportions of compost (which also violates many international guidelines).
    • Because the sand based bioretention media developed in this project had low organic matter content (less than 10 percent v/v compost, equating to 1 to 3 percent (w/w) total carbon), and also had very low clay and silt components, it was expected that plants might not grow well in them because they were thought to have little ability to store and supply water and nutrients to the plants. Therefore the authors tested two of their sand based mixes for plant growth against a rich horticultural soil mix, containing 30 percent compost by volume, known to grow vigorous plants. They grew 2 bioretention plant species for 6 months in pots of each of the three soils, with a low rate of 9 month slow release fertilizer added to the sand-based mixes only. No differences in plant growth or plant available water were observed between the plants growing in sand based mixes vs. the horticultural mix. They concluded that: “the volume of stored water that plants can access for growth is similar in all substrates, being 21 to 24 percent of the total soil volume. This is because the two commercial mixes have a large amount of water that is held very tightly (to the organic matter) and therefore inaccessible to plants… It is unlikely the three sand-based mixes developed will be any more drought prone than existing commercial mixes with high organic contents, as all store similar volumes of plant-available water per unit depth...” If these results also hold true in longer term field experiments, plants should be able to grow just as well in bioretention media with low organic matter content as in horticultural mixes designed for vigorous plant growth. Minimizing soil organic matter content is crucial to maximize nutrient reduction by bioretention systems and to minimize nutrient leaching from the soil.
  • Australian guidelines for bioretention media (FAWB 2009)
    • Based on extensive research, a bioretention BMP requires three layers of media:
      • the filter media itself (400 to 600 millimeters (15.75 to 23.62 inches) deep, or as specified in the engineering design);
      • a transition layer (100 millimeters (4 inches) deep)); and
      • a drainage layer (50 millimeters (2 inches) minimum cover over underdrainage pipe).
    • The biofiltration system will operate so that water will infiltrate into the filter media and move vertically down through the profile.
    • The biofiltration system is required to support a range of vegetation types (from ground covers to trees) that are adapted to freely draining soils with occasional wetting.
    • The material should be based on natural or amended natural soils or it can be entirely engineered… In the case of natural or amended natural soils, the media should be a loamy sand.
    • In general, the media should have an appropriately high permeability under compaction and should be free of rubbish, deleterious material, toxicants, declared plants and local weeds (as listed in local guidelines/Acts), and should not be hydrophobic.
    • The filter media should contain some organic matter for increased water holding capacity but be low in nutrient content.
    • Requires minimum 3 percent organic matter by weight; does NOT specify maximum organic content, just maximum nutrient content. NOTE: Research in North Carolina indicates that using trees with suspended pavement and soils higher in organic matter content can still attain good nutrient reduction if soil nutrient content is below North Carolina standards (Jonathan Page, NC State, 2013, personal communication).
    • Maintaining an adequate infiltration capacity is crucial in ensuring the long-term treatment efficiency of the system. The ability of a biofiltration system to detain and infiltrate incoming stormwater is a function of the filter surface area, extended detention (ponding) depth, and the hydraulic conductivity of the filter media. Most importantly, design of a biofiltration system should optimize the combination of these three design elements.
    • For a biofiltration system in a temperate climate with an extended detention depth of 100 to 300 millimeters (4 to 12 inches) and whose surface area is approximately 2 percent of the connected impervious area of the contributing catchment, the prescribed hydraulic conductivity will generally be between 100 to 300 millimeters per hour (4 to 12 inches per hour) in order to meet best practice targets. This configuration supports plant growth without requiring too much land space.
    • The infiltration capacity of the biofiltration system will initially decline during the establishment phase as the filter media settles and compacts, but this will level out and then start to increase as the plant community establishes itself and the rooting depth increases(FAWB, 2009) based on research by Hatt et al. (2009).
    • The hydraulic conductivity of potential filter media should be measured using the ASTM F1815-06 method. This test method uses a compaction method that best represents field conditions and so provides a more realistic assessment of hydraulic conductivity than other test methods. Note: if a hydraulic conductivity lower than 100 millimeters per hour (4 inches per hour) is prescribed, the level of compaction associated with this test method may be too severe and so underestimate the actual hydraulic conductivity of the filter media under field conditions. However, FAWB considers this to be an appropriately conservative test, and recommends its use even for low conductivity media.
    • PSD is of secondary importance compared to hydraulic conductivity. A material whose PSD falls within the following recommended range does not preclude the need for hydraulic conductivity testing; i.e., it does not guarantee that the material will have a suitable hydraulic conductivity. However, the following composition range (percentage w/w) provides a useful guide for selecting an appropriate material:
Texture Percent Size
Clay and silt < 3% < 0.05 mm
Very fine sand 5 to 30% 0.05 to 0.15 mm
Fine sand 10 to 30% 0.15 to 0.25 mm
Medium to coarse sand 40 to 60% 0.25 to 1.0 mm
Coarse sand 7 to 10% 1.0 to 2.0 mm
Fine gravel < 3% 2.0 to 3.4 mm
    • Clay and silt are important for water retention and sorption of dissolved pollutants; however they substantially reduce the hydraulic conductivity of the filter media. This size fraction also influences the structural stability of the material (through migration of particles to block small pores and/or slump). It is essential that the total clay and silt mix is less than 3 percent (w/w) to reduce the likelihood of structural collapse of such soils. The filter media should be well-graded; i.e., it should have all particle size ranges present from the 0.075 millimeter to the 4.75 millimeter sieve (as defined by AS1289.3.6.1 - 1995). There should be no gap in the particle size grading, and the composition should not be dominated by a small particle size range. This is important for preventing structural collapse due to particle migration.
    • Total Nitrogen (TN) Content – less than 1000 milligrams per kilogram.
    • Orthophosphate (PO43-) Content – less than 80 milligrams per kilogram. Soils with total phosphorus concentrations greater 100 miligrams per kilograms should be tested for potential leaching. Where plants with moderate phosphorus sensitivity are to be used, total phosphorus concentrations should be less than 20 milligrams per kilogram.
    • For engineered media, they recommend
      • a washed, well-graded sand with an appropriate hydraulic conductivity should be used as the filter medium.
      • the top 100 millimeters (4 inches) of the filter medium should then be ameliorated with appropriate organic matter, fertilizer and trace elements. This amelioration is required to aid plant establishment and is designed to last four weeks, the rationale being that beyond this point the plants receive adequate nutrients via incoming stormwater.

Definitions

  • Clay, silt and sand soil particles: Per USDA size designations. It is critical to NOT use testing laboratories that report results in engineering size designations such as the Unified or AASHTO systems.
  • Coarse Sand: Process washed and graded sand from regional sand suppliers.
  • Compost: Decomposed plant based biomass.
  • Bioretention Soil Mix A: A mixture of topsoil, coarse sand and compost intended to fill bio- retention planters to support the treatment ofstorm water and the growth of trees and other plants.
  • Screened Soil: Any soil run through any type of screen with a mesh size 3 square inches or smaller.
  • Soil Cells: also called structural cells; pre-engineered structural system to hold up the sidewalk and be filled with soil to support tree roots and treat stormwater.
  • Soil Peds: Clumps of soil that naturally aggregate during the soil building process.
  • Structural Soil: A mixture of stone and soil formulated to be compacted to 95 percent of maximum dry density, Standard Proctor, and support tree roots.
  • Topsoil: Fertile, friable, loamy soil, harvested from natural topsoil sources.

References


Construction guidelines

Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.
Caution: Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.

Standard keys to success in bioretention construction apply to trees for bioretention, including the following.

  • Plan for feasible temporary and permanent erosion and sediment control techniques and sequencing.
    • Plan for temporary and permanent erosion and sediment control techniques, sequencing, and pay items, and prepare a thorough SWPPP plan. Example techniques include compost logs (MnDOT 2573) and plastic sheeting (MnDOT 2575), and diversion berms (MnDOT 2573).
    • Plan to minimize or avoid soil compaction to the extent feasible. Techniques include, for example, using drivable mats, using tracked machinery, and machinery with long arms to avoid having to drive in tree trenches.
    • Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control.
  • Plan for snow storage during (if applicable) and after construction
  • Construction administration and communication with contractor
    • Effective communication during pre-construction meeting.
    • Include “check points” in specifications with timelines – points of inspection which must be approved before proceeding to next step of construction. These will vary depending on specific project,but will include, for example, approval of required submittals, and required testing, such as, for example infiltration tests.
    • Ensure checkpoints are approved prior to proceeding to next steps. Where applicable, require signature by Contractor, Designer, and Chief Inspector prior to proceeding to next step.
    • Require submittals for material to be used, including, sources and certifications where applicable.
    • Specify required tests and tolerances.

See the bioretention section for other construction guidelines and specifications.

Construction guidelines and specifications specifically for trees for bioretention

In addition to general bioretention guidelines and specifications, the following guidelines and specifications apply specifically to trees for stormwater design.

  • Tree material and installation guidelines
  • Tree soil quality and volume guidelines
  • Tree opening guidelines
    • Tree openings need to be large enough to allow for trunk flare. Minimum recommended tree opening dimension is 5 feet by 5 feet.
    • The use of tree grates is discouraged in order to protect the tree root flare.
    • Tree openings need to be protected from foot and vehicular traffic.
  • Tree spacing guidelines
    • For street trees, a minimum spacing of 30 feet on center is recommended for large trees to allow their canopies to grow to their full size. This also makes it easier to provide adequate soil volumes for each tree.
  • Guidelines for providing rootable soil volume for tree root growth and bioretention under pavement. Where there is not enough open space for traditional bioretention, several techniques exist to protect soil volume under pavement from traffic compaction so that this soil can be used both for bioretention and tree root growth. Examples of these techniques include:
    • Structural cells
    • Rock based structural soil
    • Sand based structural soil
    • Soil boxes

Each of the above techniques is described and compared below. Links to construction guidelines are also provided.

Suspended pavement

Image of completed silva cell system
Trees planted in suspended pavement on Tryon Street Mall, Charlotte, NC (Image source: The Kestrel Design Group, Inc)

In areas that do not have enough open space to grow large trees, techniques like suspended pavement can be used to extend tree rooting volume under HS-20 load bearing surfaces and create favorable conditions to grow large trees in urban areas. This rooting volume can also be used for bioretention. While suspended pavement has been built in several different ways, all suspended pavement is held slightly above the soil by a structure that “suspends” the pavement above the soil so that the soil is protected from the weight of the pavement and the compaction generated from its traffic.

One of the earliest examples of trees grown in suspended pavement is in Charlotte, North Carolina, where a reinforced concrete sidewalk was installed over the top of poured in place concrete columns. While this is an effective way to grow large trees, it is labor intensive and requires intensive surveying to ensure that column heights are precise.

A more recently developed, and less labor intensive, technique to build suspended pavement is through the use of soil cells. An example is Silva Cells, modular proprietary pre-engineered structural cells manufactured by Deeproot Green Infrastructure. Another example are the Stratacell and Stratavault systems manufactured by Citygreen Systems. The modular design allows flexibility to size the rooting/bioretention volume as needed for each site. Underground utilities can be accommodated within these systems. Because soil in a suspended pavement system is protected from compaction from loads on pavement above the cells, a wide range of soils can be used in these systems, so soil can be tailored to desired functions (e.g. tree growth and stormwater management). Construction documents and specifications for a wide range of soil cell applications can be found on the manufacturers' websites (Deeproot Green Infrastructure and Citygreen Systems).

Photo galleries below illustrate the Silva Cell and Strata Cell/Stratavault technologies.

Rock-based structural soils

Conceptual diagram of CU-Structural Soil™ including stone-on-stone compaction and soil in interstitial spaces used as a base course for pavements
Conceptual diagram of CU-Structural Soil™ including stone-on-stone compaction and soil in interstitial spaces used as a base course for pavements. Source: Urban Horticulture Institute, Cornell University
Typical streetscape with tree planted in CU structural soil
Typical streetscape with tree planted in CU structural soil. Source:Urban Horticulture Institute, Cornell University

Rock based structural soils are engineered to be able to be compacted to 95 percent Proctor density [6] without impeding root growth. Rock based structural soils are typically gap graded engineered soils with

  • stones to provide load bearing capacity and protect soil in its void spaces from compaction,
  • soil in rock void spaces for tree root growth, and
  • tackifier to keep the soil uniformly distributed in the rock void spaces (tackifier is only found in some kinds of rock based structural soil).

Stone lattice

Desired characteristics for the stone base used in rock based structural soils include the following.

  • The stones should be uniformly graded and crushed or angular for maximum porosity, compaction, and structural interface (Bassuk et al 2005).
  • Mean pore size should be large enough to accommodate root growth (Lindsey 1994).
  • Significant crushing of stone should not occur during compaction (Lindsey 1994).

According to University of California-Davis (1994), 2 inch stones would be able to support most tree roots. Because limestone has been found to crush on some projects, granite stone is recommended. If limestone is used, it should meet specifications described in the permeable pavement section of this manual.

Soil

Soil needs to have adequate nutrient and water holding capacity to provide for the tree’s needs.

Tackifier

The tackifier, if used, should be non-toxic and non-phototoxic.

Construction drawings are available on Cornell University’s website, including

  • detail and plan for using structural soil in a planting island;
  • detail and plan for using structural soil in a limited soil volume planter;
  • detail for using structural soil with permeable pavers; and
  • detail for structural soil breakout zone.

Types of structural soils

Several types of rock based structural soils have been developed, including

  • Cornell University (CU) structural soil: 80 percent stone with size ranging from 0.75 to 1.5 inches and 20 percent loam to clay loam soil with minimum 5 percent organic matter, by dry weight, and hydrogel to uniformly mix the stone and soil (Bassuk et al., 2005). Patented formula available only from licensed producers to ensure quality control. Considerable information can be found on Cornell University's website.
  • Stalite structural soil: 80 percent Stalite, a porous expanded slate rock (0.75 inches), and 20 percent sandy clay loam soil (by volume)(Xiao and McPherson 2008).
  • Swedish structural soil
  • University of California (UC) Davis structural soil

Note: University of California (UC) Davis structural soil is not designed to be load-bearing and therefore should not be compacted. This structural soil is 75 percent lava rock (0.75 inches) and 25 percent loam soil (by volume) (Xiao and McPherson, 2008). Because of the lava rock, this soil stores more stormwater than other structural soils and has a very high surface area to facilitate pollutant trapping.

Wenz provides a discussion of some structural soils, including case studies.

Design considerations for trees growing in structural soil

Day and Dickinson (2008) provide information on use of trees in structural soils, including design specifications. The following considerations should be made in using structural soils.

  • Soil pH: Care must be taken to select species tolerant of structural soil pH. For example, if limestone based structural soil is used, trees tolerant of alkaline pH must be selected, as limestone can raise the pH of soil to 8.0 or higher (Bassuk, 2010 soil debate; Urban, 2008).
  • Drainage Rate: Because rock based structural soils drain quickly (greater than 24 inches per hour), designers should select tree species tolerant of extremely well drained soils (Bassuk, 2010).
  • Volume of rock based structural soil needed for healthy tree growth: Because only 20 percent of the volume of a rock based structural soil is actually soil, a greater total volume of rock based structural soil is needed compared to growing the same size tree in a sandy loam soil. A pot study by Loh et al. (2003) found that 5 parts of structural soil were needed to provide the soil value of 1 part of loam soil. Similarly, an on-going study at Bartlett Tree labs is finding that over the past 9 years, trees growing in loam soil in suspended pavement have been consistently outgrowing trees growing in equal volumes of rock based structural soils, stalite soil, and compacted soil.

Based on the above studies, Urban (2008) recommends: “Given the extreme inefficiency of the ratio of excavated volume to soil usable by the tree, strips of structural soil less than 20 feet wide might be better constructed as soil trenches or structural cells, where more soil can be included for less cost. A 5-foot wide soil trench set of structural cells…will provide more soil usable by the tree than a 20 foot wide trench of soil/aggregate structural soil. Soil/aggregate structural soils may have applications as a transition to other options, and to add soil in places where other options may not be practical. These might include tight, contorted spaces and fills around utility lines and against foundations where full compaction is required.”

More information about rock based structural soils is available online at

Sand based structural soil

Sand based structural soils were first developed in Amsterdam when some trees were in poor condition because of an “unfavorable rooting environment” (Couenberg, 1993). Because the natural soils in Amsterdam, bog-peat, was non-load bearing, the top 2 meters of soil had been replaced with a medium coarse sand, which had insufficient nutritional value. Amsterdam soils were developed in an effort to grow better trees but still provide adequate bearing capacity for pavement bearing light loads, such as sidewalks. The Dutch studied various mixes for tree growth, soil settlement, and several other parameters. The resulting Amsterdam Tree Soil contains medium coarse sand with 4 to 5 percent organic matter and 2 to 4 percent clay by weight and also meets other criteria, including, for example, (1) the medium coarse sand must meet specific gradation requirements, (2) soil mix must be free of salt, (3) mix must contain less than 2 percent particles below 2 micrometers, and (4) amount of particles below 2 micrometers must be considerably less than the amount of organic materials (Couenberg 1993).

All medium coarse sand (the layer above the Amsterdam Tree Soil) is compacted to 95 percent to 100 percent Proctor density. Amsterdam tree soil is not compacted to 100 percent density, but “is compacted until the soil has a penetration resistance between 1.5 and 2 MegaPascal (187 to 250 pounds per square inch (PSI)) ... Comparison of soil density values after filling with soil density at 100 percent Proctor Density has shown that soil density of Amsterdam Tree Soil after filling amounts to 70 to 80 percent Proctor Density” (Couenberg 1993).

Amsterdam Tree Soil was found to settle 19 millimeters [0.75 inches] in 3 years compared to the surrounding pavement, which was acceptable according to Dutch standards (Couenberg 1993), but may not be acceptable to many communities in the US to minimize risk of litigation related to trip and fall hazards.

The standard design in Amsterdam includes the following from bottom to top.

  • Ground water table 1 to 1.2 meters below ground level
  • Saturation zone 10 to 20 centimeters
  • Layer of compacted, non-saturated sand
  • Amsterdam tree soil compacted in 2 layers of 40 to 50 centimeters
  • Compacted layer of 10 centimeters medium coarse sand for paving.
  • Pavement, typically concrete pavers 30 x 30 x 5 centimeters (Couenberg 1993)

While Amsterdam sand based soils work well in Amsterdam, trees grown in Amsterdam soils in Minnesota would likely need significant irrigation, as they have low water holding capacity. Amsterdam receives an average of 36 inches of rain per year, which is higher than the range of average yearly rainfall in Minnesota. Rainfall events are also more frequent and lower intensity in Amsterdam than in Minnesota. Summer temperatures are also higher in Minnesota than in Amsterdam, resulting in higher water needs by trees. According to Couenberg (1993), who helped develop Amsterdam soils, “the Amsterdam tree soil has been developed in an area where there is sufficient rainfall. If this soil is used in other climatic areas, adaptations to the tree pit will have to be made after the local situation has been monitored.”

Most of Amsterdam also has a high water table which is controlled to have almost no seasonal fluctuation, and groundwater wicks up into tree rooting zones by capillary action, so trees always have access to water from the ground water table. According to Urban (2008), “tests of Amsterdam planting soil in other locations in Europe without high water tables showed less promising results with overly dry soil, which required significant supplemental water.” So, in summary, sand based structural soils may be a viable in Minnesota if some settlement is acceptable with light structural loads and trees are irrigated.

For more information on Amsterdam Tree Sand see [7]

Soil boxes

Soil boxes are concrete boxes designed for bioretention. They are typically proprietary products, such as, for example, the boxes made by Filterra and Contech. Rooting volume capacity of these boxes is typically limited to large shrubs. Soil volumes provided by these boxes are typically not sufficient to grow healthy large trees. A standard 6 feet by 6 feet Filterra box, for example, provides 72 cubic feet of soil, assuming a 2 foot depth of soil. Given that the recommended soil volume for trees is 2 cubic feet of soil per 1 square foot of tree canopy, this is only enough to support a tree with a 5 foot radius canopy.

Comparison of soil volumes, open space, and underground space needed for open grown tree vs. tree in suspended pavement, rock based structural soil, sand based structural soil, and soil boxes.
Link to this table

Technique Soil volume provided per cubic foot rooting zone (not including pavement profile where applicable) Open space needed to grow a 30’ diameter canopy tree assuming 34” soil depth1 Total volume recommended per 30’ diameter canopy tree1 Structural Capacity (Traffic load supported)
Open tree pit (no pavement or foot traffic above rooting space) 1 ft3 500 2 1413 ft3 Not even foot traffic should be allowed on open tree pits
Structural cells 0.92 ft3 5 ft by 5 ft 1536 ft3 Can support vehicle loading up to AASHTO H-20 rating of 32,000 lbs. per axle (U.S. Federal Highway Bridge Standard). This rating refers to the ability of a roadway to safely accommodate 3-4 axle vehicles, such as a large semi-truck and trailer (Deeproot website)
Rock based structural soils3 0.5 to 0.75 ft2 5 ft by 5 ft 2826 to 2120 ft3 Can be used under pedestrian mall paving, sidewalks, parking lots, and low-use access roads
Sand based structural soils 1 ft3 5 ft by 5 ft 1413 ft3 No standard test data available; Amsterdam sand settled 19 mm in 3 years compared to the surrounding pavement (Couenberg 1993), which is generally not acceptable in the U.S.
Soil boxes 1 ft3 Not large enough to grow 30 ft diameter tree Not large enough to grow 30 ft diameter tree

1 Based on 2 ft3 of soil volume per 1 ft3 of canopy area, assumes Silva Cells are used for structural cells, assumes 92% void space in Silva Cells; assumes CU Structural Soil is used for rock based structural soil; assumes soil component of rock based structural soil is 20%.
2 Although a typical rock-based soil includes 80% rock and 20% soil, the effective volume of the soil is greater than 20%. The recommended ratios in the table reflect information provided in Grabowsky et al., 2009, and by Dr. Nina Bassuk, Cornell University (personal communication).



Protection of existing trees

Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.

Wounds such as open branches and torn or nicked bark can damage a tree by depleting a tree’s energy and providing entry points for disease and insects (Johnson 1999). Many jurisdictions have their own tree preservation standards and/or guidelines. Toronto, for example, has detailed tree protection specifications for construction near trees.

Underground damage to a tree

schematic showing protected root zone
Images from Johnson (1999) illustrating how to calculate the Protected Root Zone. Image courtesy of University of Minnesota Extension.

Compaction within the tree’s root zone adversely impacts the tree. Compaction can result from machinery/vehicular traffic, foot traffic, and from stockpiling materials over a tree’s root zone. Grading within the tree’s root zone, whether cutting or filling, also negatively impacts trees.

The part of the root system in which construction activities, including material storage and traffic, should be avoided is called the Protected Root Zone (PRZ). Only about half of the root system is under the tree canopy (University of Florida, 2013). Many tree roots extend beyond the dripline a distance equal to two or more times the height of the tree (Johnson 1999). The PRZ should therefore extend beyond the tree canopy (see MnDOT specfications in Appendix A for recommended PRZ).

How much of a tree’s root zone can be disturbed before a tree’s health is compromised? Johnson (1999) writes:”Just how close an activity can come without seriously threatening the survival of a tree depends on the species, the extent of damage, and the plant’s health. Some healthy trees can survive after losing 50 percent of their roots. However, other species are extremely sensitive to root cutting, even outside the dripline. ... If possible, disturb no more than 25 percent of the roots within the dripline for any tree, protect intermediate species to the dripline, and allow extra space beyond the dripline for sensitive species. For all trees, avoid needless or excessive damage. A qualified tree-care specialist can help you determine how much root interference a particular tree can tolerate.”

Older trees are more susceptible to damage from construction activities than younger trees (Cappiella et al 2006, Johnson 1999). Negative impacts from construction activities within a tree’s root zone are often not visible in the tree until 3 to 7 years after the construction has ended (Johnson 1999).

Removing trees from a site can also expose the remaining trees to increased sun and wind exposure, which can shock the remaining trees (Johnson 1999). Saving groups of trees instead of individual trees can minimize sun and wind stress (Johnson 1999).

While it can generally be assumed that tree roots are found beyond the dripline a distance equal to two or more times the height of the tree, where the exact location of roots needs to be known for some reason, ground penetrating radar technology can be used to locate exactly where roots are located without digging, for example, when tree roots are under pavement (Bassuk et al., 2011). Where tree roots are not under pavement, an airspade can be used to examine roots without damaging the tree.

Damage to trees from soil chemical changes

According to Johnson (1999) “Improper handling or disposal of materials used during construction also can harm roots. For example, wood products treated with pentachlorophenol and creosote can be deadly to tree roots; CCA-treated timber (greenish color) is a better alternative.” Changes in soil pH, for example, from concrete, alkaline clays, or limestone can also damage trees (Johnson 1999).

