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==References== | ==References== | ||
*Bakker. J.W. 1983. ''Groeiplaats en watervoorziening van straatbomen''. Groen, 39(6)205-207; OBIS, 1988. Bomen in straatprofielen – Voorbeelden – Groeiplaatsberekening. Uitgeverij van de Vereniging van de Nederlandse gemeenten, ‘s-Gravenhage 1988. 63 p. Cited in Kopinga 1991. | *Bakker. J.W. 1983. ''Groeiplaats en watervoorziening van straatbomen''. Groen, 39(6)205-207; OBIS, 1988. Bomen in straatprofielen – Voorbeelden – Groeiplaatsberekening. Uitgeverij van de Vereniging van de Nederlandse gemeenten, ‘s-Gravenhage 1988. 63 p. Cited in Kopinga 1991. | ||
− | *Barret, M., Limouzin, | + | *Barret, M., M. Limouzin, and D. Lawler. 2011. [http://www.crwr.utexas.edu/reports/pdf/2010/rpt10-05.pdf Performance Comparison of Biofiltration Designs]. World Environmental and Water Resources Congress 2011. pp. 395-404. |
− | *Bassuk, Nina. 2010. [http://www.asla.org/uploadedFiles/CMS/Meetings_and_Events/2010_Annual_Meeting_Handouts/Sat-B1The%20Great%20Soil%20Debate_Structural%20Soils%20Under%20Pavement.pdf | + | *Bassuk, Nina. 2010. [http://www.asla.org/uploadedFiles/CMS/Meetings_and_Events/2010_Annual_Meeting_Handouts/Sat-B1The%20Great%20Soil%20Debate_Structural%20Soils%20Under%20Pavement.pdf Using CU-Structural Soil to Grow Trees Surrounded by Pavement]. In The Great Soil Debate Part II: Structural soils under pavement. ASLA Annual Meeting Handout. |
− | *DTAH, Lead Consultant, ARUP, Engineering, and James Urban, Urban Trees + Soils Urban Forest Innovations, Arborist. 2013. Tree Planting Solutions in Hard Boulevard Surfaces Best Practices Manual. Project # A21065. Prepared for: City of Toronto. | + | *DTAH, Lead Consultant, ARUP, Engineering, and James Urban, Urban Trees + Soils Urban Forest Innovations, Arborist. 2013. [http://www1.toronto.ca/staticfiles/city_of_toronto/parks_forestry__recreation/urban_forestry/files/pdf/TreePlantingSolutions_BestPracticesManual.pdf Tree Planting Solutions in Hard Boulevard Surfaces Best Practices Manual]. Project # A21065. Prepared for: City of Toronto. |
*Facility for Advancing Water Biofiltration (FAWB). 2009. [http://graie.org/SOCOMA/IMG/pdf/FAWB_Filter_media_guidelines_v3_June_2009-2.pdf Biofiltration Filter Media Guidelines (Version 3.01)]. | *Facility for Advancing Water Biofiltration (FAWB). 2009. [http://graie.org/SOCOMA/IMG/pdf/FAWB_Filter_media_guidelines_v3_June_2009-2.pdf Biofiltration Filter Media Guidelines (Version 3.01)]. | ||
− | *Fassman, EA, Simcock, | + | *Fassman, EA, R. Simcock, and S. Wang. 2013. [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CCoQFjAA&url=http%3A%2F%2Fwww.aucklandcouncil.govt.nz%2FEN%2Fplanspoliciesprojects%2Freports%2Ftechnicalpublications%2FDocuments%2Ftr2013011mediaspecificationforstormwaterbioretentiondevices.pdf&ei=5F0fUuP_L-qK2wWqtYGwDg&usg=AFQjCNFp3kcfmdW_Go5LUhekP76T0n_2Yw&sig2=lDEFut2QUvSiA8P6nayUJg&bvm=bv.51495398,d.aWM Media specification for stormwater bioretention devices, Prepared by Auckland UniServices for Auckland Council. Auckland Council technical report, TR2013/011]. |
− | *Helliwell, D.R. 1986. The Extent of Tree Roots. Arboriculture Journal 10:341-347 | + | *Helliwell, D.R. 1986. ''The Extent of Tree Roots''. Arboriculture Journal 10:341-347. Updated in Letter to the Editor. Arboricultural Journal: The International Journal of Urban Forestry, Volume 16, Issue 2, 1992. |
− | *Hinman, C., and B. Wulkan. 2012. Low Impact Development | + | *Hinman, C., and B. Wulkan. 2012. [http://www.psp.wa.gov/downloads/LID/LID_manual2005.pdf Low Impact Development: Technical Guidance Manual for Puget Sound]. Publication No. PSP 2012-3. |
− | *Jenkins, J. K. G., B. M. | + | *Jenkins, J. K. G., Wadzuk, B. M., & Welker, A. L. 2010. ''Fines Accumulation and Distribution in a Storm-Water Rain Garden Nine Years Postconstruction''. Journal of Irrigation and Drainage Engineering, 136(12):862-869. |
*Kent, D., S. Shultz, T. Wyatt, and D. Halcrow. 2006. ''Soil Volume and Tree Condition in Walt Disney World Parking Lots''. Landscape Journal 25:1–06 | *Kent, D., S. Shultz, T. Wyatt, and D. Halcrow. 2006. ''Soil Volume and Tree Condition in Walt Disney World Parking Lots''. Landscape Journal 25:1–06 | ||
