image
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 completed project, Marquette Avenue, Minneapolis
Completed tree system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.
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.

This page provides guidance for operation and maintenance (O&M) of tree trenches and tree boxes. A tree trench is a bioretention practice that contains one or more trees. Tree trenches and boxes may be designed as infiltration or filtration (underdrained) systems.

Supplemental information can be found on the page called Operation and maintenance of tree trenches and tree boxes - supplemental information. Supplemental information includes the following.

  • Supplemental watering
  • Pruning
  • Staking and straightening
  • Protecting the trunk
  • Mulching
  • Fertilizers
  • Check tree safety
  • Tree health troubleshooting guidelines
  • References

Overview of typical O&M Issues

In addition to runoff reduction and stormwater treatment, street trees and tree boxes provide a range of community benefits. Trees contribute to air pollution reduction, carbon sequestration, reduction in heat island effect, habitat creation for wildlife, and can be used as tools for climate adaptation. Green stormwater infrastructure (GSI) trees are often planted in highly visible areas and are subject to public interaction, vandalism, and winter deicing, but trees typically remain resilient with proper maintenance.

O&M of tree trenches and boxes generally requires a low level of effort and expertise, and street trees can have life spans of up to 60 years (Cappiella et al., 2016). Green infrastructure managers do share common issues and concerns related to trees. The most common O&M concerns for tree trenches and tree boxes include

  • general poor health,
  • insufficient water for growth,
  • accumulation of litter and debris in the inlet, and
  • spent and lost mulch.

The sections below describe best practices to prevent or minimize these common problems.

Design phase O&M considerations

photo of tree protection with signage, Maplewood Mall
Photo of tree protection with signage, Maplewood Mall. Image Courtesy of Barr Engineering Company

Tree trenches and tree boxes have a small footprint and should be designed to treat a relatively small drainage area (0.25 acres or less per tree). Surface area of the tree trench or box and surface area to drainage area ratio are key design characteristics. Design specifications vary based on the selected filter media and filter box manufacturer (most tree trench and box systems are proprietary practices). Properly sizing and designing the tree box will help prevent O&M issues. Designers should consider

  • minimizing the surface area to drainage area ratio, based on the manufacturer guidelines;
  • properly sizing the tree box or tree trench to manage an appropriate surface area based on local precipitation patterns;
  • selecting regionally appropriately species that are tolerant to the soil and growth media (see Design Guidelines for Tree Quality and Planting); and
  • selecting species that will grow in the limited space provided by the filter box (see Design Guidelines for Tree Quality and Planting).

Designers should also consider the maintenance schedules and tasks when locating tree trenches or tree boxes on their site. The small surface area of tree boxes makes them versatile. However, they should be placed within areas that are easily accessible to ensure proper maintenance. Maintenance of tree trenches and tree boxes does not require large or heavy equipment, but routine maintenance should be expected once or twice a year. Designers can incorporate solutions to facilitate the following maintenance activities.

  • Incorporating multiple and easy access points
  • Placement near supportive companion plants to prevent diseases
  • Installing observation wells
  • Specifying the optimized soil media composition and depth to effectively trap or sequester nutrients (phosphorus in particular), and that can also support the desired species
  • Site-specific species selections that take into account sun exposure, shade, proximity to traffic corners (visibility issues), salt-tolerance, etc.
  • Providing educational signage to increase public awareness
  • Installing measures like low fencing to prevent damage from pedestrian foot traffic

Designers should consult and include any local requirements regarding green infrastructure. O&M considerations often depend on whether the practice is located on public land, private land, or in the public right of way. For example, plantings in the public right of way that conflict with any traffic safety considerations could require increased O&M, such as pruning or complete removal.

The designer should also provide a site-specific O&M plan that includes the following.

  • Construction inspection schedule and checklists
  • Post-construction routine maintenance schedule and checklists
  • Operating instructions for the practice (if applicable)

Additional information that should be included in O&M plans is described in the Post-Construction Phase O&M Considerations section. Example O&M plans are also provided below.

Construction phase O&M considerations

Proper construction methods and sequencing play a significant role in reducing O&M problems. Some key items during the construction phase include the following.

  1. Before construction begins
    1. Ensure that the contributing drainage area is fully stabilized with vegetation prior to the beginning of construction. Also make sure that impervious areas in the contributing drainage area are clean. If this is not possible, use barriers or diversions to direct stormwater flows from the contributing drainage area away from the practice.
    2. Install any needed erosion protection and sediment control in your construction site and prepare a storm water pollution prevention plan (SWPPP).
    3. Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control.
    4. Hold a pre-construction meeting to review the construction plans and the sequencing of construction.Other agenda items could include discussing the status of the pre-construction drainage, erosion, and sediment controls; and reviewing or assign points of contacts for the stormwater and/or green infrastructure key personnel.
  2. During construction
    1. Construct any pretreatment devices before installing any tree boxes or trenches. Depending on the site design and drainage pattern, this may not be necessary, but stormwater should be directed away from the boxes during the construction phase.
    2. Ensure heavy equipment does not enter the footprint of the practice to avoid compaction of the infiltration medium.
    3. Store any soil, mulch, or gravel media away from the practice footprint to avoid clogging the infiltration medium.
    4. Inspect the practice during construction to ensure that the tree trenches or tree boxes are built in accordance with the approved design and standards and specifications. Use detailed inspection checklists that include sign-offs by qualified individuals at critical stages of construction to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.
  3. After construction
    1. Verify that the tree trenches or tree boxes were built in accordance with the approved design and standards and specifications.
    2. Verify that tree is staked and supported with guy wires to promote stable growth, if necessary.
    3. Verify that the contributing drainage area is fully stabilized with vegetation prior to removing any barriers, diversions, or erosion and sediment control measures.
    4. Verify that the practice actually captures and infiltrates runoff. Conduct a full inundation test to inspect the underdrain and outflow function. For more information, see Assessing the performance of tree trenches and tree boxes.
    5. Use detailed inspection checklists that include sign-offs by qualified individuals at the completion of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.