See the references and additional resources listed below for more information on adverse effects of construction on trees.

Best practices for minimizing impacts from construction activities, such as installation of fences around trees

As described by (Cappiella et al, 2006), minimizing impacts from construction activities is a five step process:

  1. Inventory existing trees on project site
  2. Identify trees to protect
  3. Design project with tree conservation in mind
  4. Protect trees and soils during construction
  5. Protect trees after construction

Inventory existing trees on project site

Record the location, species, size, and health of each tree. The size and health of the tree will affect whether or not the tree should be preserved or cut. According to Johnson (1999), “Wilted leaves, broken or dead limbs, trunk rot, and thin tops are all symptoms of stress. Trees that are overmature, display poor form, lean heavily over future buildings, or have severe insect or disease problems should be marked for removal prior to construction.”

The extent of the inventory will depend on the needs of the specific project. Some jurisdictions have tree preservation ordinances that require an inventory and dictate the size and types of trees that must be inventoried (For examples, see [8][9][10]). The following additional features are recommended in the tree inventory (Cappiella et al 2006)

  • location of property boundary
  • location of roads
  • location of utilities
  • easements and covenants
  • topography
  • location of streams and stream buffers
  • presence of critical habitats
  • adjacent land uses
  • presence of cultural and historical sites
  • location of 100 year floodplain
  • location of wetlands
  • soil types
  • location of steep slopes

Identify trees to protect

Healthy, vigorous trees with good structure are the most likely to survive through construction activities and should be the top priority for preservation (University of Florida 2013). Johnson (1999) provides the following additional tips for selecting which trees to protect from construction activities:

  • Save the best and chip the rest. Use those wood chips to provide a blanket of protection over the root systems of trees that can be saved.
  • Understand the characteristics of your trees or get advice from a knowledgeable person. If you know about your trees you can help insure their survival and improve the future site appearance of the site.
  • Select tree species that fit the spatial constraints of the site, remembering that trees grow throughout their lives. Be sure to consider overhead powerlines.
  • Young, small trees tend to survive disturbance better than old, large trees.
  • Large trees almost never survive within five feet of a new building and should not be kept.
  • Healthy young trees that fall in the construction zone may be saved by transplanting.
  • Save a mixture of tree species to safeguard your landscape against contagious diseases or insects.
  • Improve tree survival by saving groups of trees rather than individuals.

Design project with tree conservation in mind

image of an alternative turnaround
Example of an alternative trunaround. This design utilizes vegetation and can incorporate permeable pavement into the design. Source: EPA

Cappiella et al (2006), provides the following examples of better site design techniques to conserve forests.

  • Minimize impervious cover
  • Design structural elements such as roads and utilities to minimize soil disturbance and take advantage of natural drainage patterns
  • Where possible, place several utilities in one trench in order to minimize soil disturbance
  • Reduce building footprints by building up, not out
  • Use the minimum required street and right-of-way widths
  • Use alternative turnarounds instead of cul-de-sacs
  • Use efficient street layouts
  • Consider shared driveways for residential lots
  • Use the minimum required number of parking spaces instead of creating additional spaces

Protect trees and soils during construction

Project plans shall clearly mark which trees are to be protected. Numbering and field tags can effectively help avoid cutting down the wrong trees. Value of trees to be preserved should be appraised using Guide for Plant Appraisal, and the value should be noted on large scale labels on each tree to be preserved. Labels shall be attached in a manner that does not damage the tree.

Prepare the trees for construction disturbance by making sure they are as healthy as possible before construction begins. Regularly water them before and during construction if rainfall is not adequate (i.e. whenever soil is dry 6 inches below the soil surface), and prune dead, diseased, and hazardous branches.

Install a layer of wood chips at least 12 inches deep over areas that will be used for traffic or material storage in areas that will be used for future planting, and over tree roots outside of the PRZ. Where dump trucks will be moving across such areas, increase wood chip depth to 18 inches.

Trees shall be protected in accordance with the most recent version of MnDOT specification 2572, Protection and Restoration of Vegetation. Protection measures per MnDOT specification 2572 shall remain in place throughout the duration of construction, and penalties for violation should be strictly enforced. Visit the site regularly to ensure tree protection requirements are not violated.

When possible, remove trees that are not to be preserved in winter after the leaves have fallen. When they are dormant, trees to be preserved will be less susceptible to damage from the removal of adjacent trees, and frozen ground helps protect roots.

Ensure that tree roots and soils are not exposed to adverse chemical changes during construction. Johnson (1999) recommends the following “Ask the builder about the materials to be used on the site and read product labels. Chemical spill damage can be prevented by filling gas tanks, cleaning paintbrushes and tools, and repairing mechanical equipment well outside tree PRZs. Insist that all building debris and chemical wastes be hauled away for proper disposal, and not burned or buried on the site. Finally, avoid changes in soil pH (acidity). Increases in pH are particularly dangerous to many species. Alkaline clays or limestones should not be used for fill or paving, and concrete should be mixed on a thick plastic tarp or outside the site. Mixing trucks should never be rinsed out on the site.”

If any construction damage occurs to trees, address problems as soon as possible, photograph the damage, and inform contractor immediately (Johnson 1999). Levy liquidated damages if applicable. Irrigate trees during construction whenever soil is dry 6 inches below the soil surface. Also irrigate thoroughly before and after trees receive any kind of direct damage (e.g. severed roots) (Johnson 1999).

If roots are cut, “cut cleanly to promote quick wound closure and regeneration. Vibratory plows, chain trenchers, and hand tools do a better job at this than bulldozers and backhoes. Minimize damage by avoiding excavation during hot, dry weather; keeping the plants well watered before and after digging; and covering exposed roots with soil, mulch, or damp burlap as soon as possible” (Johnson 1999).

If utilities must be installed under existing tree root zone, use installation techniques that will minimize root damage. According to Johnson (1999), “As much as 40 percent of a tree's root system could be cut during the installation of a nearby utility line. This reduces water and nutrient uptake, and may compromise the stability of the tree. If it is not possible to relocate the utility line outside the tree's PRZ, you can reduce root damage by as much as 25 percent by tunneling under the tree's root system. When digging a trench near a tree, begin tunneling when you encounter roots larger than one inch in diameter.” Another technique that can minimize root damage when installing utilities is to use an airspade to excavate the utility trench under tree root zone (University of Florida, 2013). An airspade pushes air at a very high speed, removing soil without damaging roots.

pruning branches at the branch collar (Johnson 1999)
Pruning branches at the branch collar (Johnson 1999). Image courtesy of University of Minnesota Extension.

Johnson (1999) recommends the following for pruning and wound repair: “Prune broken or dead branches cleanly at the branch collar ... To test whether a branch is dead, bend several twigs. Twigs on live branches tend to be pliable, while twigs on dead branches tend to break. Buds also can be used to evaluate branch condition. Live buds appear full and normal in color while dead ones appear shriveled or dry.

Pruning is commonly recommended for large trees that have suffered root damage. However, opinions differ over the merits of this practice. Assuming that the tree has adequate water and is not in severe decline, some experts believe that retaining maximum leaf cover is important for root regeneration and only dead limbs should be removed. Others argue that pruning selected live limbs is necessary to compensate for lost roots. Generally, it is best to follow the recommendation of your tree-care specialist experienced in construction damage to trees.

When properly done in moderation by a skilled professional, pruning may reduce wind resistance and limb failure and improve tree health and appearance. DO NOT let anyone cut off all of the top branches to the same height (topping).

The treatment of trunk wounds depends on the extent of damage. If 50 percent or more of the bark has been removed around the entire trunk, the tree will not likely survive and should be removed. If only a patch of bark has been removed leaving a few splinters, use a sharp knife to cleanly cut off the loose bark to a place on the stem where it is firmly attached. DO NOT make the wound any larger than necessary.

You do not need to use pruning paint or dressing to cover exposed wounds or pruned limbs. Except for special cases involving disease control, these products do little more than improve appearance.”

Any activity that causes open wounds on the roots or trunks of oaks between April 1 and July 1 in Minnesota renders the oaks very vulnerable to oak wilt, a lethal fungus disease. Johnson (1999) suggests the following for construction around oaks that are to be preserved: “Immediately (within minutes) cover all open wounds with any water-based paint or shellac during this period. If you suspect oak wilt, contact your city forester or private tree-care specialist. If you have oaks on your site, obtain a copy of Oak Wilt in Minnesota (Minnesota Extension Service publication MI-3174) for additional information on identifying the disease and protecting your trees.”

Protect trees after construction

image of suckering
Suckering or water sprouts are a symptom of construction damage (Johnson, 1999). Image courtesy of University of Minnesota Extension.

After construction activities are completely finished, evaluate condition of trees to be preserved and note indications of stress or damage. Johnson (1999) recommends looking for the following symptoms of construction damage: “Wilted or scorched leaves and drooping branches usually are the first signs of construction damage. In deciduous plants these symptoms may be followed by early fall coloring and premature leaf drop. Damaged conifers will drop excessive amounts of inner needles. In subsequent years you may notice yellowed or dwarfed leaves, sparse leaf cover, or dead branches. Other indicators might include flowering out of season, excessive water sprout formation on the trunk ... abnormal winter dieback, or abnormally large amounts of seed. Flower and seed production and water sprout formation are defense mechanisms for ensuring species survival and commonly indicate that the plant is experiencing extreme stress.

image showing how to determine annual growth
Annual growth is the distance between bud scale scars on twigs (Johnson 1999). Image courtesy of University of Minnesota Extension.

In addition to observing a tree's appearance, monitor its annual growth. A slightly damaged plant will grow more slowly and be less resistant to insects, diseases, and weather-related stress. Examine the annual shoot and branch growth ... Healthy trees generally will grow at least two to six inches at the ends of the branches each year. Photographs and records of the tree prior to construction also can help identify growth problems.”

Signs of soil compaction negatively impacting a tree include leaf wilt, early fall coloring, top dieback, and slow growth (Johnson 1999).

Construction and grading can also change how much water a tree receives and may cause some trees to receive too much water. Observe drainage patterns and soil moisture after construction and contact an arborist if you suspect trees may be getting too much water. Dead twigs and branches can be a sign that the tree is receiving too much water (Johnson 1999), however, do not wait for dead twigs and branches to appear to remedy the problem if you suspect a tree may be getting too much water.

Trees should be watered as needed after construction is complete to minimize water stress. As much of the PRZ as possible should be mulched to further maximize tree health. Planting the area under the tree with understory shrubs or perennial herbaceous plants instead of turf will also benefit the tree (Johnson 1999).

Signs of damage may not be visible until 3 to 7 years after construction is finished. Therefore trees should be inspected every year or two to determine if pruning, fertilization and /or pest or disease control is needed (Johnson 1999).

Post construction monitoring of soil compaction

Tree BMP’s for stormwater should be designed in a way that prevents foot or vehicular traffic, to prevent compaction. For example, low fences can be installed around tree openings to prevent compaction from foot or vehicular traffic. If for any reason it is suspected that the soil may have been compacted after construction, measure soil compaction and note whether or not compaction exceeds root limiting compaction level. Signs that the soil may have become compacted include for example:

  • Observation of people or vehicles on tree BMP soil
  • Signs of traffic on tree BMP soil (e.g. vehicular tracks)
  • Potential symptoms of compaction visible in soil (e.g. hard crust, standing water, inability to penetrate soil with a rod or shovel)
  • Potential symptoms of compaction in tree.

If soil is compacted beyond root limiting density, de-compact soil by loosening with an airspade. Then mix 4 inches of compost mixed into the soil with the airspade.

Mitigation practices following construction, such as soil ripping and use of amendments such as compost

For urban trees, a bioretention soil in accordance with the bioretention soil guidelines will be used, so it contains organic matter and will not be compacted. To promote drainage into in situ soils, it is strongly recommended that the bottom of the urban tree BMP be scarified with the teeth of a backhoe (except for tree BMP’s with bioretention volume under pavement that need a compacted base for structural stability).

Appendix A

MnDOT Specification 2572 PROTECTION AND RESTORATION OF VEGETATION as of September 2013

2572.1 DESCRIPTION

This work consists of protecting and preserving vegetation from damage and restoring vegetation damaged by the Contractor’s operations.

2572.2 MATERIALS

A Plant Materials .......................................................................................................... 2571 and 2575

B Temporary Fence

Provide temporary fence meeting the following characteristics and requirements:

(1) At least 4 ft [1.2 m] high,

(2) Conspicuous in color (see Standard Detail Sheet for Protection and Restoration of Vegetation), and

(3) Commercially available snow fence or other fencing material approved by the Engineer.

C Water ................................................................................................................................ 2571.2.C.4

D Sandy Loam Topsoil ................................................................................................................. 3877 2500’s 344

E Tree Growth Retardant (TGR)

Provide the TGR paclobutrazol or an equal approved by the Engineer.

2572.3 CONSTRUCTION REQUIREMENTS

A Protecting and Preserving

Protect and preserve the following:

(1) Specimen trees,

(2) Threatened and endangered plants listed on the Federal and state threatened and endangered species list,

(3) Vegetation as required by the contract,

(4) Trees, brush, and natural scenic elements within the right-of-way and outside the limits of clearing and grubbing in accordance with 2101.3, “Clearing and Grubbing, Construction Requirements,” and

(5) Other vegetation as directed by the Engineer.

Do not place temporary structures, store material, or conduct unnecessary construction activities within 25¼ ft [8 m] outside of the dripline of trees designated for preservation, unless otherwise approved by the Engineer.

Do not place temporary structures or store material, including common borrow and topsoil, outside of the construction limits in areas designated for preservation, as required by the contract or as approved by the Engineer.

Do not place or leave waste material on the project, including bituminous and concrete waste that would interfere with performing the requirements of 2105.3.C, “Preparation of Embankment Foundation,” or 2575, “Establishing Turf and Controlling Erosion.” The Department defines concrete waste as excess material not used on the project, including material created from grinding rumble strips. Dispose of excess material in accordance with 2104.3.D, “Disposal of Material and Debris.”

A.1 Temporary Fence

Place temporary fences to protect vegetation before starting construction. Place temporary fence at the construction limits and at other locations adjacent to vegetation designated for preservation as required by the contract or as approved by the Engineer. The Department will provide tree protection signs. Place tree protection signs in accordance with the following:

(1) Along the temporary fence at 50 ft [15.25 m] intervals,

(2) At least two signs per fence, or

(3) As directed by the Engineer.

Do not remove the fence until all work is completed or until approved by the Engineer.

Ensure the fence prevents traffic movement and the placement of temporary facilities, equipment, stockpiles, and supplies from harming the vegetation.

A.2 Clean Root Cutting

Cleanly cut tree roots at the construction limits as required by the contract or as directed by the Engineer. Immediately and cleanly cut damaged and exposed roots. Cut back damaged roots of trees designated for protection to sound healthy tissue and immediately place topsoil over the exposed roots. Immediately cover root ends exposed by excavation activities with 6 in [150 mm] of topsoil as measured outward from the cut root ends. Limit cutting to a minimum depth necessary for construction. Use a vibratory plow, or other approved root cutter in accordance with the Standard Detail Sheet for Protection and Restoration of Vegetation, before excavation. 2500’s 345

A.3 Watering

Water root-damaged trees during the growing season that root damage occurs, and water specified trees if required by the contract or directed by the Engineer. Maintain adequate but not excessive soil moisture by saturating the soil within the undisturbed portion of the dripline of impacted or identified trees to a depth of 20 in [500 mm]. Use a soil recovery probe to check the soil moisture to a depth of 20 in [500 mm], and adjust the intervals and frequency of watering in accordance with prevailing moisture and weather conditions.

A.4 Sandy Loam Topsoil

Place sandy loam topsoil instead of common borrow fill within the dripline of specimen trees as required by the contract or as directed by the Engineer. Place the topsoil to avoid over-compaction as approved by the Engineer. Establish turf consistent with the adjacent areas as approved by the Engineer.

A.5 Utility Construction

Bore under roots of trees designated for preservation for utility installations within the tree protection zone in accordance with the following: Table 2572-1 Tree Protection Zone

Bore under roots of trees designated for preservation for utility installations within the tree protection zone in accordance with the following: Table 2572-1 Tree Protection Zone

Tree diameter at 4.5 ft [1.4 m] above ground, in [mm] Minimum distance from face of tree trunk, ft [m] Minimum depth of tunnel, ft [m]
<2 [50] 2 [0.6] 2 [0.6]
2–4 [51–100] 4 [1.2] 2.5 [0.75]
>4–9 [101–225] 6 [1.8] 2.5 [0.75]
>9–14 [226–350] 10 [3.0] 3 [0.9]
>14–19 [351–480] 12 [3.6] 3.25 [1.0]
>19 [480] 15 [4.8] 4 [1.2]

Do not perform open trenching within the tree protection zone.

A.6 Blank

A.7 Pruning

Provide an arborist certified by the International Society of Arboriculture to prune trees as required by the contract or as directed by the Engineer in accordance with 2571.3.E.1, “Pruning – Top Growth and Roots.” Ensure the arborist removes dead, broken, rubbing branches, and limbs that may interfere with the existing and proposed structures.

A.8 Destroyed or Disfigured Vegetation

Restore vegetation designated on the plans for preservation that is damaged or disfigured by the Contractor’s operations at no additional cost to the Department. Restore the damaged vegetation to a condition equal to what existed before the damage. The Engineer may assess damages against the Contractor for damage to vegetation not restored to the previous condition. The Engineer will assess the value of damages to trees and landscaping at not less than the appraisal damages as specified in the Council of Tree and Landscape Appraisers Guide for Plant Appraisal. The Engineer will determine and assess damages of other vegetation.

A.9 Oak Trees

Avoid wounding of oak trees during April, May, June, and July to prevent the spread of oak wilt. If the Engineer determines that work must take place near oak trees during those months, immediately treat resulting wounds with a wound dressing material consisting of latex paint or shellac. Blend paint colors with the bark color. Maintain a supply of approved wound dressing on the project at all times during this period.

A.10 Tree Growth Retardant (TGR)

Provide an arborist certified by the International Society of Arboriculture to treat trees with the TGR as required by the contract or as directed by the Engineer. Ensure the arborist applies the TGR paclobutrazol as a basal drench or soil injection and in accordance with the label directions. Provide the Engineer with the product label and material safety data sheet for the product used.

A.11 (Blank)

A.12 Other Vegetation Protection Measures

Provide other vegetation protection measures including root system bridging, compaction reduction, aeration, irrigation systems, J-barriers for specimen tree protection, and retaining walls as required by the contract or as directed by the Engineer.

References

  • Bassuk, Nina, Jason Grabosky, Anthony Mucciardi, and Gary Raffel. 2011.Ground-penetrating Radar Locates Tree Roots in Two Soil Media Under Pavement. Arboriculture & Urban Forestry 37(4): 160–166.
  • Cappiella, Karen, Tom Schueler, and Tiffany Wright. 2006. Urban Watershed Forestry Manual Part 2. Conserving and Planting Trees at Development Sites. Second in a Three-Part Manual Series on Using Trees to Protect and Restore Urban Watersheds. Prepared for and published by: United States Department of Agriculture Forest Service Northeastern Area State and Private Forestry.
  • Johnson, Gary R. 1999. Protecting Trees from Construction Damage. University of Minnesota Extension publication WW-06135.
  • Matheny, N. and J.R. Clark. Trees and Development: A Technical Guide to Preservation of Trees During Land Development. International Society of Arboriculture (June 1, 1998).
  • University of Florida. 2013. Tree preservation during land development.


Operation and maintenance

Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.

Requiring the Owner to sign a maintenance agreement to receive stormwater credits can help ensure trees receive adequate maintenance. The following maintenance tasks are HIGHLY RECOMMENDED for using tree boxes and tree trenches for stormwater management.

Maintenance Inspections

Perform maintenance inspections per the maintenance checklist and perform actions needed per the inspection checklist (see File:Tree-Maintenance checklist-Final.xls).

Maintenance inspection checklist for trees.
Link to this table
To access an Excel version of form (for field use), click here.

Project Name:
Project Address:
Owner Name:
Owner Phone #:
Inspector Name:
Inspector Phone #:
Date of Inspection:
Weather at time of inspection:
Date of last rainfall prior to inspection:
Inspection item Inspection frequency Date last inspected Need to inspect during current inspection Describe signs of problems (if none, write "none") Action needed and deadline Date completed
Tree opening
Mulch layer less than 3" deep: needs additional mulch Yearly
Erosion *
Evidence of clogging *
Evidence of standing water *
Weeds present As needed
Accumulation of sediment, debris or trash *
Does drawdown time meet project requirements *
Inlet (Curb cut at tree opening, curb cut at catch basin, porous pavement, trench drain, or other)
Accumulation of sediment, debris or trash *
Erosion *
Pretreatment (curb cut at tree opening, catch basin, porous pavement, or other)
Accumulation of sediment, debris or trash *
Erosion *
Evidence of clogging *
Evidence of standing water *
Distribution and drainage pipes
Overflow/outlet structure *
Other
*
*
*
*

NOTES:

  • Inspect tree minimum once a month and after every major storm during first year after planting.
  • Unless otherwise notes in "minimum inspection frequency column", inspect items below minimum spring, fall, and after major storms; adjust frequency as needed based on project conditions.


Supplemental Watering

Chesapeake stormwater Network logo
The Chesapeake Stormwater Network has developed two videos that illustrate inspection and maintenance of LID BMP practices, including bioretention. NOTE: These videos provide useful tips but should not be used for compliance with Minnesota permits.

Newly planted trees need to be watered regularly. In the first three years after planting, trees typically need about 1.5 gallons of water per inch of trunk diameter whenever the soil feels dry or slightly damp in the top 6 inches (Johnson et al., 2008) and a minimum of once a week during the first growing season. However, the amount of water the tree needs will depend on many factors, including soil type, drainage and weather. Soil moisture sensors can be used to automate watering whenever the soil is dry enough to need watering.

Many people have found that one of the most reliable ways to ensure trees receive adequate water during the establishment phase is to minimize labor involved, by using watering bags or an automated watering system. Watering bags are cone shaped bags that hold about 20 gallons of water and zip around the tree trunk. They can be used for trees with a caliper between 1 and 8 inches. Once they are manually filled with water, they release water slowly directly above the root package, providing slow, deep, watering without losing any water to runoff evaporation. They are reusable, inexpensive and easy to use.

Once the tree has rooted out of the root package, watering should be tapered off to encourage the tree to grow deep, wide-spreading roots. Depending on factors such as, tree species, soil type, perviousness of the surface above the tree rooting zone, weather, and how much stormwater is directed to the tree, mature trees may also require supplemental watering during extended droughts. Urban trees are especially prone to drought as urban areas are typically warmer and there is often less pervious surface above the rooting zone of an urban tree. Soil moisture should be checked and trees watered as needed from spring until the soil freezes in the fall. Water whenever soil is dry 3 inches below the soil surface. Too much water can kill trees just as easily as not enough water, especially in compacted and degraded urban soils, so ensure trees receive enough water but are not overwatered.

To mimic the pre-settlement hydrological cycle as closely as possible by increasing both evapotranspiration and infiltration, harvest runoff, for example, from adjacent impervious surfaces, and use harvested runoff to irrigate trees. In urban areas dominated by impervious surfaces, evapotranspiration and infiltration are typically lower than in the pre-settlement hydrological cycle. Using harvested water to irrigate trees will increase both evapotranspiration and infiltration, and more closely mimic the pre-settlement hydrologic cycle.

Straightening Trees

Maintain all plants in a plumb position throughout the warranty period. Straighten all trees that move out of plumb including those not staked. Plants to be straightened should be excavated and the root ball moved to a plumb position, and then re-backfilled.

Mulching And Other Amendments

Wood mulch provides many benefits to the tree, including, for example, weed and turf suppression, and increased moisture retention. It also improves bioretention pollutant reduction function and increases the organic matter content of the soil.

Maintain a ring of mulch as wide as possible, 2 to 3 inches deep, around each tree. Ideally each tree should have at least an area two feet in diameter for each inch of tree trunk diameter, with a minimum mulch ring diameter of eight feet for trees with a stem diameter three inches or less (Gilman, 2013). This may not be feasible in urban areas. Where the tree opening does not permit an eight foot wide mulch ring, make the mulch ring as large as the tree opening permits.