− | *Kopinga, J. 1991. [http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2412&Type=2 The Effect of Restricted Volumes of Soil on the Growth and development of Street Trees]. Journal of Arboriculture 17(3): 57-63 | + | *Kopinga, J. 1991. [http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2412&Type=2 The Effect of Restricted Volumes of Soil on the Growth and development of Street Trees]. Journal of Arboriculture 17(3):57-63 |
− | *Li, H., & Davis, A. P. (2008b). ''Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies''. Journal of Environmental Engineering | + | *Li, H., & Davis, A. P. (2008b). ''Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies''. Journal of Environmental Engineering. 134(6):409-418. |
*Lindsey, P. and N. Bassuk. 1991. [http://www.hort.cornell.edu/uhi/research/articles/JArb17(6).pdf Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers]. Journal of Arboriculture 17(6):141-149. | *Lindsey, P. and N. Bassuk. 1991. [http://www.hort.cornell.edu/uhi/research/articles/JArb17(6).pdf Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers]. Journal of Arboriculture 17(6):141-149. | ||
− | *Loh, F. C. W.; Grabosky, J. C.; Bassuk, N. L. | + | *Loh, F. C. W.; Grabosky, J. C.; Bassuk, N. L. 2003. [http://www.hort.cornell.edu/uhi/research/articles/UrbForUrbGr%282%292003.pdf Growth Response of Ficus Benjamina to Limited Soil Volume and Soil Dilution in a Skeletal Soil Container Study]. In Urban For. Urban Gree. 2(1):53-62. |
*Pitt, R., S. Clark, P. Johnson, and J. Voorhees. In Press. ''Evapotranspiration and Related Calculations for Bioretention Devices''. CHI Monograph 14. | *Pitt, R., S. Clark, P. Johnson, and J. Voorhees. In Press. ''Evapotranspiration and Related Calculations for Bioretention Devices''. CHI Monograph 14. | ||
*Schoenfeld, P.H. 1975. ''De groei van Hollandse iep in the kustprovincies van Nederalnd''. Nederlands Bosbouw Tijdschrift 47:87-95. Cited in Kopinga 1991. | *Schoenfeld, P.H. 1975. ''De groei van Hollandse iep in the kustprovincies van Nederalnd''. Nederlands Bosbouw Tijdschrift 47:87-95. Cited in Kopinga 1991. | ||
*Schoenfeld, P.H., and J. van den Burg. 1984. ''Voortijdige bladval en groeiafname bij ‘Heidemij’populier in beplantingen langs autowegen''. Nederlands Bosbouw Tijdschrift 56:12-21. Cited in Kopinga 1991. | *Schoenfeld, P.H., and J. van den Burg. 1984. ''Voortijdige bladval en groeiafname bij ‘Heidemij’populier in beplantingen langs autowegen''. Nederlands Bosbouw Tijdschrift 56:12-21. Cited in Kopinga 1991. | ||
*Smiley, E. T. 2013. Bartlett Tree Research Lab, Charlotte North Carolina, Adjunct Professor Clemson Univ., unpublished data. | *Smiley, E. T. 2013. Bartlett Tree Research Lab, Charlotte North Carolina, Adjunct Professor Clemson Univ., unpublished data. | ||
− | *Smiley, E. Thomas, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006. [http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2952&Type=2 Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement]. Arboriculture & Urban Forestry 32(4): 164-169. | + | *Smiley, E. Thomas, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006. [http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2952&Type=2 Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement]. Arboriculture & Urban Forestry 32(4):164-169. |
*Urban, J. 1992. [http://auf.isa-arbor.com/request.asp?JournalID=1&ArticleID=2490&Type=2 Bringing Order to the Technical Dysfunction Within the Urban Forest]. Journal of Arboriculture 18(2): 85-90. | *Urban, J. 1992. [http://auf.isa-arbor.com/request.asp?JournalID=1&ArticleID=2490&Type=2 Bringing Order to the Technical Dysfunction Within the Urban Forest]. Journal of Arboriculture 18(2): 85-90. | ||
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)
Based on research, the recommended minimum soil volume is 2 cubic feet of rootable soil volume per square foot of mature tree canopy size. This soil is considered essential to healthy growth of trees. (See canopy size for several common tree species).