Do not pile mulch against the trunk of the tree, and place only a thin layer, if any, mulch over the root ball. According to Gilman (2013), “This keeps the trunk dry and allows rainwater, irrigation, and air to easily enter the root ball. Mulch resting on the trunk or layered too thick can kill the plant by starving it of oxygen, killing the bark, causing stem and root decay, preventing hardening off, encouraging rodent damage to the trunk, keeping soil too wet, and repelling water. Mulch on the root ball has little impact on water lost from the tree since most of the moisture that leaves the root ball does so by transpiration, not evaporation. Only a small amount (less than 10 percent) leaves the root ball by evaporation from the surface of the root ball.”

According to Gilman (2013), “No amendments of any kind are necessary in the backfill soil because extensive research clearly shows that they typically do not increase survival nor growth after planting.”

Fertilizer

Most trees generally do not need to be fertilized regularly, especially if they are receiving nutrient rich stormwater. Trees should not be fertilized with nitrogen unless diagnosis by an arborist deems it necessary. Other nutrients should be applied only if soil tests indicate the soil has insufficient nutrients.

If soil tests or diagnosis by an arborist indicate a need to fertilize,

  • do not apply more fertilizer than the soil lab or arborist recommends. Overdosing could harm the tree and leach nutrients into downstream water bodies.
  • Water well before and after fertilizing.
  • Fertilize in the fall after the tree has lost its leaves, or in the spring before buds develop.
  • If fertilizer is needed, use a slow release fertilizer.

Protecting the tree trunk

The bark of young deciduous trees should be protected with a trunk guard to prevent rabbits, mice, and deer from damaging the trunk. Trunk wounding can create long term damage. Install light colored plastic tubing, or ¼ inch mesh hardware cloth around the trunk with 1 to 4 inches of space between the guard and the trunk. The guard should extend 1 to 2 feet above the snowline for protection from small rodents, and as tall as possible for deer protection. It should be pushed into the ground or mulch about an inch to secure it but not enough to damage the roots. The guard should be in place at a minimum during winter months, but can be left in place year round if it does not touch the trunk. Enlarge or remove trunk guard once there is no longer at least 1 inch between the tree trunk and tree guard.

Removing Stakes

Staking, if used, should be removed 1 to 2 years after planting. Check staking and tree guying material at the end of the first growing season after planting. If the tree is stable without the stake and guying material, remove stake and guys. If tree is unstable, retie guying to stake and remove stake and guying at the second growing season after planting.

Pruning

Trees are pruned for safety, health and aesthetics. Johnson et al. (2008) recommend the following pruning frequency:

  • once in year 2 or 3 after planting;
  • every three years during years 4 to 10 after planting; and
  • after 10 years from planting, every 5 years for deciduous trees and as needed for conifers.

Check with your city or town to see if they have laws regarding pruning. Never prune trees or branches that are within 10 feet of utility lines; contact the utility company. Pruning guidelines can be found in

Check tree health

Check trees for damage from mowers and weed whips, vandals, and animals. Inspect leaves, branches,crown and trunk for signs of insect or disease problems. Contact an arborist if needed. Guidance on how to hire an arborist can be found on page 28 of Johnson et al. (2008).

Tree health troubleshooting guidelines (adapted from Johnson et al., 2008).
Link to this table

If you see: Potential cause: You should:
TRUNK
A flat-sided trunk at the base of the tree Encircling root restriciting the flow of water and nutrients between the roots and rest of the tree Excavate to check for encircling root
Bark damage near the bottom of the tree Rodent or string trimmer Apply mulch/trunk guard to protect from further damage
An elm tree with liquid oozing from the trunk Slime flux or wetwood Not worry about health
BRANCHES
An elm tree with bright yellow leaves on one or two branches Dutch elm disease Immediately call the university* or an arborist
Webs in the branches or webs covering the tips of branches Fall webworm or Eastern tent caterpillar Not worry about health
Many branch tips snipped off and laying on the ground Squirrel damage Not worry about health
Black clumps on branches of a cherry tree Black knot Call for advice*
Very little growth Many Call for advice*
Hole in trunk or branches Many Call for advice*
LEAVES
Leaves sticky and covered with a black velvety coating (like soot) Piercing, sucking insect and sooty mold Hose down leaves to get rid of sap
Leaves wilted Many Call for advice*
Spots on leaves Many Call for advice*
Small leaves Many Call for advice*
Sparse leaves Many Call for advice*
Yellow or brown leaves Many Call for advice*
Holes in leaves Insect feeding Not worry about health
Bumps on leaves Many Not worry about health

* Call an arborist or other qualified professional


Check tree safety

Johnson et al. (2008) recommends checking trees after storms for the following signs of potential danger:

  • broken, dead, or hanging branches;
  • cracks, fungi, and cavities;
  • weak trunk or branch unions;
  • encircling root compressing the trunk (a flat sided trunk at the ground level is a good indicator); and
  • recent lean (especially if the soil or grass has lifted on one side)

If any of the above are found, contact an arborist. Guidanceon how to hire an arborist can be found on page 28 of Johnson et al 2008, and on the International Society of Arboriculture’s website.

Check for Girdling Roots and Correct

Roots that encircle the trunk will likely cause health or safety problems. Remedy girdling roots at planting. Check for girdling roots (roots that encircle the trunk) every 4 to 5 years after planting and if girdling roots are found, contact an arborist for treatment (Johnson et al 2008). Girdling roots can be removed if caught early.

Clean root collar

Girdling roots are encouraged if root collar is covered with soil or mulch. Clean root collar once a year by removing soil and mulch until the first set of roots is exposed. Removing soil with a wet-dry vacuum (Johnson et al 2008) or air spade speeds the work without harming the roots.

References


Assessing performance

Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.

Assessing the performance of tree trenches and tree boxes includes assessing the functionality of the BMP in terms of water and/or pollutant removal and assessing the performance of trees. Both are discussed on this page.

Assessing BMP performance

Tree trenches with underdrains are designed to retain solids and associated pollutants by filtering. A typical method for assessing the performance of tree BMPs with underdrains is therefore measuring and comparing pollutant concentrations at the influent and effluent. BMPs without underdrains are more difficult to assess, although considering only potential impacts to surface waters, a properly functioning infiltration system is considered to be highly performing.

An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.

  • Level 1: Visual Inspection. This includes assessments for infiltration practices and for filtration practices. The website includes links to a downloadable checklist.
  • Level 2: Capacity Testing. Level 2 testing can be applied to both infiltration and filtration practices.
  • Level 3: Synthetic Runoff Testing for infiltration and filtration practices. Synthetic runoff test results can be used to develop an accurate characterization of pollutant retention or removal, but can be limited by the need for an available water volume and discharge.
  • Level 4: Monitoring for infiltration or filtration practices

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Additional information on designing a monitoring network and performing field monitoring are found at this link.

Assessing tree health

The health of an urban, suburban, rural, or natural forest is rarely limited to individual species alone. An assessment of forest health should be related to both the individual tree and the collection of trees, including interactions between trees.

Many metrics and methods have been developed for assessment of individual tree health. The concept of “resilience” at the individual and canopy levels is the core of the assessment tools. The majority of these evaluative methods and metrics focus on the response of the individual or evaluative unit to a disturbance regime to quantify the “resilience.” The type and capacity of response to the given disturbance and the time it takes to return to the initial qualitative equilibrium state indicate the overall resilience to the disturbance or pressure. Eichorn and Roskams (2013) cite various sources indicating that this return to “equilibrium” is not always return to the initial state, stating that, “open systems will reorganize at critical points of instability.” Determining the critical thresholds for certain pressures, disturbances, and changes the system or individual can tolerate before it cannot recover provides a proxy for tree and forest health.

The resilience of the tree individuals and canopy is often difficult to quantify directly for multiple pressures. Rather, indirect measures are often employed for inventory and monitoring of tree health. Measurements and metrics can also be taken both directly (e.g. assessing growth rings from a core) and indirectly (e.g. remote sensing of canopy leaf area). Direct and indirect methodologies are discussed and compared below. It is suggested that the base of monitoring, evaluation, and correlation of forest health be that of overall forest resilience, rather than individual tree health. The foundation of the assessment focuses on the health of the individual as a component of the collection of individuals in the forest canopy. Eichhorn and Roskams (2013) suggest using two levels of monitoring and implementation:

  • Level 1 – a large-scale systemic network of the trees within the defined forest area or region; and,
  • Level 2 – an individual- or stand-based approach using intensive monitoring plots.

These levels are not distinct in their interactions and the information gained at each level can inform the interactions and information at the other level. Interactions at each of these levels may also be correlated with and inform forest health and interactions at the national or global scale. We suggest future strategies and policy efforts to standardize, create, and implement a larger national, and possibly global, forest assessment tool for monitoring, assessing, and evaluating the health of our forest. Per the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests, implementation within the MPCA tree monitoring focuses on the following objectives of Minnesota tree condition monitoring, as a subset of the national and global forest system (Eichorn and Roskams (2013):

  • to contribute to a Minnesota-wide early warning system and to a better understanding of tree vitality, including relationships to stress factors and ecosystem disturbances;
  • to provide a periodic information on the spatial and temporal variation of tree condition in relation to stress factors;
  • to currently document and evaluate the major environmental challenges in Minnesota such as the impact of climate change on forest ecosystem stability;
  • to gain information about the impact of biotic and abiotic stressors on crown and tree condition;
  • to provide baseline data on the distribution, occurrence, and harmfulness of biotic agents or co-occurring factors in total or parts of Minnesota;
  • to validate models regarding stress or risk for trees; and
  • to contribute to decision support for forest policy and forest practice with regard to ecological sustainability of forest management.

The methodologies presented hereafter focus on these objects in order to establish a framework for a comprehensive tree monitoring system that can be added to as new methodologies and assessment tools emerge.

Methodologies for assessing tree health

Assessment of tree and forest health can be measured directly or indirectly at either the Level 1 (overall forest) or Level 2 (individual or stand) scales. Indicators of tree condition found in monitoring efforts may be assessed via qualitative and quantitative methods for assessing morphology and architecture (canopy, trunk, fruit, roots, etc.), forest composition, biotic/abiotic agents, growth rate, and age. Eichhorn and Roskams (2013) give an overview of indicators of tree health that may be focused on within a monitoring program at both Level 1 and Level 2, and the targeted areas for assessment and evaluation. A summary of variables they suggest as indicators of tree condition is presented below.

  • Primary production
    • Defoliation - estimate the leaf/needle loss in the assessable tree crown relative to a reference standard; estimates range from 0 to 100 in 5 percent classes
    • Apical shoot architecture - estimate the shoot development in relation to the standard of an adaptable tree crown from 0 to 100 in 5 percent classes
    • Fructification - estimate fruits in whole tree crown from 0 to 100 in 5 percent classes
    • Fruit biomass - calculate biomass (mass per hectare) of fruits in litterfall traps from a stand
    • Diameter growth - measure diameter growth, in centimeters, of a stand
  • Ecosystem disturbances
    • Determine occurrence and diagnose symptoms and signs of biotic and abiotic agents of whole trees
    • Record number of trees from a plot absent as a result of removal or mortality and cause(s) of absence
  • Ecosystem internal regulation
    • Measure or estimate tree age, in years, from a sample of trees
    • Measure tree-related stand structure from a sample of trees

The proposed MPCA tree and forest monitoring system and protocol presented hereafter focuses mainly on direct measurements of individual trees at a Level 1 scale, as a proxy for Level 2 interactions, using the Eichhorn-Roskams (2013) methodology described above.

Direct measurement methods typically employ tree architecture and morphology as a measure and indicator of tree health. The measurements are broken down by foliation (defoliation) of the upper crown/canopy, apical shoot architecture, and fructification. The following table breaks down the areas of evaluation and assessment within each category of tree area.

Qualitative and quantitative evaluation and assessment of trees, by area of tree.
Link to this table.

Area of tree Qualitative and quantitative evaluation and assessment
Foliage Leaves, related to ability of tree to capture light for metabolic processes (photosynthesis), particularly noting:
  • overall canopy area
  • openings in the canopy and areas of the openings
  • quality of leaves, noting any structure or color change
  • leaf drop, patchiness, or mortality
Apical shoots and overall tree architecture and morphology
  • Trunk
    • quality and location of any damage, disruptions and disturbance
    • type of response noted to disturbance (e.g. scab, open wound, healing, etc.)
    • bark quality compared to known standards, noting any quality and quantity differences from normal
    • presence of insects, insect-related activity, and infection
  • Branches
    • overall divergence from normal branching pattern (e.g. no limbs on one side of tree)
    • branch mortality or abscission, presence of and location per normal growth patterns
    • no leaf out and bud-related structures set, presence of and location per normal patterns
    • presence of insects, insect-related activity, and infection
Fructification Fruit production, as an indicator of reproductive success and health, infection or stress-related response
  • quality of fruit, noting any damage, infection or pest indications
  • quantity of fruit. Can be difficult to interpret results, as fruit abundance or deficit can indicate stress (succession-related response of reproductive proliferation and seed band inundation prior to mortality) or success/health (succession-related response of population growth due to abundant resources; excess resources and metabolic byproducts applied seed production).
Roots
  • Difficult to assess without disturbing tree, with exception of aerial root structures (not found in MN species)
  • Note any presence and location of roots and root structures above soil finished grade


Recommended monitoring protocol

This section provides a recommended monitoring (assessment) protocol. The goal of monitoring is to identify issues or concerns that require a response. Responses to monitoring findings are presented in the Operation and maintenance of tree trenches and tree boxes section of this Manual. Specific troubleshooting responses are presented in a table. This series of responses is mainly on a Level 1, or individual basis, with limited Level 2 assessment suggested via remote sensing technologies. The collection, interpretation, and response to the larger forest (Level 2) health requires collection and collaboration of results between individuals. Large-scale comparative and predictive metrics and an ecosystem-scale assessment system is suggested for future investigation to provide a standardized system for collection and comparison of Level 1 monitoring information.

Frequency of monitoring

Annual tree monitoring is suggested for the crown and apical shoots (all above-ground structures). It is suggested that this could be performed with rapid assessment tools by citizens’ monitoring programs, watersheds, municipalities, or other groups. Standardization of the monitoring information collected, date of collection, procedure for collection, and reporting of monitoring for comparison of results between individuals and over larger areas is suggested. Roman et al. (2013) provide a discussion of common practices and challenges for local urban tree monitoring programs.

Positions of collecting monitoring information

schematic showing where to assess trees
Schematic illustrating points at which to conduct monitoring/assessment of trees. Each point is fixed and the locations should be recorded using GPS.

Two points should be established and noted on a map or, preferably, a GPS (Global Positioning System) device for consistent results collection. One point (Point A) should be directly underneath the tree for assessment of leaf area coverage and canopy diameter. The second point (Point B) provides quantitative and qualitative results of tree height and vertical leaf coverage, and is suggested to be at least 50 feet from trunk to provide a fixed location for consistent evaluation as the tree grows. A GIS (Geographic Information System) coverage and GPS-based system would aid in consistent evaluation and database assembly for large-scale evaluation and monitoring assessment and responses. Leaf and other tree debris can also be evaluated at any location underneath and surrounding the tree for gathering additional information regarding tree illness, injury, or stress responses.

Collected information

Information gathered should be assembled into a standard format. The information gathered focuses on the above-ground portion of the tree and surface root presence on an individual/single-tree basis as the standard unit of measurement.

Please note: fructification can give ambiguous indications of overall health, and it should not be used as an independent indication of tree health. Fructification can be used in conjunction with the above-ground structural assessment as a component of the Level 1 (individual or stand) monitoring, but will be more effective as a Level 2 (larger-scale forest assessment) to indicate ecosystem-wide response to stressors, such as insect invasion and long-term drought or reduced soil moisture availability.

Monitoring program

Level 1 monitoring

Level 1 Tree Monitoring focuses on the individual or small unit of individual trees for a field-based assessment tool. This program allows for increased ability to be performed by a layperson or expert, but does not utilize less-accessible programs and methods such as GIS-based or model-based systems that would be more useful at a Level 2 analysis. Other methodologies – electrical conductivity within sap, chlorophyll fluorescence, glucose presence - for Level 1 monitoring were encountered in the literature review, but the tools and/or knowledge required to perform the analyses or monitoring or the detailed level of information gather were prohibitive for generalized use in this level of monitoring program (Martinez-Trinidad, et al. 2010). It is suggested that this monitoring methodology be standardized in a rapid assessment form for data collection and comparison throughout Minnesota to better perform the Level 2 analyses and provide a greater degree of statistical confidence in results due to standardized methodology.

Assessing the Tree Canopy and Above-Ground Structures

This assessment includes the leaves and leaf structures and/or needles in the tree. Take a photo from Location A and B that captures the extent of the foliage. Take multiple images if necessary, indicating the general direction of view. Complete the following assessment of the canopy, providing additional information of photo-documentation where appropriate.

Assessing the tree canopy and above-ground structures. Take a photo from Location A and B that captures the extent of the foliage. Take multiple images if necessary, indicating the general direction of view. Complete the following assessment of the canopy, providing additional information of photo-documentation where appropriate.
Link to this table.

Defoliation: a relative amount of needles or leaves are missing from the canopy as compared to a reference tree.
Is there any level of defoliation noted?

____ Yes
____ No

If defoliation is present, estimate the percent defoliation
Describe the location, relative area (ft2) of the defoliation, percent canopy/leaf loss (% of whole area), and any other notable information regarding each defoliation area noted from visual assessment at Locations A and B. Take photographs as necessary, noting the general direction of view.
Apical Shoot Architecture: the architecture of the most recent growth of branches in the canopy where the majority of leaves are located and arranged. Answer all of the following questions when examining the apical shoot architecture from Locations A and B and then rate the tree using the Apical shoot scoring system for the Apical Shoot Architecture.
What is the estimated length of a typical apical shoot (inches)?
Are the upper-most apical shoots alive, as indicated by color and twig turgor pressure (e.g. not dried and brittle in appearance)?

____ Yes
____ No

What is the color of the typical apical shoot?

___ Light brown
___ Dark brown
___ Green
___ Yellow
___ Red
___ Other (specify)

Is there any presence of the following in the apical branch growth? Mark all that apply, and indicate general location of the noted issues.

___ spear-shaped twigs
___ short twigs
___ lack of bud structures (dormant season only for deciduous trees)
___ a large numbers of twigs emerging from the tips of the next lower level of branching
___ a lack of branch growth in one area or on one side of the tree

Describe any of the noted issues.
Fructification: the fruits and fruiting bodies of trees can indicate much about the health or lack thereof of the individual. Answer all of the following, per the Fructification scoring system.
Is fruit present on the tree? Only note the presence of new fruit from this year, and not “old” fruit from the previous year, as would be distinguished as wrinkled or shriveled in appearance.

___Yes
___ No

Describe the location, relative area covered, and any other notable information regarding the fruiting from Locations A and B. Please take photographs as necessary, noting the general direction of view.
Roots: the majority of the root system should be below ground and relatively difficult to assess and monitor, however, the presence and effects of circling/girdling roots may provide symptoms in tree morphology. Answer all of the following.
Is there a lack of branching or a flat side observed on the tree, or are there any girdling roots observed around the main trunk at or above the soil surface? Note the presence of these features that would indicate root-related issues.

___ Yes
___ No

Describe the location and any other notable information regarding the presence of roots above ground from Locations A and B. Take photographs as necessary, noting the general direction of view.


Assessing biotic and abiotic damages

These pressures are witnessed by signs (direct evidence of a damaging factor) and symptoms (indirect results or evidence of the damaging factor (e.g. leaf proliferation following a windstorm)) of tree health due to biotic and abiotic (environmental) influences. Assess the following damages due to biotic and abiotic factors using the following questions and the Signs and symptoms scoring table and the Symptoms and specification table.

  • Leaves/needles show signs or symptoms of damage due to biotic or abiotic factors. Note the number and location of these areas and describe the nature (e.g. color, size, affected part of the tree) of each affected area.
  • Describe any of the observed issues designated in the scoring system.
Crown distance assessment

The distance between individual canopies can provide positive and negative aspects to tree health. Some positive aspects of canopies being in close proximity to their neighbors is collective support from wind and other abiotic factors, whereas some negative aspects are increased disease transmissivity, shading, and competition for finite moisture and other resources.

  • Rate the overall crown distance between adjacent trees in each perpendicular direction and the monitored individual using the Crown distance scoring system system.
  • Describe any of the observed issues within or among the canopy relating to the adjacent canopies in the scoring system.

Level 2 monitoring

Larger-scale monitoring is difficult to assess with standard assessment tools. Large-scale monitoring of crown coverage can be performed by remote sensing methods. These methods include utilizing National Land Cover Data (NCLD) analyses, high resolution land cover gained from satellite and/or aerial sourced-photography that is interpreted by pattern for land cover, and aerial photography (Eichhorn and Roskams 2013; USDA USFS Northern Research Station (Date Unknown)). These digital models are better adapted for these large scale monitoring efforts, using base data acquired in Level 1 Monitoring in conjunction with climatic inputs, aerial imagery, and other remotely-sensed data that might not be otherwise available or useful at a site or individual scale or level of monitoring. An effective monitoring and overall assessment of a Level 2 scale system was performed for Los Angeles using USFS iTree software, examining and assessing not only the forest health for 30+ years, but the additional impacts and ecosystem services related to the urban forest on the health of the resident population (McPherson, et al. 2011). The application of this protocol to potential Level 2 monitoring by the MPCA warrants further investigation and implementation into monitoring protocol, data assembly, interpretation, trend analysis, composition assessment, resilience analysis, and resource allocation and prioritization.

Several free programs exist to assist in collecting and interpreting the results of Level 2 forest analyses, including Growth Simulator SILVA (Technische Universitat Munchen, Germany), FVS (USFS), TIPSY (BC CANADA Ministry of Forests, Lands, and Natural Resource Operations), and iTree (USFS). iTree is a free program with a number of sub-programs that has been generated by the US Forest Service (USFS) to assist in the measurement of a number of tree-related inputs such as evapotranspiration, canopy intereception, water use, carbon sequestration, and other factors. iTree Canopy is suggested for potential use in this Level 2 analysis due to the relative ease of inputs and availability of data required, as it is based on the Google maps imagery which is updated frequently and reflects current patterns using a relatively high-resolution satellite image source. This program may provide a standardized base of comparison and interpretation of results of Level 1 assessments to provide assessment and guidance for Level 2 monitoring, pattern and trend analysis, and large-scale responses by scientists, policy-makers, government officials, and citizens alike.

Scoring tables

References

  • Churchill, D. J., A. J. Larson, M. C. Dahlgreen, J. F. Franklin, P. F. Hessburg, and J. A. Lutz. 2013. Restoring forest resilience: From reference spatial patterns to silvicultural prescriptions and monitoring. Forest Ecology and Management 291:442-457.
  • Eichhorn, J., and P. Roskams.2013. Chapter 8 - Assessment of Tree Conditions. Developments in Environmental Science 12:139-167.
  • Gomez-Baggethun, E., and D. N. Barton. 2013. Classifying and valuing ecosystem services for urban planning. Ecological Economics 86:235-245.
  • Hermans, C., M. Smeyers, R. M. Rodriguez, M. Eyletters, M. Strasser, and J.-P. Delhaye. 2003. Quality assessment of urban trees: A comparative study of physiological characterisation, airborne imaging and on site fluorescence monitoring by the OJIP-test. Journal of Plant Physiology 160:81-90.
  • Hofman, J., I. Stokkaer, L. Snauwaert, and R. Samson. 2012. Spatial distribution assessment of particulate matter in an urban street canyon using biomagnetic leaf monitoring of tree crown deposited particles. Environmental Pollution. 1-10.
  • Imbert, D., and J. Portecop. 2008. Hurricane disturbance and forest resilience: Assessing structural vs. functional changes in a Caribbean dry forest. Forest Ecology and Management 255:3494-3501.
  • International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests, 2010). ICP Forests Manual.
  • Jim, C.-Y. 2005. Monitoring the performance and decline of heritage trees in urban Hong Kong. Journal of Environmental Management. 74:161-172.
  • Jim, C.-Y. 2004. Spatial differentiation and landscape-ecological assessment of heritage trees in urban Guangzhou (China). Landscape and Urban Planning 69:51-68.
  • Jim, C.-Y., and H. Zhang. 2013. Defect-disorder and risk assessment of heritage trees in urban Hong Kong. Urban Forestry & Urban Greening, In Press.
  • Jim, C.-Y., and H. Zhang. 2013. Species diversity and spatial differentiation of old-valuable trees in urban Hong Kong. Urban Forestry & Urban Greening 12:171-182.
  • Martinez-Trinidad, T., W. T. Watson, M. A. Arnold, L. Lombardini, and D. N. Appel. 2010. Comparing various techniques to measure tree vitality of live oaks. Urban Forestry & Urban Greening 9:199-203.
  • McPherson, E. G., J. R. Simpson, Q. Xiao, and C. Wu. 2011. Million trees Los Angeles canopy cover and benefit assessment. Landscape and Urban Planning 99:40-50.
  • Nagendra, H., and D. Gopal. 2010. Street trees in Bangalore: Density, diversity, composition and distribution. Urban Forestry & Urban Greening 9:129-137.
  • Niinemets, U. 2010. Responses of forest trees to single and multiple environmental stresses from seedlings to mature plants: Past stress history, stress interactions, tolerance, and acclimation. Forest Ecology and Management 260:1623-1639.
  • Nowak, D. J., and E. J. Greenfield. 2012. Tree and impervious cover change in U.S. cities. Urban Forestry & Urban Greening 11:21-30.
  • Pretzsch, H., P. Biber, and J. Dursky. 2002. The single tree-based stand simulator SILVA: construction, application and evaluation. Forest Ecology and Management 162:3-21.
  • Redfern, D. B., and R. C. Boswell. 2004. Assessment of crown condition in forest trees: comparison of methods, sources of variation and observer bias. Forest Ecology and Management 188:149-160.
  • Richardson, J. J., and L. M. Moskal. 2014. Uncertainty in urban forest canopy assessment: Lessons from Seattle, WA, USA. Urban Forestry & Urban Greening. 13:1:152-157.
  • Roman, L.A., E. G. McPherson, B. C. Scharenbroch, J. Bartens. 2013. Identifying Common Practices and Challenges for Local Urban Tree Monitoring Programs Across the United States. Arboriculture & Urban Forestry 2013. 39(6): 292–299
  • Roy, S., J. Byrne, and C. Pickering. 2012. A systematic quantitative review of urban tree benefits, costs, and assessment methods across cities in different climate zones." Urban Forestry & Urban Greening 11:351-363.
  • USDA USFS Northern Research Station. A Guide to Assessing Urban Forests. Online Factsheet/Guide, Syracuse, NY: USDA USFS Northern Research Station.


Calculating credits

Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.
Recommended pollutant removal efficiencies, in percent, for tree trench/tree box BMPs. Sources. NOTE: removal efficiencies are 100 percent for water that is infiltrated.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
85 link to table link to table link to table 50 35 95 80
Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.

Credit refers to the quantity of stormwater or pollutant reduction achieved toward meeting a runoff volume or water quality goal either by an individual Best Management Practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how tree trench/tree box practices can achieve stormwater credits. Tree systems with and without underdrains are both discussed, with separate sections for each type of system as appropriate.

Overview

Tree trenches and tree boxes are specialized bioretention practices. They are therefore terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Tree systems consist of an engineered soil layer designed to treat stormwater runoff via filtration through plant and soil media, evapotranspiration from trees, or through infiltration into underlying soil. Pretreatment is REQUIRED for all bioretention facilities, including tree-based systems, to settle particulates before entering the BMP. Tree practices may be built with or without an underdrain. Other common components may include a stone aggregate layer to allow for increased retention storage and an impermeable liner on the bottom or sides of the facility if located near buildings, subgrade utilities, or in karst formations.

Pollutant removal mechanisms

schematic of pollutant reductions from tree trench with an underdrain BMP
Schematic illustrating how pollutant reductions (TSS, dissolved and particulate P) are calculated for a tree trench system-tree box.

Like other bioretention practices, tree trenches and tree boxes have high nutrient and pollutant removal efficiencies (Mid-America Regional Council and American Public Works Association Manual of Best Management Practice BMPs for Stormwater Quality, 2012). Tree practices provide pollutant removal and volume reduction through filtration, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption; the extent of these benefits is highly dependent on site specific conditions and design. In addition to phosphorus and total suspended solids (TSS), which are discussed in greater detail below, tree practices treat a wide variety of other pollutants.

Removal of phosphorus is dependent on the engineered media. Media mixes with high organic matter content typically leach phosphorus and can therefore contribute to water quality degradation. The Manual provides a detailed discussion of media mixes, including information on phosphorus retention.

Location in the treatment train

Stormwater treatment trains are multiple Best Management Practice (BMPs) that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Tree trenches and tree boxes can be incorporated anywhere in the stormwater treatment train but are most often located in upland areas of the treatment train. The strategic distribution of tree BMPs help control runoff close to the source where it is generated.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Tree practices generate credits for volume, TSS, and TP. Practices with underdrains do not substantially reduce the volume of runoff but may qualify for a partial volume credit as a result of evapotranspiration, infiltration occurring through the sidewalls above the underdrain, and infiltration below the underdrain piping. Tree practices are effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and approach

In developing the credit calculations, it is assumed the tree practice is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the tree section of the Manual.

Warning: Pre-treatment is required for all filtration and infiltration practices

In the following discussion, the water quality volume (VWQ) is delivered instantaneously to the BMP. The VWQ is stored within the filter media. The VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch times the new impervious surface area. For MIDS, VWQ is 1.1 inches times the impervious surface area.

Volume credit calculations - no underdrain

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil, evapotranspiration (ET) from trees, and interception of rainfall by the tree canopy. The total volume credit, V in cubic feet, is given by

<math> V = V_{inf_b}\ + V_{ET}\ + V_I </math>

where

Vinf is the volume of captured water that is infiltrated, in cubic feet;
VET is the volume of captured water that is lost to evapotranspiration, in cubic feet; and
VI is the volume of precipitation intercepted by the tree canopy, in cubic feet.

Interception credit

Water intercepted by a tree canopy may evaporate or be slowly released such that it does not contribute to stormwater runoff. An interception credit is given by a simplified value of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous and coniferous tree species.

  • Ic coniferous = 0.087 inches (2.2 millimeters)
  • Ic deciduous = 0.043 inches (1.1 millimeters)

This credit is per storm event.

Infiltration and ET credits

schematic showing terms used for credit calculations for tree trench
Schematic illustrating terms used for calculating credits for a tree trench system.

The infiltration and ET credits are assumed to be instantaneous values entirely based on the capacity of the BMP to capture, store, and transmit water in any storm event. Because the volume is calculated as an instantaneous volume, the water quality volume (VWQ) is assumed to be instantly stored in the bioretention media. The volume of water between saturation and field capacity is assumed to infiltrate through the bottom of the BMP. The volume credit (Vinfb) for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by

<math> V_{inf_b} = (n-FC)\ D_M\ (A_M + A_B)\ / 2 </math>

where

n is the porosity of the media in cubic feet per cubic foot;
FC is the field capacity of the media in cubic feet per cubic foot;
AM is the area at the surface of the media, in square feet;
AB is the area at the bottom of the media, in square feet; and
DM is the media depth within the BMP, in feet.

Vinfb should be calculated to infiltrate within a specific drawdown time. The construction stormwater permit has a 48 hour drawdown requirement (24 hours is recommended for discharges to trout streams).

ET is calculated as the volume of water between field capacity and the permanent wilting point. Two calculations are needed to determine the evapotranspiration (ET) credit. The smaller of the two calculated values will be used as the ET credit.

The first calculation is the volume of water available for ET. This equals the water stored between field capacity and the wilting point. Note this calculation is made for the entire thickness of the media.

The second calculation is the theoretical ET. The theoretical volume of ET lost (Lindsey and Bassuk, 1991) per day per tree is given by

<math>ET = (CP) (LAI) (E_{rate}) (E_{ratio})*3</math>

Where:

CP is the canopy projection area (square feet);
LAI is the Leaf Area Index;
Erate is the evaporation rate per unit time (feet per day);
Eratio is the evaporation ratio; and
3 accounts for the number of days over which ET occurs (the average number of days between rain events in Minnesota).
Caution: The theoretical ET must be adjusted if the actual soil volume is less than the recommended volume. See the adjustment calculation below.

The canopy projection area (CP) is the perceived tree canopy diameter at maturity and is given by

<math>CP = Π (d/2)^2</math>

where d is the diameter of the canopy as measured at the dripline (feet).

CP varies by tree species. Please refer to the Tree Species List for these values. Default values can be used in place of calculating CP. Defaults for CP are based on tree size and are

  • 315 for a small tree;
  • 490 for a medium sized tree; and
  • 707 for a large tree.

The leaf area index (LAI) should be stratified by type into either

  • deciduous tree species (LAI = 3.5 for small trees, 4.1 for medium-sized trees, and 4.7 for large trees), or
  • coniferous tree species (LAI = 5.47).

These values are based on collected research for global leaf area from 1932-2000 (Scurlock, Asner and Gower, 2002).

The evaporation rate (Erate) per unit time can be calculated using a pan evaporation rate for the given area, as available at NOAA. This should be estimated as a per day value.

The evaporation ratio (Eratio) is the equivalent that accounts for the efficiency of the leaves to transpire the available soil water or, alternately, the stomatal resistance of the canopy to transpiration and water movement. This is set at 0.20, or 20 percent based on research by Lindsey and Bassuk (1991). This means that a 1 square centimeter leaf transpires only about 1/5 as much as 1 square centimeter of pan surface.

If the soil volume is less than the recommended volume, the theoretical ET must be adjusted. Since the recommended soil volume equals 2 times the canopy project area (CP), the adjustment term is given by

<math>Adjustment = (S_v)/(2 CP)</math>

Where Sv is the actual soil volume in cubic feet. Multiply the theoretical ET by the adjustment term to arrive at the true value for theoretical ET.

It is recommended that calculations be based over a three day period. To determine the credit, compare the volume of water available for ET to the theoretical ET over a 3 day period. The credit is the smaller of these two values.

Recommended values for porosity, field capacity and wilting point for different soils.1
Link to this table.

Soil Hydrologic soil group Porosity 2 (volume/volume) Field capacity (volume/volume) Wilting point (volume/volume) Porosity minus field capacity (volume/volume)3 Field capacity minus wilting point (volume/volume)4
Sand A (GM, SW, or SP) 0.43 0.17 0.025 to 0.09 0.26 0.11
Loamy sand A (GM, SW, or SP) 0.44 0.09 0.04 0.35 0.05
Sandy loam A (GM, SW, or SP) 0.45 0.14 0.05 0.31 0.09
Loam B (ML or OL) 0.47 0.25 to 0.32 0.09 to 0.15 0.19 0.16
Silt loam B (ML or OL) 0.50 0.28 0.11 0.22 0.17
Sandy clay loam C 0.4 0.07
Clay loam D 0.46 0.32 0.15 0.14 0.17
Silty clay loam D 0.47 to 0.51 0.30 to 0.37 0.17 to 0.22 0.16 0.14
Sandy clay D 0.43 0.11
Silty clay D 0.47 0.05
Clay D 0.47 0.32 0.20 0.15 0.12

1Sources of information include Saxton and Rawls (2006), Cornell University, USDA-NIFA, Minnesota Stormwater Manual
2Soil saturation is assumed to be equal to the porosity.
3This value may be used to represent the volume of water that will drain from a bioretention media.
4This value may be used to estimate the amount of water available for evapotranspiration


The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and water quality volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


Volume credit calculations - underdrain

Volume credits for a tree system with an underdrain include the ET and interception credits discussed above and an infiltration credit. The main design variables impacting the infiltration volume credit include whether the underdrain is elevated above the native soils and if an impermeable liner on the sides or bottom of the basin is used. Other design variables include media top surface area, underdrain location, basin bottom area, total depth of media, soil water holding capacity and media porosity, and infiltration rate of underlying soils. The total volume credit (Vinf), in cubic feet, is given by

<math> V_{inf} = V_{inf_b}\ + V_{inf_s}\ + V_U + V_{ET}\ + V_I </math>

where:

Vinfb = volume of infiltration through the bottom of the basin (cubic feet);
Vinfs = volume of infiltration through the sides of the basin (cubic feet);
VU = volume of water stored beneath the underdrain that will infiltrate into the underlying soil (cubic feet);
VET = volume of captured water that is lost to evapotranspiration, in cubic feet; and
VI = volume of precipitation intercepted by the tree canopy, in cubic feet.

Volume credits for ET and canopy interception remain the same as shown above

Volume credits for infiltration through the bottom of the basin (Vinfb) are accounted for only if the bottom of the basin is not lined and the BMP permanently removes a portion of the stormwater runoff via infiltration through sidewalls or beneath the underdrain piping. As long as water continues to draw down, some infiltration will occur through the bottom of the BMP. However, it is assumed that when an underdrain is included in the installation, the majority of water will be filtered through the media and exit through the underdrain. Because of this, the drawdown time is likely to be short. Volume credit for infiltration through the bottom of the basin is given by

<math> V_{inf_B} = A_B\ DDT\ I_R/12 </math>

where

IR = design infiltration rate of underlying soil (inches per hour);
AB = surface area at the bottom of the basin (square feet); and
DDT = drawdown time for ponded water (hours).
Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

The Construction Stormwater permit requires drawdown within 48 hours and recommends 24 hours when discharges are to a trout stream. With a properly functioning underdrain, the drawdown time is likely to be considerably less than 48 hours.

Volume credit for infiltration through the sides of the basin is accounted for only if the sides of the basin are not lined with an impermeable liner. Volume credit for infiltration through the sides of the basin is given by

<math> V_{inf_s} = (A_M - A_U)\ DDT\ I_R/12 </math>

where

AM = the area at the media surface (square feet); and
AU = the surface area at the underdrain (square feet).
Information: The MIDS calculator assigns a default value of 0.06 inches per hour, equivalent to a D soil, to IR. This is based on the assumption that most water will drain to the underdrain, but that some loss to underlying soil will occur. A conservative approach assuming a D soil was thus chosen.

This equation assumes water will infiltrate through the entire sideslope area during the period when water is being drawn down. This is not the case, however, since the water level will decline in the BMP. The MIDS calculator assumes a linear drop in water level and thus divides the right hand term in the above equation by 2.

Volume credit for media storage capacity below the underdrain (VU) is accounted for only if the underdrain is elevated above the native soils. Volume credit for media storage capacity below the underdrain is given by

<math> V_U = (n-FC)\ D_U\ (A_U + A_B)/2 </math>

where

AB = surface area at the bottom of the media (square feet);
n = media porosity (cubic feet per cubic foot);
FC is the field capacity of the soil, in cubic feet per cubic foot; and
DU = the depth of media below the underdrain (feet).

This equation assumes water between the soil porosity and field capacity will infiltrate into the underlying soil. Water stored below the underdrain should infiltrate within a specified drawdown time. The construction stormwater permit has a 48 hour requirement for drawdown (24 hours is recommended when discharges are to trout streams).

The ET and infiltration credits are assumed to be instantaneous values based on the design capacity of the BMP for a specific storm event. Instantaneous volume reduction, also termed event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools. Assuming an instantaneous volume will somewhat overestimate actual storage when the majority of water is being captured by the underdrains.

The volume of water passing through underdrains can be determined by subtracting the volume loss (V) from the volume of water instantaneously captured by the BMP. No volume reduction credit is given for filtered stormwater that exits through the underdrain, but the volume of filtered water can be used in the calculation of pollutant removal credits through filtration.

Example calculation

A parking lot is developed and will contain tree trenches containing red maple (Acer rubrum). The tree trench has 1000 cubic feet of sandy loam per tree. Note that the following calculations are on a per tree basis. Total volume credit for the BMP will equal the per tree value times the number of trees, assuming all trees are of the same relative size (large in this case).

Infiltration credit

The infiltration credit is given by

<math>(soil volume) (porosity - field capacity) = 1000 * 0.31 = 310 cubic feet</math>

Evapotranspiration credit

Using the tree morphology table, red maple is a large tree with a mature canopy of 30 feet. The available storage volume is given by

<math>Soil volume (field capacity - wilting point) = 1000 * 0.09 = 90 cubic feet</math>

The theoretical ET volume is given by

<math>(CP) (LAI) (E_{rate}) (E_{ratio}) (adjustment) (3 days) = 707 * 4.7 * 0.02 * 0.2 * (1000/(2 * 707)) * 3 = 28.2 cubic feet</math>

The smaller value is the theoretical ET (28.2 cubic feet), so that is the volume credit. Note that if the recommended soil volume of 1414 cubic feet had been used the credit would be 39.9 cubic feet.

To make this calculation we used the default value of 707 for CP and the soil volume information from the table above. The evaporation rate (Erate) of 0.24 inches per day (0.02 feet per day) was from data collected at the Southwest Research and Outreach Center in Lamberton, Minnesota.

Interception credit

The interception credit is given by

<math>707 (0.043/12) = 2.5 cubic feet</math>

The division by 12 converts the calculation to feet.

Total credit

The total credit is the sum of the infiltration, ET and interception credits and equals (310 + 28.2 + 2.5) or 340.7 cubic feet.

Total suspended solids credit calculations

schematic of pollutant reductions from tree trench with an underdrain BMP
Schematic illustrating how pollutant reductions (TSS, dissolved and particulate P) are calculated for the tree trench system-tree box with an underdrain BMP in the MIDS calculator. If there is no underdrain, pollutant removal for infiltrated water is 100 percent.

TSS reduction credits correspond with volume reduction through infiltration/ET and filtration of water captured by the tree BMP and are given by

<math> M_{TSS} = M_{TSS_{i+ET}} + M_{TSS_f} </math>

where

MTSS = TSS removal (pounds);
MTSSi+ET = TSS removal from infiltrated and evapotranspired water (pounds); and
MTSSf = TSS removal from filtered water (pounds).

Pollutant removal for infiltrated and evapotranspired water is assumed to be 100 percent. The event-based mass of pollutant removed through infiltration and ET, in pounds, is given by

  • underdrain - <math> M_{TSS_{i+ET}} = 0.0000624\ (V_{inf_b} + V_{inf_s} + V_U + V_{ET})\ EMC_{TSS} </math>
  • no underdrain - <math> M_{TSS_{i+ET}} = 0.0000624\ V_{WQ}\ EMC_{TSS} </math>

where

EMCTSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on EMC values for TSS, link here. If there is no underdrain, the water quality volume (VWQ)) is used in this calculation.

Removal for the filtered portion is less than 100 percent. The event-based mass of pollutant removed through filtration, in pounds, is given by

<math> M_{TSS_f} = 0.0000624\ (V_{total} - (V_{inf_b} + V_{inf_s} + V_U))\ EMC_{TSS}\ R_{TSS} </math>

where

Vtotal is the total volume of water captured by the BMP (cubic feet); and
RTSS is the TSS pollutant removal percentage for filtered runoff.

The Stormwater Manual provides a recommended value for RTSS of 0.85 (85 percent removal) for filtered water, while the MIDS calculator provides a value of 0.65 (65 percent). Alternate justified percentages for TSS removal can be used if proven to be applicable to the BMP design.

The above calculations may be applied on an event or annual basis and are given by

<math> M_{TSS_f} = 2.72\ F\ V_{annual}\ EMC_{TSS}\ R_{TSS} </math>

where

F is the fraction of annual volume filtered through the BMP; and
Vannual is the annual volume treated by the BMP, in acre-feet.

Phosphorus credit calculations

Total phosphorus (TP) reduction credits correspond with volume reduction through infiltration/ET and filtration of water captured by the tree BMP and are given by

<math> M_{TP} = M_{TP_{i+ET}} + M_{TP_f} </math>

where

MTP = TP removal (pounds);
MTPi+ET = TP removal from infiltrated and evapotranspired water (pounds); and
MTPf = TP removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration and ET, in pounds, is given by

  • underdrain - <math> M_{TP_{i+ET}} = 0.0000624\ (V_{inf_b} + V_{inf_s} + V_U + V_{ET})\ EMC_{TP} </math>
  • no underdrain - <math> M_{TP_{i+ET}} = 0.0000624\ V_{WQ})\ EMC_{TP} </math>

where

EMCTP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter).

The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

The filtration credit for TP in an underdrained system assumes removal rates based on the soil media mix used and the presence or absence of amendments. Soil mixes with more than 30 mg/kg phosphorus (P) content are likely to leach phosphorus and do not qualify for a water quality credit. If the soil phosphorus concentration is less than 30 mg/kg, the mass of phosphorus removed through filtration, in pounds, is given by

<math> M_{TP_f} = 0.0000624\ (V_{total} - (V_{inf_b} + V_{inf_s} + V_U + V_{ET}))\ EMC_{TP}\ R_{TP} </math>

Information: Soil mixes C and D are assumed to contain less than 30 mg/kg of phosphorus and therefore do not require testing

Again, assuming the phosphorus content in the media is less than 30 milligrams per kilogram, the removal efficiency (RTP) provided in the Stormwater Manual is a function of the fraction of phosphorus that is in particulate or dissolved form, the depth of the media, and the presence or absence of soil amendments. For the purpose of calculating credits it can be assumed that TP in storm water runoff consists of 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). The removal efficiency for particulate phosphorus is 80 percent. The removal efficiency for dissolved phosphorus is 20 percent if the media depth is 2 feet or greater. The efficiency decreases by 1 percent for each 0.1 foot decrease in media thickness below 2 feet. If a soil amendment is added to the BMP design, an additional 40 percent credit is applied to dissolved phosphorus. Thus, the overall removal efficiency, (RTP), expressed as a percent removal of total phosphorus, is given by

<math> R_{TP} = (0.8 * 0.55) + (0.45 * ((0.2 * (D_{MU_{max=2}})/2) + 0.40_{if amendment is used})) * 100 </math>

where

the first term on the right side of the equation represents the removal of particulate phosphorus;
the second term on the right side of the equation represents the removal of dissolved phosphorus; and
DMUmax=2 = the media depth above the underdrain, up to a maximum of 2 feet.

The following table can be used to calculate phosphorus credits.

Phosphorus credits for bioretention systems with an underdrain.
Link to this table

Particulate phosphorus Dissolved phosphorus
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, particulate credit = 80% of the particulate fraction (assumed to be 55% of total P)
  • If no or unknown, particulate credit = 0%


TP removal credit

  • Particulate fraction (55% of TP) * removal rate for that fraction (80%) = 0.55 * 0.80 = 0.44 or 44%
1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2)
  • If no or unknown, credit = 0%

2. Does the system include approved P-sorbing soil amendments2?

  • If yes, additional 40% credit


TP removal credit

  • TP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent
  • Adjust TP removal if depth is less than 2 feet
  • Adjust TP removal if dissolved credit is higher due to use of P-sorbing soil amendments

1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include

  • 5% by volume elemental iron filings above IWS or elevated underdrain
  • minimum 5% by volume sorptive media above IWS or elevated underdrain
  • minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
  • other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results

Example calculations

Example 1 Assume the following:

  • A tree trench with an underdrain has 1 foot of media above the underdrain
  • 50 percent of annual runoff is infiltrated into the underlying soil
  • 40 percent of annual runoff is captured by the underdrain
  • 10 percent of annual runoff bypasses the BMP
  • Media Mix A is used and soil phosphorus is 32 milligrams per kilogram
  • Water Treatment Residuals, 7 percent by weight, have been mixed into the top 15 centimeters of the media.

The credits are as follows

  • 100 percent credit for infiltrated runoff = 50 percent of annual runoff = 50 percent of annual phosphorus load
  • For water that is captured by the underdrain
    • The media is Mix A with a P content greater than 30 milligrams per kilogram, resulting in no credit for particulate or dissolved phosphorus
    • A P-sorbing amendment has been added to the media and meets the requirements for a credit of 40 percent. The credit applies to the dissolved portion of phosphorus, which is 45 percent of total phosphorus. The credit is therefore 40 percent times 45 percent times the annual runoff volume of 40 percent, resulting in a credit of 7 percent of total annual P (0.4 * 0.45 * 0.4).
  • No credit for water that bypasses the BMP
  • The total credit is 57 percent of the annual P load.

Example 2 Assume the following:

  • A tree trench with an underdrain has 1 foot of media above the underdrain
  • 50 percent of annual runoff is infiltrated into the underlying soil
  • 40 percent of annual runoff is captured by the underdrain
  • 10 percent of annual runoff bypasses the BMP
  • Media Mix C is used

The credits are as follows

  • 100 percent credit for infiltrated runoff = 50 percent of annual runoff = 50 percent of annual phosphorus load
  • For water that is captured by the underdrain
    • The media is Mix C resulting in 80 percent credit for particulate phosphorus. Since particulate P is 55 percent of total P, the credit is 0.80 * 0.55 * 0.40 = 18 percent. The value of 0.4 in the equation accounts for 40 percent of the annual runoff volume.
    • The media mix is C and there is 1 foot of media above the underdrain. The credit is 0.2 * 1/2 * 0.45 = 5 percent. The 1/2 adjusts for the thickness of media above the underdrain and the 0.45 accounts for 45 percent of total phosphorus being in dissolved form.
  • No credit for water that bypasses the BMP
  • The total phosphorus credit is 73 percent of the annual P load (50 + 18 +5).