Soil volume affects the stormwater volume credits for tree trenches and tree boxes. Stormwater volume credits for tree trenches and tree boxes include
The first of these credits, storage of water below an underdrain, is a direct function of soil volume. The evapotranspiration (ET) credit is indirectly related to soil volume. If only rock-based structural soil is used, the ET credit is limited to water storage in the media. The rock component of the structural soil should not be included in the volume calculation for ET. The decrease in stormwater volume available for ET is assumed to be linear with decreases in soil volume below the minimum recommended volume. For example, if a rock-based structural soil is used that contains 80 percent rock and 20 percent soil, only the 20 percent soil component counts toward the ET volume credit. This approach is utilized within the Minimal Impact Design Standards (MIDS) calculator.
For specific information on stormwater volume and pollutant credits for tree trenches and tree boxes, click here.
An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above. The results of this review are presented below.
Minimum soil volume needed to grow healthy trees has been studied several ways, including
Each of these techniques indicates similar ranges of minimum soil volume needed:
To put these numbers in perspective in relation to tree size and typical street tree spacing:
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 |
|
Emeryville, CA |
|
Toronto, Ontario, Canada |
|
Markham and Oakville, Ontario, Canada; Burnaby MetroTown Development Area, British Columbia, Canada |
|
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 |
|
Winnipeg, Manitoba, Canada, Tree Planting Details and Specifications, Downtown Area and Regional Streets |
|
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) |
|
Minnesota B3 Guidelines |
|
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 alone, it appears that approximately 1.5 times the amount of Cornell University (CU) structural soil is needed to grow the same size tree growing in sandy loam (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 vs. loam 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 in the following soil types and volumes:
There were no statistically significant differences in above ground growth between trees grown in small loam soil pots and trees grown in large pots with 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.
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).
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 (see Section 6.4, page 203) has a separate section on Urban trees (Hinman and Wulkan, 2012) 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.
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 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 |
|
|
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 |
|
|
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 |
|
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 |
|
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 |
|
High infiltration rates, relatively inexpensive | As compost breaks down, nutrients available for plants decreases |
F | Not in original manual |
|
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.
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.
Schedule all utility installations prior to beginning work in this section.
The following submittals are Highly Recommended.
Submit soil testing results from an approved soil-testing laboratory for each soil mix for approval. Soil suppliers that regularly prepare Bioretention Soil Mix D may submit past testing of current production runs to certify that the mix to be supplied meets the requirements, provided the testing results are less than 12 months prior to the submission date.
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 that the product meets the requirements of the specification.
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.
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.
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.
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.
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.
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 shall meet the requirements of the US Composting Council “Architecture/Design Specifications for Compost Use”, section “Compost as a Landscape Backfill Mix Component”, with the following additional requirements:
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.
Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:
Suggested mix ratio ranges are
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 Owner based on plant material specified and testing recommendations.
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:
Examine the surface grades and soil conditions for any circumstances that might be detrimental to soil drainage.
Install Soil Cells in accordance with the manufacturers requirements including all accessories, geotextile, geogrid, and aggregate layers.
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.
Install and compact Structural Soil in accordance with the manufacturer's requirements.
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.
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.
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.