Methods for calculating credits

Tree trenches and tree boxes are specialized bioretention BMPs. This section provides specific information on generating and calculating credits from bioretention BMPS, including tree-based systems, for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

  1. Quantifying volume and pollution reductions based on accepted hydrologic models
  2. The Simple Method and MPCA Estimator
  3. MIDS Calculator
  4. Quantifying volume and pollution reductions based on values reported in literature
  5. Quantifying volume and pollution reductions based on field monitoring

Credits based on models

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits. The available models described below are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

  • Model name and version
  • Date of analysis
  • Person or organization conducting analysis
  • Detailed summary of input data
  • Calibration and verification information
  • Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for bioretention BMPs, including tree-based systems. In using this table, use the sort arrow on the table to select Infiltrator BMPs or Filter BMPs, depending on the type of tree BMP and the terminology used in the model.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Link to this table
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SELECT X X X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.


The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of mean pollutant concentrations and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Warning: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

Download MPCA Estimator here: File:MPCA Estimator.xlsx

A quick guide for the estimator is available Quick Guide: MPCA Estimator tab.

MIDS Calculator

mids logo
Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including tree systems with an underdrain and without an underdrain. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) entering the BMP. Concentration reductions resulting from treatment can be converted to mass reductions if the volume of stormwater treated is known.

Designers may use the pollutant reduction values reported in this manual or may research values from other databases and published literature. Designers who opt for this approach should

  • select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
  • select a pollutant removal reduction from literature that studied a BMP with site characteristics and climate similar to the device being considered for credits;
  • review the article to determine that the design principles of the studied BMP are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
  • give preference to literature that has been published in a peer-reviewed publication.
Information: Tree trenches and tree boxes are bioretention practices, but there is limited information in the literature on pollutant removal in tree-based systems. The following references provide information for bioretention systems, which can be applied to tree-based practices

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits for bioretention systems. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed bioretention device, considering such conditions as watershed characteristics, bioretention sizing, soil infiltration rates, and climate factors.

  • International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals
    • Compilation of BMP performance studies published through 2011
    • Provides values for TSS, Bacteria, Nutrients, and Metals
    • Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels
  • Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon
    • Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations
    • Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics
    • Applicable to filters, wet ponds, porous pavements, soakage trenches, flow-through stormwater planters, infiltration stormwater planters, vegetated infiltration basins, swales, and treatment wetlands
  • The Illinois Green Infrastructure Study
    • Figure ES-1 summarizes BMP effectiveness
    • Provides values for TN, TSS, peak flows / runoff volumes
    • Applicable to permeable pavements, constructed wetlands, infiltration, detention, filtration, and green roofs
  • New Hampshire Stormwater Manual
    • Volume 2, Appendix B summarizes BMP effectiveness
    • Provides values for TSS, TN, and TP removal
    • Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
  • Design Guidelines for Stormwater Bioretention Facilities. University of Wisconsin, Madison
    • Table 2-1 summarizes typical removal rates
    • Provides values for TSS, metals, TP, TKN, ammonium, organics, and bacteria
    • Applicable for bioretention
  • BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
    • Appendix B provides pollutant removal performance curves
    • Provides values for TP, TSS, and zinc
    • Pollutant removal broken down according to land use
    • Applicable to infiltration trench, infiltration basin, bioretention, grass swale, wet pond, and porous pavement
  • Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report
    • Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
    • Applicable to wet basins, stormwater wetlands, bioretention filter, sand filter, infiltration trench, and filter strips/grass swales

Credits based on field monitoring

Field monitoring may be used to calculate stormwater credits in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

  • Establish the objectives and goals of the monitoring.
    • Which pollutants will be measured?
    • Will the monitoring study the performance of a single BMP or multiple BMPs?
    • Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    • Will the results be compared to other BMP performance studies?
    • What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
  • Plan the field activities. Field considerations include:
    • Equipment selection and placement
    • Sampling protocols including selection, storage, delivery to the laboratory
    • Laboratory services
    • Health and Safety plans for field personnel
    • Record keeping protocols and forms
    • Quality control and quality assurance protocols
  • Execute the field monitoring
  • Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

  • Chapter 2: Designing the Program
  • Chapters 3 & 4: Methods and Equipment
  • Chapters 5 & 6: Implementation, Data Management, Evaluation and Reporting
  • Chapter 7: BMP Performance Analysis
  • Chapters 8, 9, & 10: LID Monitoring
Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes the following chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP:

  • Chapter 4: Stormwater Characterization
    • 4.2: General Characteristics and Pollutant Sources
    • 4.3: Sources of Stormwater Quality data
  • Chapter 8: Performance Evaluation
    • 8.1: Methodology Options
    • 8.5: Evaluation of Quality Performance for Individual BMPs
    • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
  • Chapter 10: Hydrologic Evaluation
    • 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices.

In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The information contained in this White Paper would be of use to those considering the monitoring of a manufactured BMP. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include:

  • Chapter 4: Monitoring Methods and Equipment
  • Chapter 5: Analytical Methods and Laboratory Selection
  • Chapter 6: Monitoring Site Selection
  • Chapter 8: Equipment Installation and Maintenance
  • Chapter 10: Pre-Storm Preparation
  • Chapter 11: Sample Collection and Handling
  • Chapter 12: Quality Assurance / Quality Control
  • Chapter 13: Laboratory Reports and Data Review
  • Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice, involving:

  • Level 1: Visual Inspection
  • Level 2: Capacity Testing
  • Level 3: Synthetic Runoff Testing
  • Level 4: Monitoring
  • Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other pollutants

In addition to TSS and phosphorus, bioretention BMPs can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that bioretention BMPs are effective at reducing concentrations of pollutants, including metals, and bacteria. A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Relative pollutant reduction from bioretention systems for metals, nitrogen, bacteria, and organics.
Link to this table

Pollutant Constituent Treatment capabilities1
Metals2 Cadmium, Chromium, Copper, Zinc, Lead High
Nitrogen2 Total nitrogen, Total Kjeldahl nitrogen Low/medium
Bacteria2 Fecal coliform, e. coli High
Organics Petroleum hydrocarbons3, Oil/grease4 High

1 Low: < 30%; Medium: 30 to 65%; High: >65%
2 International Stormwater Database, (2012)
3 LeFevre et al., (2012)
4 Hsieh and Davis (2005).


References and suggested reading

To see how some other cities are calculating tree credits, see Cities That are Pioneers in Developing Stormwater Credit Systems for Trees (Shanstrom, 2014)

  • Brown, Robert A., and William F. Hunt III. 2010. Impacts of media depth on effluent water quality and hydrologic performance of undersized bioretention cells. Journal of Irrigation and Drainage Engineering 137, no. 3: 132-143.
  • Brown, R. A., and W. F. Hunt. 2011. Underdrain configuration to enhance bioretention exfiltration to reduce pollutant loads. Journal of Environmental Engineering 137, no. 11: 1082-1091.
  • Bureau of Environmental Services. 2006. Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland. Oregon. Bureau of Environmental Services, Portland, Oregon.
  • California Stormwater Quality Association. 2003. California Stormwater BMP Handbook-New Development and Redevelopment. California Stormwater Quality Association, Menlo Park, CA.
  • Chris, Denich, Bradford Andrea, and Drake Jennifer. 2013. Bioretention: assessing effects of winter salt and aggregate application on plant health, media clogging and effluent quality. Water Quality Research Journal of Canada. 48(4):387.
  • Caltrans. 2004. BMP Retrofit Pilot Program Final Report. Report No. CTSW-RT-01-050. Division of Environmental Analysis. California Dept. of Transportation, Sacramento, CA.
  • CDM Smith. 2012. Omaha Regional Stormwater Design Manual. Chapter 8 Stormwater Best Management Practices. Kansas City, MO.
  • Davis, Allen P., Mohammad Shokouhian, Himanshu Sharma, and Christie Minami. 2001. Laboratory study of biological retention for urban stormwater management. Water Environment Research, 73, no. 1:5-14.
  • Davis, Allen P., Mohammad Shokouhian, Himanshu Sharma, and Christie Minami. 2006. Water quality improvement through bioretention media: Nitrogen and phosphorus removal. Water Environment Research 78, no. 3: 284-293.
  • Davis, Allen P., Mohammad Shokouhian, Himanshu Sharma, Christie Minami, and Derek Winogradoff. 2003. Water quality improvement through bioretention: Lead, copper, and zinc removal. Water Environment Research 75, no. 1: 73-82.
  • DiBlasi, Catherine J., Houng Li, Allen P. Davis, and Upal Ghosh. 2008. Removal and fate of polycyclic aromatic hydrocarbon pollutants in an urban stormwater bioretention facility. Environmental science & technology 43, no. 2: 494-502.
  • Dorman, M. E., H. Hartigan, F. Johnson, and B. Maestri. 1988. Retention, detention, and overland flow for pollutant removal from highway stormwater runoff: interim guidelines for management measures. Final report, September 1985-June 1987. No. PB-89-133292/XAB. Versar, Inc., Springfield, VA (USA).
  • Geosyntec Consultants and Wright Water Engineers. 2012. Urban Stormwater BMP Performance Monitoring. Prepared under Support from U.S. Environmental Protection Agency, Water Environment Research Foundation, Federal Highway Administration, Environmental and Water Resource Institute of the American Society of Civil Engineers.
  • Gulliver, J. S., A. J. Erickson, and P.T. Weiss. 2010. Stormwater treatment: Assessment and maintenance. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN.
  • Hathaway, J. M., W. F. Hunt, and S. Jadlocki. 2009. Indicator bacteria removal in storm-water best management practices in Charlotte, North Carolina. Journal of Environmental Engineering 135, no. 12: 1275-1285.
  • Hong, Eunyoung, Eric A. Seagren, and Allen P. Davis. 2006. Sustainable oil and grease removal from synthetic stormwater runoff using bench-scale bioretention studies. Water Environment Research 78, no. 2: 141-155.
  • Hsieh, Chi-hsu, and Allen P. Davis. 2005. Evaluation and optimization of bioretention media for treatment of urban storm water runoff. Journal of Environmental Engineering 131, no. 11: 1521-1531.
  • Hunt, W. F., A. R. Jarrett, J. T. Smith, and L. J. Sharkey. 2006. Evaluating bioretention hydrology and nutrient removal at three field sites in North Carolina. Journal of Irrigation and Drainage Engineering 132, no. 6: 600-608.
  • Jaffe, et. al. 2010. The Illinois Green Infrastructure Study. Prepared by the University of Illinois at Chicago, Chicago Metropolitan Agency for Planning. Center for Neighborhood Technology, Illinois-Indiana Sea Grant College Program.
  • Jurries, Dennis. 2003. Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal. Quality, State of Oregon, Department of Environmental Quality (Ed.).
  • Lefevre G.H., Hozalski R.M., Novak P. 2012. The role of biodegradation in limiting the accumulation of petroleum hydrocarbons in raingarden soil. Water Res. 46(20):6753-62.
  • Leisenring, M., J. Clary, and P. Hobson. 2012. International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals. July: 1-31.
  • Li, Houng, and Allen P. Davis. 2009. Water quality improvement through reductions of pollutant loads using bioretention. Journal of Environmental Engineering 135, no. 8: 567-576.
  • Komlos, John, and Robert G. Traver. 2012. Long-term orthophosphate removal in a field-scale storm-water bioinfiltration rain garden. Journal of Environmental Engineering 138, no. 10: 991-998.
  • Mid-America Regional Council, and American Public Works Association. 2012. Manual of best management practices for stormwater quality.
  • New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B. Concord, NH.
  • North Carolina Department of Environment and Natural Resources. 2007. Stormwater Best Management Practices Manual. North Carolina Department of Environment and Natural Resources, Raleigh, North Carolina.
  • Passeport, Elodie, William F. Hunt, Daniel E. Line, Ryan A. Smith, and Robert A. Brown. 2009. Field study of the ability of two grassed bioretention cells to reduce storm-water runoff pollution. Journal of Irrigation and Drainage Engineering 135, no. 4: 505-510.
  • Ohrel, R. 2000. Simple and Complex Stormwater Pollutant Load Models Compared: The Practice of Watershed Protection. Center for Watershed Protection, Ellicott City, MD. Pages 60-63
  • Oregon State University Transportation Officials. Dept. of Civil, Environmental Engineering, University of Florida. Dept. of Environmental Engineering Sciences, GeoSyntec Consultants, and Low Impact Development Center, Inc. 2006. Evaluation of Best Management Practices for Highway Runoff Control. No. 565. Transportation Research Board.
  • Schueler, T.R., Kumble, P.A., and Heraty, M.A. 1992. A Current Assessment of Urban Best Management Practices: Techniques for Reducing Non-Point Source Pollution in the Coastal Zone. Metropolitan Washington Council of Governments, Washington, D.C.
  • TetraTech. 2008. BMP Performance Analysis. Prepared for US EPA Region 1, Boston, MA.
  • Torres, Camilo. 2010. Characterization and Pollutant Loading Estimation for Highway Runoff in Omaha, Nebraska. M.S. Thesis, University of Nebraska, Lincoln.
  • United States EPA. 1999. Stormwater technology fact sheet-bioretention. Office of Water, EPA 832-F-99 12.
  • Water Environment Federation. 2014. Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices. A White Paper by the National Stormwater Testing and Evaluation of Products and Practices (STEPP) Workgroup Steering Committee.
  • WEF, ASCE/EWRI. 2012. Design of Urban Stormwater Controls. WEF Manual of Practice No. 23, ASCE/EWRI Manuals and Reports on Engineering Practice No. 87. Prepared by the Design of Urban Stormwater Controls Task Forces of the Water Environment Federation and the American Society of Civil Engineers/Environmental & Water Resources Institute.
  • Weiss, Peter T., John S. Gulliver, and Andrew J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices Final Report.. Published by: Minnesota Department of Transportation.
  • Wossink, G. A. A., and Bill Hunt. 2003. The economics of structural stormwater BMPs in North Carolina. Water Resources Research Institute of the University of North Carolina.


Case studies

Maplewood Mall, Maplewood Minnesota

photo of pipe and trench system at Maplewood Mall
Excavated tree trench with the aggregate base and pipe system, Maplewood Mall. Image Courtesy of Barr Engineering Company
photo of soil being washed into tree trench, Maplewood Mall
Soil being washed into the angular granite rock, Maplewood Mall. Image Courtesy of Barr Engineering Company.
photo of tree protection with signage, Maplewood Mall
Photo of tree protection with signage, Maplewood Mall. Image Courtesy of Barr Engineering Company
photo of Maplewood Mall with completed tree trenches and parked cars
Photo of Maplewood Mall with completed tree trenches and parked cars. Image Courtesy of Barr Engineering Company.
Green Infrastructure: Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.

The Kohlman Lake TMDL (Total Maximum Daily Load) report calls for a 22 percent reduction in watershed loading of phosphorus. In response to the TMDL, the Maplewood Mall recently underwent a comprehensive redesign of its 35 acre parking lot. The four-phase construction process of this redesign took place between 2009 and 2012. This parking lot was designed by Barr Engineering Company on behalf of the Ramsey-Washington Metro Watershed District (who sponsored the project) and Simon Property Group (who owns Maplewood Mall). This redesign was completed with the intent of capturing one inch of runoff from 90 percent of the parking lot area, while reducing at least 90 percent of the sediment load and 60 percent of the phosphorus load leaving the site.

To help meet the proposed stormwater goals, this project included 375 trees planted across the project site. Two hundred trees were placed in tree trenches and 175 trees were placed in rainwater gardens. This case study will focus on the planting methods for the trees planted within the tree trenches.

Each tree placed in a tree trench was planted using a Stockholm Tree Trench Method for Stormwater. The Stockholm Tree Trench Method uses angular granite rock to bear the load of the pavement profile and vehicular traffic above. The void space between the angular granite was filled with soil, which is washed into the angular granite. The soil does not need to be compacted to support loads. This non-compacted soil and remaining void space between the angular granite are available for use by trees as both a growing medium and space for gaseous exchange to and from the roots.

Approximately 820 cubic feet of soil/angular granite and clear angular granite was provided per tree on average. The average installed cost per tree using this method was $18,000. This cost includes soil treatment and all other aspects of the tree trench installation such as storm sewers, asphalt pavement profile, concrete curb, tree grates, cages, etc. The trees received supplemental watering via 15 gallon capacity gator bags for the first two years of growth. Following the two-year establishment period, the trees will only receive water from above and below during rainfall events. At the time of the writing of this case study, October 2013, the planted trees are in good to excellent condition. When initially planted, the tree canopy covered less than 1 percent of the site. After 20 years growth, the trees are projected to cover 17 percent of the site with canopy.

The designer stated that in future work using similar planting techniques they would modify their maintenance protocol to include biannual fertilization at the tree root ball for the first two years of growth (per the advice from the street tree specialists in Stockholm, Sweden) and use larger capacity gator bags to better facilitate watering during the first two years of growth.

The stormwater goals of this project were to

  • determine the effectiveness of the tree trench systems in reducing pollutant loading and volume;
  • determine the effectiveness of the overall project in reducing pollutant loading and volume; and
  • monitor the use of the cistern during the first full season of its operation in order to determine water use and the frequency of cleaning required in the future.

In 2011 and 2012, Barr Engineering Company worked with the Ramsey-Washington Metro Watershed District to develop a monitoring plan and to create an XPSWMM model to determine the reduction in runoff volume as a result of this project.

The following activities were planned for 2013.

  • monitor rain gage levels;
  • monitor the east and south locations for water quality and quantity from post-project conditions;
  • record water levels in the entrance cistern while it is online;
  • monitor tree grove level loggers;
  • model (using rain gage data) to determine the reduction in runoff volume due to project;
  • analyze water quality data and tree grove level logging data; and
  • develop a monitoring report to assess the effectiveness of the project.

At the time of this case study, the analysis of the data was still being completed and the monitoring report had not been released.

Project Summary

  • Owners: Ramsey-Washington Metro Watershed District (project sponsors) and Simon Property Group (owners of Maplewood Mall)
  • Designers: Barr Engineering Company
  • Year of completion: 2012
  • Tree Planting Method: Stockholm Soil Trench Method
  • Tree Species Used in Planting Method: Discovery Elm (Ulmus davidiana (Discovery)), Skyline Honey Locust (Gleditsia tricanthos var. inermis (Skyline)), Espresso Kentucky Coffeetree (Gymnocladus dioicus (Espresso)), Common Hackberry (Celtis occidentalis), and Swamp White Oak (Quercus bicolor)
  • Number of trees in Planting Method: 200
  • Approximate Installed Cost Per Tree: $18,000
  • Stormwater Goals: capturing one inch of runoff from 90 percent of the parking lot area, while reducing at least 90 percent of the sediment load and 60 percent of the phosphorous load leaving the site.
  • Is the Site Accessible: Yes

For more information

Central Corridor Green Line Integrated Tree Trench System, St. Paul and Minneapolis, Minnesota

schematic cross-section of Central Corridor Light Rail tree system
Cross section of the tree system installed for the Central Corridor Light Rail Transit project in St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
photo of soils used for Light Rail project, St. Paul
Soils used for the Central Corridor Light Rail Transit project, St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
profile  section of the tree system for the Light Rail project in St. Paul
Profile section of the tree system for the Central Corridor Light Rail Transit project in St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
photo of permeable pavement for Light Rail project, St. Paul
Permeable pavement used for the Central Corridor Light Rail Transit project, St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
detail of Central Corridor tree system, St. Paul
Detail cross-section for the Central Corridor Light Rail Transit project, St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
photo of soil placement for Light Rail Corridor project
Placement of soil for the Central Corridor Light Rail Transit project, St. Paul, Minnesota. Image courtesy of the Capitol Region Watershed District.
photo of trees prior to planting for the Light Rail project, St. Paul, MN
Trees prior to planting for the Central Corridor Light Rail Transit project, St. Paul, MN. Image courtesy of the Capitol Region Watershed District.
photo for tree trench system, Central Corridor Light rail project
Photo of the completed tree system for the Central Corridor Light Rail Transit project, St. Paul, Minnesota. Image courtesy of the Capitol Region Watershed District.

The Central Corridor Light Rail Transit (LRT) line is located within a highly-urbanized and heavily-paved corridor connecting downtown St. Paul and downtown Minneapolis. This corridor is made up of 111 acres of impervious surface. The stormwater runoff from the impervious surfaces ultimately drains to the Mississippi River via the municipal storm drain system and carries both sediment and pollutant loads to the river. Portions of the river are impaired for turbidity, nutrients, and bacteria.

As this project is located within the jurisdiction of the Capitol Region Watershed District (CRWD), it was required to comply with the CRWD's rules for stormwater management. These rules require that the site capture and retain the first inch of rainfall on site.

Four different green infrastructure practices (tree trench, underground infiltration trench, rain garden, stormwater planter) were used in conjunction to help meet the requirements outlined by CRWD, but this case study focus on the integrated tree trench systems. These systems stretch out along both sides of the corridor for over five miles of the LRT corridor. The tree trench systems are approximately 10 feet wide, with a 3 foot deep profile of CU structural soil and 1.5 feet of clear aggregate below that. These trenches provide approximately 792,000 cubic feet of CU structural soil in total for the project (approximately 634 cubic feet of CU structural soil per tree).

These tree trench systems are layered systems of trees, porous pavers, CU Structural Soil, drainage rock, and a 12 inch perforated PVC pipe that directs runoff from the street into the clear rock in the integrated tree trench systems. Where possible and appropriate, these systems are paired with existing soils with high infiltration rates. In addition to the structural components mentioned above, approximately 1,250 new trees have been planted within the integrated tree trenches. The total bid for the integrated tree trench system was $3.29 million (approximately $2,632 per tree). Please note that this bid was lump-sum and is likely low in regard to final costs. The engineer's estimate for the system was approximately $6.0 million (approximately $4,800 per tree).

The goals of this green infrastructure project, including the integrated tree trench systems, are

  • treating stormwater runoff to remove sediments and other pollutants;
  • reducing the quantity and rate of stormwater runoff entering the Mississippi River;
  • infiltrating significant portions of the street and sidewalk runoff;
  • increasing the amount of pervious surface in the right of way;
  • providing a non-irrigated water source to the proposed trees; and
  • enhancing livability/beauty of the corridor with streetscape enhancements and plantings.

It is estimated that the green infrastructure practices will reduce runoff volume by 3 acre feet per storm event and 102 acre-feet annually (which includes total storage volume within the tree trench systems, pavers, and CU Soils plus an estimated 0.15 inch reduction per square foot of canopy for a 20-year-old tree for each species). Estimated reductions in pollutants are 83 pounds for phosphorus and 15,134 pounds for TSS.

Monitoring occurs along the integrated tree trench systems and includes

  • collection of water level and drawdown information with pressure transducer;
  • use of temperature sensors 1 and 2.5 feet below the pavers;
  • use of watermark moisture sensors at 0.5. 1.5. 2.0. and 3.0 foot depths (electrical resistance granular matrix sensor); and
  • use of a watermark Monitor Data Logger.

Project Summary

  • Owners: The Metropolitan Council
  • Designers: AECOM, Kimley-Horn, HZ United
  • Year of completion: 2013
  • Tree Planting Method: Integrated tree trench system using CU structural soil and drainage rock.
  • Sample of Tree Species Used in Planting Method: Armstrong Red Maple (Acer Rubrum (Armstrong)), Autumn Spire Red Maple (Acer Ruburm (Autumn Spire)), Sienna Glen Maple (Acer fremanii (Sienna)), Sensation Maple (Acer negundo (Sensation)), Skyline Honeylocust (Gleditsia tricanthos var. inermis (Skyline)), Princeton Sentry Ginkgo (Ginkgo biloba (Princeton Sentry)), Kentucky Coffee Tree (Gymnocladus dioicus), Spring Snow Crabapple (Malus x (Spring Snow)), Swamp White Oak (Quercus bicolor), Ivory Silk Lilac (Syringa reticulate (Ivory Silk)), Boulevard Linden (Tilia americana (Boulevard)), Redmond Linden (Tilia americana (Redmond)), Cathedral Elm (Ulmus (Cathedral)), Discovery Elm (Ulmus davidiana (Discovery)), Valley Forge Elm (Ulmus Americana (Valley Forge)), Princeton Elm (Ulmus Americana (Princeton)), Accolade Elm (Ulmus x Morton (Accolade)), Regal Prince Oak (Quercus x robar (Long)).
  • Number of trees in Planting Method: 1,250
  • Approximate Installed Cost Per Tree: $2,632-$4,800, estimated
  • Stormwater Goals: Retain the 1-inch rainfall on site
  • Is the Site Accessible: Yes

For more information, see:

Tryon Street Mall Trees, Charlotte, NC

photo of trees in Tryon Mall
26 year old trees in the Tryon Street Mall. Image Courtesy of The Kestrel Design Group, Inc.

Planted in 1985, the Tryon Street Mall Trees are some of the oldest trees planted using suspended pavement. The pavement around the trees is held slightly above the soil by 3 foot tall underground pillars so that the soil under the pavement is not compacted beyond root-limiting densities by the pavement loads. In 2013, the willow oaks planted had an average diameter at breast height of 18 inches and an average height of 71 feet (Smiley, 2013). In addition to growing big trees, the system modeled a 10 percent reduction in peak flows (peak storm event) to the City’s stormwater system (EPA, 2013). As of 2013, almost 30 years after planting, the trees are flourishing.

Project Summary

  • Designers: Don McSween and Tom Perry
  • Year of completion: 1985
  • Tree Planting Method: Suspended pavement
  • Tree Species Used in Planting Method: Willow Oak (Quercus phellos)
  • Number of trees in Planting Method: 154
  • Cubic feet of soil per tree: 700 c.f. per tree, shared
  • Is the Site Publicly Accessible: Yes

MARQ2, Minneapolis, Minnesota

photo of silva cell installation, Marquette Avenue, Minneapolis
Tree trench showing Silva Cell system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.
photo of soil placement into silva cell installation, Marquette Avenue, Minneapolis
Soil placement into Silva Cell system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.
photo of tree planting, Marquette Avenue, Minneapolis
Tree planting, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.
photo of completed project, Marquette Avenue, Minneapolis
Completed tree system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.

The MARQ2 project (Marquette Ave and 2nd Ave Busways) originated in response to the Minneapolis Ten Year Transportation Plan, to improve transit service by redefining lanes, lane widths, and sidewalk zones and widths. Stakeholders requested that healthy trees be part of the project.

To maximize the trees’ health, lifespan, as well as maximize stormwater and other ecological services, the designers maximized rootable soil volume for each tree using Silva Cells, a proprietary soil cell system that supports suspended pavement. Soil cells are modular structures that provide uncompacted soil volumes under paved surfaces and can support up to HS20 loading (US Standard Bridge Loading), making it possible to provide urban trees with large soil volumes even in urban areas with little open space. The uncompacted soil volume in soil cells can also be used for stormwater treatment, creating a bioretention system under paved surfaces. Using structural cells with bioretention soil, stormwater becomes an asset as a way to water the trees. The trees in turn will help cleanse and abstract storm water runoff.

The trees and soil cells in this project collect runoff from the sidewalks along 2 of Minneapolis’ main downtown streets (Marquette and 2nd Avenues) through pervious pavers that drain into the underlying structural cells. One of the soil cell groups also collects roof runoff from adjacent building scuppers.

While the amount of runoff treated per tree varies from block to block and from tree to tree, on average, each tree pit collects runoff from about a 300 square foot watershed. With 167 trees, this adds up to an estimated 50,118 square feet or 1.15 acres of sidewalk runoff captured. The loam soil in the structural cells has enough capacity to capture runoff from a 1 inch rain event from 5 times as much impervious surface as it currently captures. In other words, the soil in the structural cells has capacity to capture 1 inch of rain from 5.75 acres of impervious surface. The City of Minneapolis is reserving this extra soil stormwater holding and infiltration capacity for future routing of street runoff to the cells. As trees grow larger, they will also contribute to stormwater capture through interception and evapotranspiration.

Project Summary

  • Owners:City of Minneapolis
  • Designers: Short Elliott Hendrickson Inc. and URS Corporation
  • Year of completion: 2009
  • Tree Planting Method: Suspended pavement with Silva Cells
  • Tree Species Used in Planting Method:Autumn Blaze Maple (Acer x freemanii (Jeffersred)), Crimson Spire Oak (Quercus alba x Q. robur (Crimschmidt)), Emerald Lustre Maple (A. platanoides (Emerald Lustre)), Skyline Honeylocust (Gleditsia triacanthos (Skycole)), Hophornbeam (Ostrya virginiana), Japanese Tree Lilac (Syringa reticulata), Greenspire Linden (Tilia americana (Greenspire)), Swamp White Oak (Quercus bicolor)
  • Number of trees in Planting Method: 167
  • Approximate Installed Cost Per Tree: $8,038
  • Cubic feet of soil per tree: average of 588 c.f. per tree
  • Stormwater Goals: maximize stormwater benefits
  • Is the Site Publicly Accessible: Yes, however only the pervious pavers are visible at the surface.  The Silva cells are not visible.

Mills Woods Sports Site, Edmonton, Alberta

photo millwoods sport center stratacell system
Stratacell system being installed for the Mills Woods Sports site, showing perforated pipe through base of the trench. Photo courtesy Citygreen.
photo millwoods sport center stratacell system
Stratacell system being installed for the Mills Woods Sports site, Showing the washed rock filled area for the filter/water storage system. The area in the forefront to be filled with soil for tree installation. Photo courtesy Citygreen.

A water harvesting project was conducted by the City of Edmonton at Mill Woods Sports site. This harvesting system has been designed to function as a passive recreation facility for a small lake which collects water and provides irrigation to the surrounding sports fields. The StrataCell® structural soil cell system was installed in a continuous trench along the centre of the south parking lot to provide a filter/storage system for runoff water and to provide soil volume for 6 trees installed within the matrix. The water runoff drains into the center of the parking lot through permeable surfacing, then filtered/stored within the StrataCell® system filled with washed rock. Once the water has filtered through the matrix, it reaches a perforated pipe which is tied into the storm sewer and transferred to the storm water lake located on the south-east side of the park. The perforated pipe continues beneath trees to transfer the water to the storm line.

Project Summary

  • Designers: Stantec Consulting
  • Year of completion: 2012
  • Tree Planting Method: Trees are planted in Stratacell system.
  • Number of trees in Planting Method: 6
  • Is the Site Publicly Accessible: Yes

Garrison Concourse Overlook Site Restoration and ADA Improvements, City of Garrison, Minnesota

Project Summary: The Garrison Concourse Overlook Site Restoration and ADA Improvement Project directs runoff from the access road to a center island/historic interpretive area through ADA curb cuts that also double up as stormwater curb cuts. Turf in the interpretive area is planted into an 80 percent sand/20 percent compost bioretention soil mix. Trees are planted into this area in MnDOT select topsoil borrow that extends to a 5 foot radius from each tree trunk. A 6 inch deep fine filter aggregate layer that spans under the bioretention soil and topsoil borrow provides for drainage to the underdrain, which is wrapped in coarse filter aggregate.

Project plans (File:Garrison inf pond.pdf) exist, including a SWPPP plan, estimated quantities, tree protection, and tree planting details.

Project Summary

  • Designers: MacDonald & Mack Architects
  • Year of completion: In Progress
  • Tree Planting Method: Trees are planted in turf open space. Each tree is planted in a 10 foot diameter circle of MnDOT topsoil borrow, within bioretention areas of turf planted in an 80 percent sand/20 percent compost soil mix.
  • Tree Species Used in Planting Method: Princeton Elm (Ulmus Americana (Princeton))
  • Number of trees in Planting Method: 8
  • Is the Site Publicly Accessible: Yes


Soil amendments for phosphorus

Principal mechanisms for phosphorus (P) removal in bioretention are the filtration of particulate-bound P and chemical sorption of dissolved P (see Hunt et al., 2012). Most stormwater control measures (SCMs) capture particulate P by settling or filtration, but leave dissolved P (typically phosphates) untreated. This untreated P accounts on average for 45 percent of total phosphorus in stormwater runoff and can be up to 95 percent of the total phosphorus, depending on the storm event (Erickson et al., 2012). Dissolved phosphorus is bioavailable and represents a significant concern for surface water quality.

Phosphorus sorbing materials contain a metal cation (typically di or trivalent) that reacts with dissolved phosphorus to create an insoluble compound by adsorption or precipitation or both (Buda et al., 2012). Soil components and amendments that have been shown to be effective in increasing chemical sorption of dissolved P include

Caution: Acceptable amendments include the following.
  • 5 percent by volume elemental iron filings above IWS or elevated underdrain;
  • minimum 5 percent by volume sorptive media above IWS or elevated underdrain;
  • minimum 5 percent by weight water treatment residuals (WTR) to a depth of at least 10 centimeters; and
  • other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results

Buda et al. (2012) provide a literature review of P-sorption amendments. Characteristics of ideal P-sorption amendments include low cost, high availability, low toxicity for soil and water resources, potential for reuse as a soil amendment once fully saturated, and no toxicity to plants, wildlife, or children. It is also crucial that soil amendments not negatively impact soil infiltration rate and the ability to grow vigorous plants. Some P sorptive amendments, such as water treatment residuals (WTRs), are waste products turned into a resource to reduce P in bioretention (or agricultural) soils. Results from much of the research to date on use of P-sorbing materials to reduce nutrients in stormwater effluent are promising, but much remains to be learned about lifespan and long term effects of P-sorbing materials on soils and plants.

Benefits

P sorptive amendments have been shown to provide effective P retention for the expected lifetime of bioretention facilities (e.g. Lucas and Greenway, 2011; O’Neill and Davis, 2012a and 2012b). The presence of healthy vegetation plays a crucial role in extending P reduction lifespan of amendments.

Types of P-sorbing materials

The primary P-sorbing chemicals are calcium (Ca), aluminum (Al) and iron (Fe). These are found in a variety of materials.

Limestone or calcareous sand

Combinations of C 33 sand with limestone or calcareous sand were tested in laboratory columns by Erickson et al. (2007). Limestone or calcareous sand showed strong retention of phosphorus but clogged the columns, resulting in hydraulic failure. On-going field studies are looking at the potential for calcium-based systems to remove phosphorus. Examples include studies by Ramsey-Washington Watershed District to determine effectiveness of spent lime and a permeable limestone barrier, and a study by Riley-Purgatory-Bluff Creek Watershed District to determine the effectiveness of a spent lime system. Long-term monitoring of these systems will provide useful information for determining if calcium-based systems can provide effective treatment for dissolved phosphorus.

Drinking Water Treatment Residuals (WTS)

Drinking-water treatment residuals are primarily sediment, metal (aluminum, iron or calcium) oxide/hydroxides, activated carbon, and lime removed from raw water during the water purification process (Agyin-Birikorang et al., 2009). WTRs are increasingly being used to control phosphorus in soils where phosphorus leaching may be problematic for water quality. Kawczyinski and Achtermann (1991) reported that landfilling is the predominant disposal method, followed by land application, sanitary sewer disposal, direct stream discharge, and lagooning. WTRs contain high concentrations of amorphous aluminum (Al) or iron (Fe), making them potential amendments for sorbing soil phosphorus.

Aluminum-based Water Treatment Residuals (WTRs)

O’Neill and Davis (2012a and 2012b) recommend a bioretention soil media of 5 percent WTR, 3 percent triple-shredded hardwood bark mulch, and 92 percent loamy sand for P reduction on the basis of batch, minicolumn, and large column studies. The life expectancy for this media was 20 years. In a comparison of bioretention soil medias (BSM’s) with varying fines concentrations, they found that increasing the concentration of sand (i.e. decreasing fines) improved P reduction. They also found that hardwood bark mulch, a source of organic matter typically low in P, further improved P reduction (O’Neill and Davis 2012a). The authors contend that an oxalate-extractable aluminum-, iron-, and phosphorus-based metric, the oxalate ratio, can be used to predict P sorption capacity, and suggest that a media oxalate ratio of 20 to 40 is expected to meet P adsorption requirements for nutrient sensitive watersheds. This media adsorbed 88.5 percent of the applied P mass, compared to a non-WTR amended control media for which effluent P mass increased 71.2 increased.

O'Neill and Davis (2012b) state “This media consistently produced total phosphorus effluent mean event concentrations less than 25 micrograms per liter and exhibited a maximum effluent concentration of only 70 micrograms per liter”. Concentrations of P as low as 25 micrograms P per liter may be necessary to reduce eutrophication risk depending on receiving water conditions (U.S. Environmental Protection Agency (US EPA, 1986) in O’Neill and Davis, 2012a). References to additional studies are found in O’Neill and Davis (2012a and 2012b).

Iron-based Water Treatment Residuals (WTRs)

As reviewed in O’Neill and Davis (2012 a), one study of iron based WTRs found iron based WTRs to be ineffective to P reduction because they solubilized and released all adsorbed P in reducing conditions, but another more recent study found this may not be the case. According to Dr. Allen Davis (University of Maryland), iron based water treatment residuals “should work just as well, maybe better than Al. The concern with Fe is that if the media becomes anaerobic due to flooding or any other reason, the Fe can be reduced and will dissolve. It adds another layer of complexity to the system.” This concern can be addressed by designing the bioretention practice to ensure the layer where P sorbtion will occur stays aerobic.

Iron filings

Research by Erickson et al. (2012) suggests that the lifespan for iron enhanced sand filtration (5 percent iron) with a typical impervious area ratio should be at least 30 years. Dissolved phosphorus capture should be greater than 80 percent for more than 30 years (Erickson, 2010). Many agricultural studies have also found several forms of iron enhancements to be effective to capture P (e.g. Chardon et al., 2012; Stoner et al. 2012; literature review in Buda et al. 2012). Research showing that native iron-rich soils also have high P sorption capacity further supports giving dissolved P removal credit (e.g. Lucas and Greenway, 2011). Stenlund (2013 personal communication) has observed that adding iron to soil causes the soil to harden to a rock like medium, and recommends augering holes for plant growth into soils that have been amended with iron.

Imbrium Sorptive®MEDIA

Imbrium Sorptive®MEDIA, a proprietary P sorbing amendment available from Contech, is an engineered granular media containing aluminum oxide and iron oxide that demonstrates substantial capacity for adsorption of dissolved phosphorus from stormwater runoff. A recent study reported results from monitoring P reduction of 5 bioretention mesocosms with varying concentrations of Imbrium Sorptive®MEDIA (Balch et al 2013). The study is summarized below.

Five individual bioretention cells were monitored, each with 50 cm (20 inches) depth of soil that consisted of sand and 15 percent peat moss. The authors state “Four of [the cells] had different concentrations of Sorbtive® Media (3, 5, 10 and 17 percent by volume). The fifth cell contained only the sand/peat soil mix and no amendment, and therefore represented a control that provided the ability to determine how much phosphorus was retained by the sand/peat mix alone. The total volume of spiked artificial stormwater applied to each cell approximated the volume of cumulative runoff generated in this region [Canada] over a two-year period by a drainage area five times the size of a bioretention cell. At every phosphorus concentration, all the cells amended with Sorbtive® Media demonstrated much higher percent removal of phosphorus compared to the control cell with no Sorbtive® Media. The performance gap between the amended cells and the control cell widened as the phosphorus concentration increased. At the 0.2 percent target phosphorus concentration, mean dissolved phosphorus removal ranged 79 to 92 percent for the amended cells compared to 54 percent for the control cell. At the 0.8 percent target phosphorus concentration, mean dissolved phosphorus removal ranged 86 to 98 percent for the amended cells compared to 20 percent for the control cell. In the final week of the study, with 0.8 percent target phosphorus concentration in the artificial stormwater, percent removal of dissolved phosphorus was 82 percent for the 3 percent amendment, 97 to 98 percent for the 5, 10, and 17 percent amendments, and 11 percent for the control. These results demonstrate that the Sorbtive® Media maintained high phosphorus adsorptive capacity throughout the study, especially at the 5 percent and greater amendment levels.”

Researchers estimate that the lifespan for Imbrium should be at least 10 to 30 years, depending on P loading and performance goals (Garbon, 2013 personal communication; Contech Engineering, 2013). Contech Engineering (2013) estimated 45 percent dissolved P removal at 20 years after initial installation of 5 percent Sorptive media by volume.

Field studies with Imbrium are also underway in Wisconsin (Bannerman, 2013 personal communication). Additionally, Imbrium media has been used in an upflow filter on a North Carolina wet pond, resulting in greater than 80 percent removal of dissolved P during ten monitored storm events (Winston, 2013 personal communication).

To our knowledge, no field installations with Imbrium Sorptive®MEDIA have been monitored long term. Field studies to monitor long term performance of bioretention with P sorbing amendments are recommended to monitor clogging potential and P reduction performance over the bioretention lifespan.

Examples of other innovative applications

Using P-sorptive amendments to reduce effluent P content from BMP’s is a newly emerging field. Some applications of P-sorptive amendments that are promising but for which there is not sufficient research to recommend them as standard practices are discussed below.

Using by-products like gypsum, mining residuals, or drinking water treatment residuals in filters

Several researchers have developed ditch filters with P-sorbing materials to intercept surface and subsurface flow ditch water to trap dissolved P. The filters can be replaced as needed when the P-sorption sites are full (Schneider, 2013; Stoner et al., 2012). They report that “Overall, by-products that are elevated in oxalate Al or Fe, WS Ca [water soluble calcium], and BI [buffer index] serve as the best P sorbents in P removal structures, and screening for these properties allows comparison between materials for this potential use. The flow-through approach described in this paper for predicting design curves at specific [retention time] and inflow P combinations aids in predicting how much P can be removed and how long a specific material will last until P saturation if the P loading rate for a specific site is known.” (Stoner et al., 2012)

Researching the use of such filters on effluent from bioretention systems is recommended, as this would likely be an effective technique for P reduction in bioretention systems on projects where use of filters and ability to replace them as needed is realistic and desirable. For research on by-products, testing of composition and leaching of potentially harmful chemicals (e.g. dissolved metals) should be undertaken to ensure public health.

Using drain pipes enveloped in Fe-coated sand

Groenenberg et al. (2013) tested the performance of a pipe drain enveloped with Fe-coated sand, a side product of the drinking water industry with a high ability to bind P from the (agricultural) drainage water. They report that “The results of this trial, encompassing more than one hydrological season, are very encouraging because the efficiency of this mitigation measure to remove P amounted to 94 percent. During the trial, the pipe drains were below the groundwater level for a prolonged time. Nevertheless, no reduction of Fe(III) in the Fe-coated sand occurred, which was most likely prevented by reduction of Mn oxides present in this material. The enveloped pipe drain was estimated to be able to lower the P concentration in the effluent to the desired water quality criterion for about 14 years. Manganese oxides are expected to be depleted after 5 to 10 years. The performance of the enveloped pipe drain, both in terms of its ability to remove P to a sufficiently low level and the stability of the Fe-coated sand under submerged conditions in the long term, needs prolonged experimental research.” Application of this technique could also potentially be effective for reducing P in effluent from bioretention systems with underdrains. Unlike the filter application described in Schneider (2013), though, the iron around the pipe cannot easily be removed and replaced when the P binding sites are full. However, depending on P, Ca, and iron concentrations, there may be enough P sorption sites to last the lifespan of the bioretention system. This application is similar to bioretention systems currently being tested by Bannerman in Wisconsin (Bannerman, 2013 personal communication)

Rototilling Water Treatment Residuals into existing bioretention facilities

O’Neill and Davis (2012b) also suggest that established bioretention facilities could be retrofitted for increased P reduction by rototilling WTRs into the media, as agricultural surface application has been shown to be effective. Bioretention facilities may need to be re-planted after roto-tilling WTRs into the media, however, as rototilling would likely damage roots of existing vegetation. Alternatively perhaps a different way could be found to incorporate WTRs into existing bioretention facilities, such as, perhaps by air spading out some of the existing soil around existing vegetation, and replacing the soil that was removed with bioretention soil media amended with WTR’s. This technique could perhaps be used to renew P sorption capacity of bioretention facilities when P sorption sites are filled.

Applicability

  • Removal of dissolved phosphorus requires a comparatively high hydraulic retention time, and therefore a deeper media (Hsieh et al., 2007 in Hunt et al 2012). Media depth should therefore be at least 0.6 meters, with 0.9 meters recommended (Hunt et al., 2012).
  • Infiltration rates between 0.007 and 0.028 millimeters per second (1 to 4 inches per hour) work best, as this increases the hydraulic retention time, allowing for more sorption to occur (Hunt et al 2012).
  • If the media is saturated where phosphorus is stored, P is likely to leach out. So if an internal water storage (IWS) layer is used, it should be located below the P-sequestering portion of the media. Therefore, a 0.45 to 0.6 meter (1.5 to 2 foot) separation is recommended between the top of the IWS layer and the media surface (Hunt et al 2012). The P-sorptive amendment should be located at least 0.5 feet above the top of the IWS zone (Winston, 2013).

Life cycle properties

P sorptive amendments have been shown to provide effective P retention for the expected lifetime of bioretention facilities (e.g. Lucas and Greenway, 2011; O’Neill and Davis, 2012a and 2012b).

Maintenance needs

Soil amendments to enhance P sorption typically do not increase bioretention maintenance needs. Water treatment residuals (WTR’s) are fine textured, so systems with WTR’s should be designed to minimize clogging. Hinman and Wulkan (2012) recommend adding shredded bark at 15 percent by volume for each 10 percent WTRs added by volume to compensate for the fine texture of WTRs.

Iron filings can be obtained with a size distribution similar to sand. Erickson et al (2012) found that hydraulic conductivity of a sand filter was not negatively affected when operated for a year with up to 10.7 percent iron filings, which is enough iron to capture a significant percent of dissolved P.

Cost information

Soil amendments to enhance P sorption are a relatively low cost technique to improve long term dissolved P removal. Steel wool, for example, has been found to increase the material cost by 3 to 5 percent (Erickson et al., 2007). Iron filings cost less than steel wool per unit weight because they require less manufacturing to produce (Erickson et al., 2012). Since WTRs are byproducts of the water treatment process, they can often be procured for little or no cost.

References

  • Agyin-Birikorang, Sampson, George A. O'Connor,Lee W. Jacobs, Konstantinos C. Makris, and Scott R. Brinto. 2007. Long-Term Phosphorus Immobilization by a Drinking Water Treatment Residual. J. ENVIRON. QUAL. 36:1:316-323.
  • Beck, D.A., G.R. Johnson, and G.A. Spolek. 2011. Amending green roof soil with biochar to affect runoff water quantity and quality. Environmental Pollution 159(2011):2111-8.
  • Buda, A.R., G. F. Koopmans, R. B. Bryant, and W. J. Chardon. 2012. Emerging Technologies for Removing Nonpoint Phosphorus from Surface Water and Groundwater: Introduction. J. Environ. Qual. 41:621–627.
  • Chardon, W.J., J. E. Groenenberg, E. J. M. Temminghoff, and G. F. Koopmans. 2012. Use of Reactive Materials to Bind Phosphorus. J. Environ. Qual. 41:636–646.
  • Contech Engineering. 2013. Sorbtive® Media AI 28x48 for Phosphorus Treatment. Application: Bioretention Soil Amendment 20-Year Service Life Performance Estimates.
  • Erickson, A., J. Gulliver, and P. Weiss. 2007. Enhanced Sand Filtration for Storm Water Phosphorus Removal. J. Environ. Eng. 133(5), 485–497.
  • Erickson, A. 2010. Iron Enhanced Sand Filtration For Stormwater Phosphorus Removal. Presentation given February 23rd, 2010.
  • Erickson, A.J., J.S. Gulliver, and P.T. Weiss. 2012. Capturing phosphates with iron enhanced sand filtration. Water Research. 46(9): 3032–3042.
  • Groenenberg JE, W.J. Chardon, G.F. Koopmans. 2013. Reducing phosphorus loading of surface water using iron-coated sand. Journal of Environmental Quality. 42(1):250-9.
  • Hinman, C., and B. Wulkan. 2012. Low Impact Development. Technical Guidance Manual for Puget Sound. Publication No. PSP 2012-3.
  • Hunt, W., Davis, A., and R. Traver. 2012. Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design. J. Environ. Eng. 138(6): 698–707.
  • Kawczyinski, E., Achtermann, V. 1991. A water industry database report on residuals handling. In Proc. of the AWWA/WEF Joint Residuals Conf. Durham, NC. 11-14 Aug. American Water Works Association. Denver, Colorado. p. 6b-1 to 6b-5.
  • Lucas, W. C. and M. Greenway. 2011. Phosphorus Retention by Bioretention Mecocosms Using Media Formulated for Phosphorus Sorption: Response to Accelerated Loads. Journal of Irrigation and Drainage Engineering. 137(3): 144-152.
  • O’Neill, S. W., and A. P. Davis, A. P. 2012a. Water treatment residual as a bioretention amendment for phosphorus. I. Evaluation studies. J. Environ. Eng. 138(3): 318–327.
  • O’Neill, S. W., and A. P. Davis. 2012b. Water treatment residual as a bioretention amendment for phosphorus. II. long-term column studies. J. Environ. Eng., 138(3), 328–336.
  • Schneider, C. Re-using byproducts in agricultural fields. 2013.CSA News. Crop Science Society of America, Soil Society of America, American Society of Agronomy. April 2013 issue.
  • Stoner, D., C. Penn, J. McGrath, and J. Warren. 2012. Phosphorus Removal with By-Products in a Flow-Through Setting. J. Environ. Qual. 41:654–663.


Requirements, recommendations and information for using tree trench/box without underdrain in the MIDS calculator

Symbol for tree trench system-treebox
Symbol used for the tree trench system/box (w/o underdrain) BMP in the MIDS calculator. Note the symbol shows multiple trees since tree trench systems contain multiple trees.

For a tree trench system/box without an underdrain, stormwater runoff captured by the BMP is infiltrated into the underlying soil between rain events or lost through evapotranspiration. A small portion of precipitation is also intercepted by trees in the BMP. All pollutants in the captured and intercepted water are credited as being reduced. Pollutants in the stormwater that bypasses the BMP are not reduced. The user should be aware of the difference between a tree trench system and a tree box.

  • Tree trench system is a BMP that includes multiple trees. This BMP is commonly used in areas where pavement overlies the trench system. Runoff from the impermeable surface or through a permeable pavement surface is delivered underground to the underlying media in which the trees are planted. This Manual includes case studies and a discussion of types of tree BMPs.
  • Tree box (also called soil box) typically includes a single tree. They are typically proprietary products or are included in bioretention BMPs. If a tree is included in a bioretention BMP, we recommend using the bioretention BMP in the MIDS calculator instead of this BMP.

MIDS calculator user inputs for tree trench system/box

schematic of watershed tab for tree trench in MIDS calculator
Screen shot of the Watershed tab for tree trench system/box BMP in the MIDS calculator. In this example, 1 acre of Forest/open space, 1 acre of managed Turf, and 1 acre of Impervious Cover drain to the BMP. The BMP is being routed to a constructed stormwater pond.
Screen shot of the BMP Parameters tab for tree trench in MIDS calculator
Screen shot of the BMP Parameters tab for tree trench system/box BMP in the MIDS calculator.
Screen shot of the BMP Summary tab for tree trench in MIDS calculator
Screen shot of the BMP Summary tab for tree trench system/box BMP in the MIDS calculator.

For Tree trench system/box (w/o underdrain) BMPs, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.

  • Watershed tab:
    • BMP Name: this cell is auto-filled but can be changed by the user.
    • Routing/downstream BMP: if this BMP is part of a treatment train and water is being routed from this BMP to another BMP, the user selects the name of the BMP to which water is being routed from the dropdown box. All water must be routed to a single downstream BMP. Note that the user must include the BMP receiving the routed water in the Schematic or the BMP will not appear in the dropdown box.
    • BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP. Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for three Land Cover types (Forest/Open Space, Managed Turf, and Impervious Cover). The surface area of the BMP should be included as a managed turf land cover under the hydrologic soils group of the native soils located under the BMP. Units are in acres.
  • BMP Parameters tab:
    • Media surface area (AM): This is the surface area at the surface of the media. For a tree trench system this is the cumulative area for all similar trees in the system. Similar trees are trees of the same type (deciduous or coniferous) and size (large, medium, or small tree). See Plant lists for trees for more information. Units are in square feet.
    • Bottom surface area (AB): This is the surface area at the bottom of the media within the BMP. It represents the area where the engineered media changes to native soils. For a tree trench system this is the cumulative area for all similar trees in the system. Similar trees are trees of the same type (deciduous or coniferous) and size (large, medium, or small tree). See Plant lists for trees for more information. Units are in square feet.
    • Media depth (DM): This is the depth of the engineered media between the media surface and the native soils. Units are in feet.
    • Media field capacity minus wilting point (FC - WP): This is the amount of water between field capacity and the permanent wilting point stored in the media. This is water often considered to be available for uptake by plants. If multiple types of media are used in the BMP, this value should be a weighted average of the soil water storage values of the media. Values for field capacity and wilting point based on soil type can be found here. Units are in cubic feet of water per cubic feet of media. The recommended range for this value is 0.05 to 0.17.
    • Media porosity minus field capacity (n - FC): This is the amount of water stored in the media between media porosity (soil saturation) and field capacity that is thus available for infiltration into the underlying soils. If multiple types of media are used in the BMP, this value should be a weighted average of the soil water storage values of the media. Values for porosity and field capacity based on soil type can be found here. Units are in cubic feet of pore space per cubic feet of media. The recommended range for this value is 0.15 to 0.35.
    • Tree type (most common): The user selects the type of tree planted in the tree trench/box from a drop down menu. The user can select a tree type of deciduous or coniferous. If both deciduous and coniferous trees are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
    • Tree size (average for all trees): The user selects the size of tree planted in the tree trench/box system. The user can select small, medium or large. Tree size for different tree types are listed in the tree species list. If multiple tree sizes are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
    • Number of trees: The user enters the total number of trees planted in the tree trench/tree box system.
    • Underlying soil - Hydrologic Soil Group: The user selects the most restrictive soil (lowest hydraulic conductivity) within 5 feet below the soil/media interface in the tree trench/box. There are 14 soil options that fall into 4 different Hydrologic Soil Groups (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. These correspond with soils and infiltration rates contained in this Manual. Once a soil type is selected, the corresponding infiltration rate will populate in the Infiltration rate of underlying soils field. The user may also select User Defined. This selection will activate the User Defined Infiltration Rate cell allowing the user to enter a different value from the values in the predefined selection list. The maximum allowable infiltration rate is 1.63 inches per hour.
    • Required drawdown time: This is the time in which the stormwater captured by the BMP must drain into the underlying soil/media. The user must select from predefined values of 48 or 24 hours. The MPCA Construction Stormwater General Permit requires drawdown within 48 hours, but 24 hours is Highly Recommended when discharges are to a trout stream. The calculator uses the Infiltration rate of underlying soils and the Media depth (DM) times porosity to check if the BMP is meeting the drawdown time requirement. The user will encounter an error and be required to enter a new Media depth (DM) if the stormwater stored in the BMP cannot drawdown in the required time.
  • BMP Summary tab: The BMP Summary tab summarizes the volume and pollutant reductions provided by the specific BMP. It details the performance goal volume reductions and annual average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary are the total volume and pollutant loads received by the BMP from its direct watershed, from upstream BMPs, and a combined value of the two. Also included in the summary are the volume and pollutant load reductions provided by the BMP, along with the volume and pollutant loads that exit the BMP through the outflow. This outflow load and volume is what is routed to the downstream BMP, if one is defined in the Watershed tab. Finally, percent reductions are provided for the percent of the performance goal achieved, percent annual runoff volume retained, total percent annual particulate phosphorus reduction, total percent annual dissolved phosphorus reduction, total percent annual TP reduction, and total percent annual TSS reduction.

Model input requirements and recommendations

If the following requirements for inputs into the MIDS calculator are not met, then an error message will inform the user to change the input to meet the requirement.

  • The Bottom surface area must be equal to or smaller than the Media surface area.
  • The Number of trees must be 1 or more.
  • The infiltration rates of the underlying soils cannot exceed 1.63 inches per hour.
  • The basin must meet the user-specifed drawdown time requirement. The drawdown time requirement is checked by comparing the user defined drawdown time with the calculated drawdown time (DDTcalc), given by

<math>DDT_{calc} = D_M/(n - FC) / (I_R/12)</math>

where
DM is the media depth (ft);
n - FC is media porosity minus field capacity (ft3/ft3); and
IR is the infiltration rate of the native soils (in/hr).
If DDTcalc is greater than the user-specified required drawdown time then the user will be prompted to enter a new media depth or infiltration rate of the underlying soils.

Methodology

Required treatment volume

Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP plus any water routed to the BMP from an upstream BMP. This stormwater is delivered to the BMP instantaneously.

Volume reduction

The Volume reduction capacity of BMP [V] is calculated using BMP inputs provided by the user. For this BMP, the Volume reduction capacity [V] is equal to the sum of the Volume reduction stored in soil media (infiltration), the Volume reduction of BMP from ET (VET), and the Volume reduction of BMP from interception (VI).

The total instantaneous Volume reduction capacity of BMP [V] due to these three mechanisms is calculated using BMP design inputs provided by the user. This Volume reduction capacity of BMP [V] is then compared to the Required treatment volume in order to determine the Volume of retention provided by BMP, which is the instantaneous volume credit that can be claimed for that BMP.

Volume reduction stored in soil media (infiltration)

Stormwater runoff will flow into the media of the tree trench and fill the pores of the soil, eventually reaching water saturation. Water will then drain from the soils through infiltration into the underlying soils until the water content in the media reaches field capacity. The volume of water stored in the media between saturation and field capacity is the capture volume of the BMP. The capture volume (V) is given by

<math>V = (A_M + A_B)/2) * (n - FC) * D_M</math>

where

AM is the media surface area (ft2);
AB is the surface area at the bottom of the basin (ft2);
n - FC is the media porosity minus field capacity (ft3/ft3); and
DM is the media depth (ft).

The stored water must drain within the specified drawdown time. The underlying soil controls the infiltration rate. The user must input the soil with the most restrictive hydraulic conductivity in the 5 feet directly below the BMP media.

Volume reduction of BMP from interception (VI)

The second mechanism contributing to the Volume reduction capacity of BMP is interception. Water intercepted by a tree canopy may evaporate or be slowly released such that it does not contribute to stormwater runoff. An interception credit is estimated through a simplification of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous (Ic = 0.043 in) and coniferous (Ic = 0.086 in) tree species. To determine the interception volume of an individual tree, the Ic is multiplied by the canopy projection area (CP), which is the perceived tree canopy diameter at maturity. Although CP varies by tree species and size, for the MIDS calculator CP is determined solely based on the size of the tree (CP = 315 square feet for a small tree, 490 square feet for a medium sized tree, and 707 square feet for a large tree). The volume of water lost through interception (VI) in cubic feet is given by

<math>V_I = I_C/12 * CP * N</math>

where

IC = the interception capacity for an individual tree (in);
CP is the canopy projection area for an individual tree (ft2); and
N is the total number of trees planted in the tree trench.

Volume reduction of BMP from ET (VET)

The final mechanism contributing to the Volume reduction capacity of BMP is evapotranspiration (ET) of a portion of the water stored in the media between field capacity and wilting point. The volume of water lost through evapotranspiration (VET) is assumed to be the smaller of two calculated values, potential ET and measured ET.

Potential ET (ETpot)

Potential ET (ETpot) is equal to the amount of water stored in the media between field capacity and the wilting point, and is given by

<math>ET_{pot} = D_M * (A_M + A_B)/2 * (FC - WP)</math>

where
DM is the total media depth (ft);
AM is the surface area of the media (ft2);
AB is the surface area at the bottom of the basin (ft2); and
(FC – WP) is the difference between field capacity and wilting point (ft3/ft3).
Measured ET (ETmea)

Measured ET (ETmea) is the amount of water lost to ET as measured using available data. For this BMP in the MIDS Calculator, ETmea is calculated based on the leaf area index (LAI); canopy projection (CP); an average pan evaporation rate; an adjustment to the pan evaporation rate that accounts for leaf and stomata effects; the average time between rain events in MN; the soil volume available for tree rooting (Sv); and the number of trees in the BMP. Measured ET is given by

<math>ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)</math>

where
N is the number of trees in the BMP;
CP is the canopy projection area (ft2);
LAI is the leaf area index;
Erate is the pan evaporation rate for a given area (set at 0.02 ft/day);
Eratio is an adjustment that accounts for leaf and stomata effects relative to evaporation from a pan surface (set at 0.20, or 20 percent);
Three days is the typical time period between precipitation events in Minnesota based on analysis of rainfall data.

The "adjustment" factor is a multiplier that accounts for BMP conditions where the soil volume that is less than the recommended volume. Since the recommended soil volume equals 2 times the canopy projection area (CP), the adjustment term is the lesser between 1 and the following term

<math>(adjustment) = S_v / (2*CP)</math>

where
Sv is the actual soil volume available for each individual tree (ft3), and is given by

<math>S_v = (0.5*(A_M + A_B) * D_M) / N</math>

where
DM is the total media depth (ft);
AM is the surface area of the media (ft2);
AB is the surface area at the bottom of the basin (ft2); and
N is the number of trees planted in the tree trench/box.

The LAI is stratified by tree type and tree size. For coniferous trees, the LAI = 5.47. For deciduous trees, the LAI = 3.5 for small trees; 4.1 for medium sized trees; and 4.7 for large trees. These values are based on collected research for global leaf area from 1932-2000 (Scurlock, Asner and Gower, 2002). The pan evaporation rate is set at 0.02 ft/day, which is based on evaporation data collected at the University of Minnesota Southwest Experiment Station at Lamberton, Minnesota. The Eratio term accounts for the reduced efficiency of the leaves to transpire the available soil water or, alternately, the stomatal resistance of the canopy to transpiration and water movement, relative to evaporation from a pan surface. This is set at 0.20, or 20 percent, based on research by Lindsey and Bassuk (1991). This means that a 1 square centimeter leaf transpires only about 1/5 as much as a 1 square centimeter pan surface.

Comparison of volume reduction capacity with volume performance goal

The MIDS calculator compares the Volume reduction capacity of BMP [V] with the Required treatment volume, and the lesser of the two values is used to populate the Volume of retention provided by BMP. This comparison between potential and actual treatment volumes ensures that the BMP does not claim more credit than is due based on the actual amount of water routed to it. The Volume of retention provided by BMP is the actual volume credit the BMP receives toward the instantaneous performance goal. For example, if the BMP is oversized the user will only receive volume credit for the Required treatment volume routed to the BMP.

Annual volume retention is assessed by converting the instantaneous Volume reduction capacity of BMP [V] to an annual volume reduction percentage. This is accomplished through the use of performance curves developed from a range of modeling scenarios. These performance curves use the Volume reduction capacity of BMP [V], the infiltration rate of the underlying soils, the percent imperviousness of the contributing watershed area, and the size of the contributing watershed to calculate the Percent annual runoff volume retained and annual Retention volume provided by BMP.

Recommended values for porosity, field capacity and wilting point for different soils.1
Link to this table.

Soil Hydrologic soil group Porosity 2 (volume/volume) Field capacity (volume/volume) Wilting point (volume/volume) Porosity minus field capacity (volume/volume)3 Field capacity minus wilting point (volume/volume)4
Sand A (GM, SW, or SP) 0.43 0.17 0.025 to 0.09 0.26 0.11
Loamy sand A (GM, SW, or SP) 0.44 0.09 0.04 0.35 0.05
Sandy loam A (GM, SW, or SP) 0.45 0.14 0.05 0.31 0.09
Loam B (ML or OL) 0.47 0.25 to 0.32 0.09 to 0.15 0.19 0.16
Silt loam B (ML or OL) 0.50 0.28 0.11 0.22 0.17
Sandy clay loam C 0.4 0.07
Clay loam D 0.46 0.32 0.15 0.14 0.17
Silty clay loam D 0.47 to 0.51 0.30 to 0.37 0.17 to 0.22 0.16 0.14
Sandy clay D 0.43 0.11
Silty clay D 0.47 0.05
Clay D 0.47 0.32 0.20 0.15 0.12

1Sources of information include Saxton and Rawls (2006), Cornell University, USDA-NIFA, Minnesota Stormwater Manual
2Soil saturation is assumed to be equal to the porosity.
3This value may be used to represent the volume of water that will drain from a bioretention media.
4This value may be used to estimate the amount of water available for evapotranspiration


Pollutant Reduction

Schematic of pollutant reductions from tree trench BMP
Schematic illustrating how pollutant reductions (TSS, dissolved and particulate P) are calculated for the tree trench system-tree box BMP in the MIDS calculator.

Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting Volume reduction capacity of BMP, which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves (add link to addendum) developed from multiple modeling scenarios. The performance curves use Volume reduction capacity of BMP [V], the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above Required treatment volume will not provide additional credit towards the performance goal volume, it may provide additional pollutant reduction.

All pollutants associated with the reduced volume of water are captured for a 100 percent removal. Water that bypasses the BMP through the overflow is not treated for a 0 percent removal. A schematic of the removal rates can be seen to the right.

NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.

Routing

A tree trench/tree box BMP can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause water to be rerouted back to the tree trench/tree box BMP. All BMPs can be routed to a tree trench/tree box BMP except for a swale side slope BMP.

Assumptions for tree trench system/box

The following general assumptions apply in calculating the credit for a tree trench/box. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.

  • The tree trench system has been properly designed, constructed and will be properly maintained.
  • Stormwater runoff entering the tree trench/box has undergone pretreatment.
  • Stormwater captured by the BMP enters the BMP media instantaneously. This will slightly underestimate actual infiltration since some water will infiltrate through the basin bottom and sidewalls during a rain event, thus creating more volume for storage in the BMP.
  • Evapotranspiration is independent of plant type, plant density and weather conditions.

Example application in the MIDS calculator (Version 2)

schematic of site for calculator example
Schematic showing the site layout for a tree trench system with no underdrain. The site consists of 2.2 acres, of which 1.4 acre is an impervious parking lot and 0.8 acre is turf. Note the turf area includes the area of the BMP. The entire site is on HSG B soils.
schematic showing dimensions for calculator example
Schematic showing the dimensions of the tree trench system. The surface area of the system is 5600 square feet and the bottom surface area is 3230 square feet. The media depth is 4 feet.

A tree trench system is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious area (turf area and the tree trench BMP area). All of the runoff from the watershed will be treated by the tree trench system. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface area of the tree trench basin is 5600 square feet at the media surface. The area at the media-soil interface is 3320 square feet. The total media depth will be 4 feet. Following the MPCA Construction Stormwater General Permit requirement, the water in the media of the tree trench needs to drawdown in a 48 hour time period. The media will be Media Mix D, which is a loamy sand composition resulting in a difference between the media wilting point and field capacity of 0.05 cubic feet per cubic foot and a difference between the media porosity and field capacity of 0.35 cubic feet per cubic foot. The tree trench will be planted with 10 medium sized deciduous trees. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious area in type B soils. The pervious area includes the turf area and the area of the tree trench basin.

Step 2: Fill in the site specific information into the “Site Information” tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. The Managed turf area includes the turf area and the area of the tree trench basin. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the “Tree trench system/Box (w/o underdrain)” icon into the “Schematic Window”

screen shot of results tab
MIDS calculator screen shot for Results tab for tree trench with no underdrain.
screen shot of results tab
MIDS calculator screen shot for Results tab for tree trench with no underdrain.

Step 4: Open the BMP properties for the tree trench by right clicking on the “Tree trench system/Box (w/o underdrain)” icon and selecting “Edit BMP properties”, or by double clicking on the “Tree trench system/Box (w/o underdrain)” icon.

Step 5: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Tree trench system/Box (w/o underdrain)” BMP.

Step 6: Determine the watershed characteristics for the tree trench. For this example the entire site is draining to the tree trench. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed turf in B soils).

Step 7: Enter in the BMP design parameters into the “BMP parameters” tab. Tree trench systems requires the following entries.

  • Surface area at media surface which is 5600 square feet
  • Bottom surface area (area at media-soil interface) which is 3230 square feet
  • Media depth which is 4 feet
  • Media field capacity minus wilting point which is 0.05 cubic feet per cubic foot
  • Media porosity minus field capacity which is 0.35 cubic feet per cubic foot
  • Tree type is Deciduous
  • Tree Size is Medium
  • Number of Trees is 10
  • Underlying soil – Hydrologic Soil Group which is SM (HSG B; 0.45 in/hr) from the dropdown box
  • Required drawdown time (hrs) which is 48 from the dropdown box

Step 8: Click on “BMP Summary” tab to view results for this BMP.

Step 9: Click on the “OK” button to exit the BMP properties screen.

Step 10: Click on “Results” tab to see overall results for the site.

Requirements

image illustrating separation distance to bedrock or seasonal high water table
Measurement of depth from the bottom of the infiltration BMP to the seasonally high water table or bedrock. Note that there must be a minimum of 2 feet separation when soils beneath the BMP are ripped, with a total separation distance of 3 feet or more. Infiltration BMPs include any BMP that allows water to infiltrate into the underlying soil.
Warning: The following are requirements of the Minnesota Construction Stormwater General Permit
  • 3 foot separation from the bottom of the tree system to the seasonal high water table
  • Use the most restrictive infiltration rate within 5 feet of the bottom of the BMP
  • For measured infiltration rates, apply a safety factor of 2
  • Pretreatment

Recommendations

Caution: The following are recommendations for inputs into the MIDS calculator
  • Drawdown time of 24 hours when the discharge is to trout streams
  • Field tested infiltration rates rather than table values

Information

Information: The following information may be useful in determining inputs for the MIDS calculator

Links to MIDS pages


Requirements, recommendations and information for using tree trench/box with underdrain in the MIDS calculator

symbol for tree trench system-treebox with underdrain
Symbol used in the MIDS calculator to represent the tree trench system-tree box BMP with an underdrain. Note the symbol shows multiple trees since tree trench systems contain multiple trees.

For a tree trench system/box with an underdrain at the bottom, most of the stormwater captured by the BMP is lost to the underdrain. However, some stormwater infiltrates through the basin bottom and sidewalls if these do not have an impermeable liner. Evapotranspiration (ET) and interception also occur from the trees planted in the system. For a tree trench/box system with an elevated underdrain, in addition to volume losses through the sidewalls and through evapotranspiration and interception, a portion of the water stored in the media between the underdrain and the native soils is infiltrated. In a tree trench/box BMP with an underdrain, all pollutants in infiltrated water are removed, while pollutants are removed through filtration for the water that flows through an underdrain. All pollutants in water lost to ET and interception are removed.

The user should be aware of the difference between a tree trench system and a tree box.

  • Tree trench system is a BMP that includes multiple trees. This BMP is commonly used in areas where pavement overlies the trench system. Runoff from the impermeable surface or through a permeable pavement surface is delivered underground to the underlying media in which the trees are planted. This Manual includes case studies and a discussion of types of tree BMPs.
  • Tree box (also called soil box) typically includes a single tree. They are typically proprietary products or are included in bioretention BMPs. If a tree is included in a bioretention BMP, we recommend using the bioretention BMP in the MIDS calculator instead of this BMP.

MIDS calculator user inputs for Tree trench system/box

schematic of watershed tab for tree trench in MIDS calculator
Schematic of the Watershed tab for tree trench system-tree box BMP in the MIDS calculator. In this example, 1 acre of Forest/open space, 1 acre of managed Turf, and 1 acre of Impervious Cover drain to the BMP. The BMP is being routed to a constructed stormwater pond.
schematic of BMP Summary tab for tree trench with underdrain in MIDS calculator
Schematic of the BMP Summary tab for tree trench system-tree box with an underdrain BMP in the MIDS calculator.
screen shot of a BMP Summary tab
Screen shot of the BMP Summary tab for a BMP. Note this is a generic image and the BMP name and icon will appear on the screen in the MIDS calculator.

For Tree trench system/tree box with an underdrain BMPs, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.

  • Watershed tab
    • BMP Name: this cell is auto-filled but can be changed by the user.
    • Routing/downstream BMP: if this BMP is part of a treatment train and water is being routed from this BMP to another BMP, the user selects the name of the BMP from the dropdown box to which water is being routed. All water must be routed to a single downstream BMP. Note that the user must include the BMP receiving the routed water in the Schematic or the BMP will not appear in the dropdown box.
    • BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP. Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for three Land Cover types (Forest/Open Space, Managed Turf and impervious). The surface area of the BMP should be included as a managed turf land cover under the hydrologic soils group of the native soils located under the BMP. Units are in acres.
  • BMP Parameters tab
    • Is the underdrain elevated above native soils?: This is a YES/NO question. Answering YES means the underdrain is elevated within the media. This creates storage capacity between the underdrain and the native soils. Answering NO means that the underdrain is not elevated within the media and is directly above the native soils with no storage capacity below the underdrain.
    • Are the sides of the basin lined with an impermeable liner?: This is a YES/NO question. Answering YES means the sides of the basin are lined, preventing water from infiltrating into the native soils. Answering NO means the sides are not lined and infiltration is allowed through the side of the basin into the native soils.
    • Is the bottom of the basin lined with an impermeable liner?: This is a YES/NO question. Answering YES means the bottom of the basin is lined, preventing water from infiltrating into the native soils. Answering NO means the bottom is not lined and infiltration is allowed through the bottom of the basin into the native soils.
    • Media surface area (AM): This is the surface area at the surface of the engineered media. The user inputs this value in square feet. For a tree trench system this is the cumulative area for all similar trees in the system. Similar trees are trees of the same type (deciduous or coniferous) and size (large, medium, or small tree). See Plant lists for trees for more information.
    • Surface area at underdrain (AU): This is the surface area of the BMP at the invert elevation of the underdrain. If the response to Is the underdrain elevated above native soils? is set to “No” then this cell will become inactive and populated with the Bottom surface area value. The user inputs this value in square feet.
    • Bottom surface area (AB): This is the surface area at the bottom of the media within the BMP. Therefore, this is the area at the surface of the underlying soil. The user inputs this value in square feet. For a tree trench system this is the cumulative area for all similar trees in the system. Similar trees are trees of the same type (deciduous or coniferous) and size (large, medium, or small tree). See Plant lists for trees for more information.
    • Media depth (DM): This is the media depth between the media surface and the native soils (i.e. thickness of the engineered media). Units are in feet.
    • Depth below underdrain (DU): This is the depth of the media between the underdrain invert and the native soils. If the response to Is the underdrain elevated above native soils? is set to NO, then this cell will become inactive and populated with a 0. The user inputs this value in feet.
    • Media field capacity minus wilting point (FC - WP): This is the amount of water between field capacity and the permanent wilting point stored in the media. This is water often considered to be available for uptake by plants. If multiple types of media are used in the BMP, this value should be an average of the media installed above the underdrain. Values for field capacity and wilting point based on soil type can be found here. The user inputs this value in cubic feet of water per cubic feet of media.
    • Media porosity minus filed capacity (n - FC): This is the amount of water stored in the media between media porosity (soil saturation) and field capacity. This is the amount of water that is stored in the media and infiltrated into the underlying soils. If multiple types of media are used in the BMP, this value should be an average of the media installed. Values for porosity and field capacity based on soil type can be found here. The user inputs this value in cubic feet of pore space per cubic feet of media.
    • Tree Type: The user selects the type of tree planted in the tree trench/box from a drop down menu. The user can select a tree type of deciduous or coniferous. If both deciduous and coniferous trees are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
    • Tree Size: The user selects the size of tree planted in the tree trench/box system. The user can select small, medium or large. Tree size for different tree types are listed in the tree species list. If multiple tree sizes are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
    • Number of trees: The user enters the total number of trees planted in the tree trench/tree box system.
    • Planting media mix: The user selects the type of media mix installed for planting from a predefined list of Media mixes: Media mix A (water quality blend), Media mix B (enhanced filtration blend), Media mix C (North Carolina State University water quality blend), Media mix D (tree mix), or Other. This value is used to determine the annual phosphorus load reduction credit.
    • Is the P content of the media less than 30 mg/kg?: This is a YES/NO question. The P content of the planting media should be tested using the Mehlich 3 test or an acceptable alternative method. Select YES if the P content of the planting media is less than 30 milligrams per kilogram and NO if it is greater. P content testing is not needed for planting media C or D; therefore, this item will automatically populate to YES if one of those two media types are selected. This value is used to determine the annual phosphorus load reduction credit.
    • Is a soil amendment used to attenuate phosphorus?: This is a YES/NO question. Answer YES if the filter media contains soils amendments to enhance phosphorus sorption and NO if amendments are not used. This value is used to determine the annual phosphorus load reduction credit.
    • Underlying soil - Hydrologic Soil Group: The user selects the most restrictive soil (lowest hydraulic conductivity) within 5 feet of the soil/media interface in the tree trench/box. There are 14 soil options that fall into 4 different Hydrologic Soil Groups (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. These correspond with soils and infiltration rates contained in this Manual. Once a soil type is selected, the corresponding infiltration rate will populate in the Infiltration rate of underlying soils field. The user may also select User Defined. This selection will activate the User Defined Infiltration Rate cell allowing the user to enter a different value from the values in the predefined selection list. The maximum allowable infiltration rate is 1.63 inches per hour.
    • Required drawdown time (hrs): This is the time in which the stormwater captured by the BMP must drain into the underlying soil/media. The user may select from predefined values of 48 or 24 hours. The MPCA Construction Stormwater General Permit requires drawdown within 48 hours, but 24 hours is Highly Recommended when discharges are to a trout stream. The calculator uses the underlying soil infiltration rate and the Depth below underdrain to check if the BMP is meeting the drawdown time requirement. The user will encounter an error and be required to enter a new Depth below underdrain if the stormwater stored in the BMP cannot drawdown in the required time.
  • BMP Summary Tab: The BMP Summary tab summarizes the volume and pollutant reductions provided by the specific BMP. It details the performance goal volume reductions and annual average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary are the total volume and pollutant loads received by the BMP from its direct watershed, from upstream BMPs and a combined value of the two. Also included in the summary, are the volume and pollutant load reductions provided by the BMP, in addition to the volume and pollutant loads that exit the BMP through the outflow. This outflow load and volume is what is routed to the downstream BMP if one is defined in the Watershed tab. Finally, percent reductions are provided for the percent of the performance goal achieved, percent annual runoff volume retained, total percent annual particulate phosphorus reduction, total percent annual dissolved phosphorus reduction, total percent annual TP reduction, and total percent annual TSS reduction.

Model input requirements and recommendations

The following are requirements or recommendations for inputs into the MIDS calculator. If the following are not met an error message will inform the user to change the input to meet the requirement.

  • The water underneath the underdrain must meet the drawdown time requirement specified. The drawdown time requirement is checked by comparing the user defined drawdown time with the calculated drawdown time (DDTcalc). DDTcalc is given by

<math>DDT_{calc} = (D_U) / (I_R/12)</math>

where
DU = is the media depth, feet; and
IR = the infiltration rate of the native soils, inches/hour.
If DDTcalc is greater than the user defined required drawdown time then the user will be prompted to enter a new depth below the underdrain or infiltration rate.
  • Infiltration rates of the underlying soils are restricted to being 1.63 inches per hour or less.
  • Surface areas must be equal to or less than all surface areas at higher elevations.
  • The Depth below the underdrain cannot be greater than the Total media depth.
  • The number of trees must be 1 or more.

Methodology

Required Treatment Volume

Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP plus any water routed to the BMP from an upstream BMP. This stormwater is delivered to the BMP instantaneously.

Volume Reduction

The volume reduction achieved by a BMP compares the volume capacity of the BMP to the required treatment volume. The Volume reduction capacity of BMP [V] is calculated using BMP inputs provided by the user. For this BMP, the volume reduction credit methodology is determined by the location of the underdrain.

Underdrain located at BMP bottom: If the underdrain is located at the bottom of the BMP, then the Volume reduction capacity of BMP [V] is determined based on infiltration into the bottom of the BMP (Vinf_b), infiltration into the side slopes of the BMP (Vinf_s), evapotranspiration in the planting media above the underdrain (VET), and interception from the tree canopy (VI).

Even with an underdrain present, under saturated media conditions some water will infiltrate through the native soils as water in the basin draws down. The volume of water lost through the bottom (Vinf_b) of the BMP is given by

<math>V_{Inf_B} = I_R * (DDT) *A_B /(12in/ft)</math>

where

IR is the infiltration rate into the native soils of 0.06 inches per hour;
AB is the surface area at the bottom of the BMP in ft2; and
DDT is the drawdown time in hours.

The default infiltration rate is set at 0.06 inches per hour to represent a D soil. This rate was selected because it is assumed most of the stormwater will pass through the underdrain before it can infiltrate through the bottom of the BMP. This may be a conservative assumption if underdrains are small, spaced far apart, and the underlying soil has an infiltration rate greater than 0.06 inches per hour. Conversely, more closely spaced or larger underdrains may allow the basin to drain in less than the required drawdown time, resulting in a slight overestimation of infiltration loss through the basin bottom. If the user specifies that an impermeable liner is present at the bottom of the BMP, then no credit is given for infiltration into the bottom soils.

Under saturated conditions within the filter media, water will infiltrate through the sides of the basin as the stormwater draws down through the underdrain. Stormwater lost from a sloped sidewall (Vinf_s) is considered to infiltrate vertically into the surrounding soil. The volume of water infiltrated through the sidewalls is given by

<math>V_{Inf_S} = I_R * (DDT/2) * (A_M - A_U ) / (12in/ft) </math>

where

AM is the surface area at the media surface in ft2; and
AU is the surface area at the underdrain in ft2.

The drawdown time is reduced by a factor of 2 to account for the drop in water level within the BMP over the drawdown period. The drop in water level is therefore considered to be linear over the drawdown time. A conservative default infiltration rate of 0.06 inches per hour is used because it is assumed that most of the stormwater will pass through the underdrain before it can infiltrate through the side walls of the BMP. If the user specifies that an impermeable liner is present on the sides of the BMP, then no credit is given for infiltration into the side soils.

The third mechanism contributing to the Volume reduction capacity of BMP is interception. Water intercepted by a tree canopy may evaporate or be slowly released such that it does not contribute to stormwater runoff. An interception credit is given by a simplified value of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous and coniferous tree species. The volume of water lost through interception (VI) in cubic feet is given by

<math>V_I = I_C/12 * CP * N</math>

where

IC is the the interception capacity for an individual tree, inches;
CP is the canopy projection area for an individual tree, ft2; and
N is the total number of trees planted in the tree trench.

The interception capacity (IC) is determined based on data presented by Breuer et al. (2003) for deciduous and coniferous tree species (IC = 0.087 inched for coniferous trees and 0.043 inches for deciduous trees).

The canopy projection area (CP) is the perceived tree canopy diameter at maturity and varies by tree species. Canopy projection is determined based on the size of the tree (CP = 315 square feet for a small tree, 490 square feet for a medium sized tree, and 707 square feet for a large tree). See the morphology information for different tree species.

The final mechanism contributing to the Volume reduction capacity of BMP is evapotranspiration (ET). The water stored in the media between field capacity and wilting point is available for evapotranspiration. The volume of water lost through evapotranspiration (VET) is assumed to be the smaller of two calculated values of potential ET and measured ET.

  • Potential ET (ETpot) is equal to the amount of water stored in the media between field capacity and the wilting point. ETpot is given by

<math>ET_{pot} = [D_M * (A_M + A_B)/2 * (FC - WP)]</math>

where
DM is the total media depth;
AM is the surface area of the media;
AB is the surface area at the bottom of the BMP in ft2; and
(FC – WP) is the difference between field capacity and wilting point.
  • Measured ET (ETmea) is the amount of water lost to ET as measured using available data. Measured ET is given by

<math>ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)</math>

where
N is the number of trees in the BMP;
CP is the canopy projection area, in square feet;
LAI is the leaf area index;
Erate is the pan evaporation rate for a given area, in feet per day;
Eratio accounts for the efficiency of the leaves to transpire the available soil water;
Three days is a typical time period between precipitation events in Minnesota based on analysis of rainfall data; and
The adjustment accounts for soil volume that is less than the recommended volume.

The following assumptions apply to the above equation.

  • The LAI is stratified by tree type and tree size. For coniferous trees the LAI = 5.47. For deciduous trees LAI = 3.5 for small trees, 4.1 for medium sized trees, and 4.7 for large trees. These values are based on collected research for global leaf area from 1932-2000 (Scurlock, Asner and Gower, 2002).
  • Erate is set to 0.02 ft/day which is based on evaporation data collected at the Southwest Research and Outreach Center in Lamberton, Minnesota.
  • Eratio represents the stomatal resistance of the canopy to transpiration and water movement, relative to evaporation from a pan surface. This is set at 0.20, or 20 percent based on research by Lindsey and Bassuk (1991). This means that a 1 square centimeter leaf transpires only about 1/5 as much as 1 square centimeter of pan surface.
  • Since the recommended soil volume equals 2 times the canopy project area (CP), the adjustment term is given by Adjustment=(Sv)/(2*CP) where Sv is the actual soil volume available for each individual tree, in cubic feet. SV is given by (((Am+Ab)/2 * Dm)/N) where N is the number of trees planted in the tree trench/box.

Measured ET and potential ET are compared and the volume lost to ET is the smaller of the two values.

Elevated Underdrain: If the underdrain is elevated above the bottom of the BMP, then the volume reduction credit is determined based on the storage capacity in the media between the underdrain and the native soils, infiltration through the sides of the BMP above the underdrain (Vinf_s), evapotranspiration in the planting media (VET), and interception of rainfall from the tree canopy (VI).

When the underdrain is elevated, storage capacity becomes available in the media between the underdrain and the native soils. The storage capacity credit replaces the credit given for infiltration into the bottom of the BMP below the underdrain (VInf_B). The volume of water captured below the underdrain equals the following

<math>V = [(A_U + A_B)/2 * (n - FC) * D_U]</math>

where

AU is the surface area at the underdrain in ft2;
AB is the surface area at the bottom of the basin in ft2;
(n - FC) is the media porosity – field capacity of the soils; and
DU is the depth of the media below the underdrain in ft.

The stored water must drain within the specified drawdown time. The underlying soil controls the infiltration rate. The user must input the soil with the most restrictive hydraulic conductivity in the 5 feet directly below the basin.

In addition to the credit given for the storage capacity below the underdrain, a tree trench system with an elevated underdrain also receives volume reduction credit for infiltration into the sloped sidewall as well as evapotranspiration and interception. Credit is given following the same methods described when the underdrain is located at the bottom of the BMP (see discussion above).

The Volume of retention provided by BMP is the amount of volume credit the BMP provides toward the performance goal. This value is equal to the Volume reduction capacity of BMP [V], calculated using the above method, as long as the volume reduction capacity is less than or equal to the Required treatment volume. If Volume reduction capacity of BMP [V] is greater than Required treatment volume, then the BMP volume credit is equal to Required treatment volume. This check makes sure the BMP is not getting more credit than the amount of water it receives. For example, if the BMP is oversized the user will only receive credit for Required treatment volume routed to the BMP.

Recommended values for porosity, field capacity and wilting point for different soils.1
Link to this table.

Soil Hydrologic soil group Porosity 2 (volume/volume) Field capacity (volume/volume) Wilting point (volume/volume) Porosity minus field capacity (volume/volume)3 Field capacity minus wilting point (volume/volume)4
Sand A (GM, SW, or SP) 0.43 0.17 0.025 to 0.09 0.26 0.11
Loamy sand A (GM, SW, or SP) 0.44 0.09 0.04 0.35 0.05
Sandy loam A (GM, SW, or SP) 0.45 0.14 0.05 0.31 0.09
Loam B (ML or OL) 0.47 0.25 to 0.32 0.09 to 0.15 0.19 0.16
Silt loam B (ML or OL) 0.50 0.28 0.11 0.22 0.17
Sandy clay loam C 0.4 0.07
Clay loam D 0.46 0.32 0.15 0.14 0.17
Silty clay loam D 0.47 to 0.51 0.30 to 0.37 0.17 to 0.22 0.16 0.14
Sandy clay D 0.43 0.11
Silty clay D 0.47 0.05
Clay D 0.47 0.32 0.20 0.15 0.12

1Sources of information include Saxton and Rawls (2006), Cornell University, USDA-NIFA, Minnesota Stormwater Manual
2Soil saturation is assumed to be equal to the porosity.
3This value may be used to represent the volume of water that will drain from a bioretention media.
4This value may be used to estimate the amount of water available for evapotranspiration


Pollutant reduction

schematic of pollutant reductions from tree trench with an underdrain BMP
Schematic illustrating how pollutant reductions (TSS, dissolved and particulate P) are calculated for the tree trench system-tree box with an underdrain BMP in the MIDS calculator.

Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting Volume reduction capacity of BMP, which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves developed from multiple modeling scenarios. The performance curves use Volume reduction capacity of BMP [V], the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above Required treatment volume will not provide additional credit towards the performance goal volume, it may provide additional pollutant reduction.

A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater. Stormwater captured by the tree trench/box system but not infiltrated or consumed through ET/interception is assumed to flow through the filter media and out the underdrain. A constant 68 percent removal rate is applied to the filtered stormwater for TSS reduction.

Information: The Minnesota Stormwater Manual provides a TSS credit of 80 percent for biofiltration practices, which includes tree trench and tree box. A literature review suggests 80 percent may be high, but the Construction Stormwater (CSW) General Permit requires 80 percent reduction of TSS for filtration practices. Therefore, the CSW permit target cannot be achieved using the MIDS calculator. We created a MIDS calculator Excel file that utilizes 80 percent removal for biofiltration, tree trench, and tree box practices. Access the file at File:Corrected July 14.xls.

The removal rates of the filtered stormwater for annual particulate phosphorus and dissolved phosphorus depend on the answers given to the three user inputs: Planting media mix, Is the P content of the media less than 30 mg/kg? and Is a soil amendment used to attenuate phosphorus?

Particulate Phosphorus: The particulate phosphorus credit given is either 0 percent or 45 percent depending on the media mix used and the P content of the media.

  • If Media Mix C or D is used, the annual particulate phosphorus reduction credit is 45 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is 30 milligrams per kilogram or less, the annual particulate phosphorus reduction credit is 45 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is greater than 30 milligrams per kilogram, the annual pollutant phosphorus reduction credit is 0 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus has not been determined, the annual particulate phosphorus credit is 0 percent of the filtered water volume.

Dissolved Phosphorus: The dissolved phosphorus credit given is between 0 percent and 60 percent depending on the media mix, the media P content, and if the media was amended to attenuate phosphorus.

  • If Media Mix C or D is used, or if a media mix other than C or D is used and soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is 30 milligrams per kilogram or less, the annual dissolved phosphorus credit (%) applied to the filtered water volume is calculated by

<math>credit = 20 * (D_M - D_U) / (2 ft)</math>

where
(DM - DU) represents the media depth above the underdrain.

The credit is calculated as a percent reduction with a maximum value of 20 percent for media depths above the underdrain greater than 2 feet. If the media depth above the underdrain is less than 2 feet the credit is reduced equivalently.

  • If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is greater than 30 milligrams per kilogram, the annual dissolved phosphorus credit is 0 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus has not been determined, the annual dissolved phosphorus credit is 0 percent of the filtered water volume.

An additional annual dissolved phosphorus credit of 40 percent of the filtered water volume may be received if phosphorus-sorbing amendments are used. Acceptable amendments include the following:

  • 5 percent by volume elemental iron filings above the internal water storage (IWS) layer or elevated underdrain;
  • minimum 5 percent by volume sorptive media above IWS layer or elevated underdrain; and
  • minimum 5 percent by weight water treatment residuals (WTR) to a depth of at least 3.9 inches (10 centimeters).

An additional annual dissolved phosphorus credit commensurate with the research results can be applied if other phosphorus-sorptive amendments are proposed that have supporting third party research results showing dissolved phosphorus reduction for at least a 20-year lifespan.

The removal rates of the filtered stormwater for annual particulate phosphorus and dissolved phosphorus are summarized in the following table. NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.

Phosphorus credits for bioretention systems with an underdrain.
Link to this table

Particulate phosphorus Dissolved phosphorus
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, particulate credit = 80% of the particulate fraction (assumed to be 55% of total P)
  • If no or unknown, particulate credit = 0%


TP removal credit

  • Particulate fraction (55% of TP) * removal rate for that fraction (80%) = 0.55 * 0.80 = 0.44 or 44%
1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2)
  • If no or unknown, credit = 0%

2. Does the system include approved P-sorbing soil amendments2?

  • If yes, additional 40% credit


TP removal credit

  • TP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent
  • Adjust TP removal if depth is less than 2 feet
  • Adjust TP removal if dissolved credit is higher due to use of P-sorbing soil amendments

1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include

  • 5% by volume elemental iron filings above IWS or elevated underdrain
  • minimum 5% by volume sorptive media above IWS or elevated underdrain
  • minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
  • other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results

Routing

A tree trench/tree box BMP can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause water to be rerouted back to the tree trench/tree box BMP. All BMPs can be routed to a tree trench/tree box BMP except for a swale side slope BMP.

Assumptions for tree trench system/box (with underdrain)

The following general assumptions apply in calculating the credit for a tree trench/box. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.

  • The tree trench system has been properly designed, constructed and will be properly maintained.
  • Stormwater runoff entering the tree trench/box has undergone pretreatment.
  • Stormwater captured by the BMP enters the BMP media instantaneously. This will slightly underestimate actual infiltration since some water will infiltrate through the basin bottom and sidewalls during a rain event, thus creating more volume for storage in the BMP.
  • Evapotranspiration is independent of plant type, plant density and weather conditions.

Example application in the MIDS calculator (Version 2)

schematic of site for calculator example
Schematic showing the site layout for a tree trench system with an underdrain. The site consists of 2.2 acres, of which 1.4 acre is an impervious parking lot and 0.8 acre is turf. Note the turf area includes the area of the BMP. The entire site is on HSG B soils.
schematic showing dimensions for calculator example
Schematic showing the dimensions of the tree trench system. The surface area of the system is 5600 square feet, the surface area at the invert of the underdrain is 3948 square feet and the bottom surface area is 3230 square feet. The media depth is 4 feet, with 1 foot of media between the underdrain and native soils.

An unlined tree trench system with an elevated underdrain is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious area (turf area and the tree trench BMP area). All of the runoff from the watershed will be treated by the tree trench system. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface area of the tree trench basin is 5600 square feet at the media surface. The surface area at the invert of the underdrain will be 3948 square feet. The area at the media-soil interface is 3320 square feet. The total media depth will be 4 feet with 1 foot of media between the underdrain and native soils. Following the MPCA Construction Stormwater General Permit requirement, the water in the media of the tree trench needs to drawdown in a 48 hour time period. The media will be Media Mix D, which is a loamy sand composition resulting in a difference between the media wilting point and field capacity of 0.05 cubic feet per cubic foot and a difference between the media porosity and field capacity of 0.35 cubic feet per cubic foot. The tree trench will be planted with 10 medium sized deciduous trees. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious area in type B soils. The pervious area includes the turf area and the area of the tree trench basin.

Step 2: Fill in the site specific information into the “Site Information” tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. The Managed turf area includes the turf area and the area of the tree trench basin. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the “Tree trench system/Box (with underdrain)” icon into the “Schematic Window”

screen shot of results tab
MIDS calculator screen shot for Results tab for tree trench with an underdrain.
screen shot of results tab
MIDS calculator screen shot for Results tab for tree trench with an underdrain.

Step 4: Open the BMP properties for the tree trench by right clicking on the “Tree trench system/Box (with underdrain)” icon and selecting “Edit BMP properties”, or by double clicking on the “Tree trench system/Box (with underdrain)” icon.

Step 5: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Tree trench system/Box (with underdrain)” BMP.

Step 6: Determine the watershed characteristics for the tree trench. For this example the entire site is draining to the tree trench. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed turf in B soils).

Step 7: Enter in the BMP design parameters into the “BMP parameters” tab. Tree trench system with an underdrain requires the following entries.

  • Is the underdrain elevated above native soils – Yes;
  • Are the sides of the basin lined with an impermeable liner – No;
  • Is the bottom of the basin lined with an impermeable liner – No;
  • Surface area at media surface which is 5600 square feet;
  • Surface area at underdrain which is 3948 square feet;
  • Bottom surface area (area at media-soil interface) which is 3230 square feet;
  • Total media depth which is 4 feet;
  • Depth below underdrain which is 1 foot;
  • Media field capacity minus wilting point which is 0.05 cubic feet per cubic foot;
  • Media porosity minus field capacity which is 0.35 cubic feet per cubic foot;
  • Tree type is Deciduous;
  • Tree Size is Medium;
  • Number of Trees is 10;
  • Planting media mix which is Media Mix D;
  • Is the P content of the media less than 30 mg/kg which autofills to “Yes” for Media Mix D;
  • Is a soil amendment used – No;
  • Underlying soil – Hydrologic Soil Group which is SM (HSG B; 0.45 in/hr) from the dropdown box; and
  • Required drawdown time (hrs) which is 48 from the dropdown box.

Step 8:Click on “BMP Summary” tab to view results for this BMP.

Step 9: Click on the “OK” button to exit the BMP properties screen.

Step 10: Click on “Results” tab to see overall results for the site.

Requirements

image illustrating separation distance to bedrock or seasonal high water table
Measurement of depth from the bottom of the infiltration BMP to the seasonally high water table or bedrock. Note that there must be a minimum of 2 feet separation when soils beneath the BMP are ripped, with a total separation distance of 3 feet or more. Infiltration BMPs include any BMP that allows water to infiltrate into the underlying soil.
Warning: The following are requirements of the Minnesota Construction Stormwater General Permit
  • 3 foot separation from the bottom of the tree system to the seasonal high water table
  • Use the most restrictive infiltration rate within 5 feet of the bottom of the BMP
  • For measured infiltration rates, apply a safety factor of 2
  • Pretreatment

Recommendations

Caution: The following are recommendations for inputs into the MIDS calculator
  • Drawdown time of 24 hours when the discharge is to trout streams
  • Field tested infiltration rates rather than table values

Information

Information: The following information may be useful in determining inputs for the MIDS calculator

Links to MIDS pages


This page was last modified on 18 November 2014, at 09:29.

Minnesota Pollution Control Agency | 651-296-6300, 800-657-3864 | Assistance | Web site policy