Post-construction phase O&M

Post-construction maintenance is performed on tree trenches and tree boxes to maintain proper infiltration, filtration, and to promote healthy vegetation. Immediately after construction, short-term maintenance goals include efforts to establish and keep the tree healthy. Not all trees are guaranteed to survive the establishment phase, so care should be taken to inspect the health following construction. Important post-construction considerations are provided below.

  • A site-specific Operations and Maintenance Plan should be prepared by the designer prior to putting the stormwater practice into operation. This plan should provide any operating procedures related to the practices. The plan should also provide clear maintenance expectations, activities, and schedules. Include photos if possible. Be clear who is responsible for maintenance and the type of expertise needed for distinct O&M activities. The O&M plan should include an anticipated budget for O&M activities. The O&M plan should also include an example O&M inspection checklist and an example maintenance report. Example O&M plans are provided here and a schedule of general maintenance activities is provided in the adjacent table.
  • A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority. Example maintenance agreements are provided here.
  • Inspection and maintenance activities are distinct and can be done as separate activities or together. Inspection will typically assess the practice for any O&M issues, whereas maintenance will address the O&M issues identified by the inspection. A dedicated inspection effort on a large number of BMPs can help prioritize maintenance activities.
  • Maintenance activities should be careful not to cause compaction. No vehicles or stockpiling should be allowed within the footprint of the practice. Foot traffic should be kept to a minimum.
  • BMP areas generally should not be used as dedicated snow storage areas.

See the adjacent tables for a schedule of general maintenance activities and common problems and troubleshooting.

Overview and schedule of general maintenance activities for tree trenches and tree boxes
Link to this table

First Year of Operation
Activity Frequency Time period Level of effort O&M benefita
Check that there is no ponding within the box or trench. At least twice after storm events > 0.5 inches Within the first 6 months 1-2 hours 1,5
Check for evidence of clogging in the media. At least twice after storm events > 0.5 inches Within the first 6 months 1-2 hours 1
Supplemental watering, during drier periods, particularly if keeping stormwater offline trees are established. 1/week initially During first two months of the growing season 1-2 hours 2,3,4,5,6
As needed First growing season 1-2 hours 2,3,4,5,6
Remove any weeds present As needed First growing season 1-2 hours 2,3,4,5,6
Remove any trash or debris As needed First growing season 1-2 hours 1,2,3,4,5,6
Add mulch if layer is less than 3 inchesb As needed First growing season 1-2 hours 4,5
Clean root collar Annually During growing season 1-2 hours 1
Remove stakes As needed End of growing season 1-2 hours 5
Inspect tree health Biannually and after large storms First growing season 1-2 hours 1,2,5,6
After First Year of Operation
Activity Frequency Time period Level of effort O&M benefita
Check that there is no ponding within the box or trench. Biannually Any time when ground is not frozen 1-2 hours 1,5
Check for evidence of clogging Biannually In spring and fall 1-2 1
Supplemental watering during drier periods As needed or when soil is dry 3 inches below the surface Dry periods until roots are established 1-2 hours 2,3,4,5,6
Remove any weeds present As needed During growing season 1-2 hours 2,3,4,5,6
Remove any trash or debris As needed Any time 1-2 hours 1,2,3,4,5,6
Add mulch if layer is less than 3 inches Annually During growing season 1-2 hours 4,5
Remove stakes (if still present) Once End of second growing season 1-2 hours 5
Prune excess growth
  • Once in year 2 or 3,
  • Every three years between years 4-10,
  • Every five years as needed after 10 years
End of winter or early spring 1-2 hours 2,5,6
Inspect tree health and safety Biannually and after large storms During growing season 1-2 hours 1,2,5,6
After 5+ Years of Operation (non-routine maintenance)
Activity Frequency Time period Level of effort O&M benefita
After long term operation of the practice, some occasional and infrequent maintenance activities might be required, such as tree or media replacement. As needed As needed Could be significant depending on the activity 1,2,3,4,5,6

aKey to Maintenance Benefits:

  1. Proper stormwater flow and infiltration
  2. Creation and maintenance of wildlife habitat
  3. Creation and maintenance of pollinator habitat
  4. Nutrient cycling and storage
  5. Aesthetics and public enjoyment
  6. Carbon sequestration
bNote that many practitioners are minimizing the use of mulch or using alternatives to mulch to control weeds. Using mulch can cause clogging of inlet, outlet, and bypass pipes, and can introduce invasive species such as jump worms. Alternatives to mulch include ground vegetation such as clover or sedges, or arranging plantings in more dense configurations so as to minimize use of mulch.


Common problems and how to troubleshoot them for tree trenches and tree boxes
Link to this table

Symptom Possible causes Solution
Standing water within the infiltration area for more than 48 hours This might be because a pretreatment is no longer working, or there are excessive sediment loads due to erosion or high sediment loads from the contributing area.
  • If applicable, clean the pretreatment area with more frequency.
  • Scrape, clean or vacuum the infiltration area.
  • Aerate the infiltration area
  • Stabilize erosion in the drainage area.
  • Contact manufacturer.
Rainwater does not appear to flow to the infiltration/filtration area Leaves, sediment, trash, or plant debris may be blocking the flow path to the inlet Remove these materials on a regular basis
Tree is not growing or shows signs of poor health during inspections
  • Species selection is inappropriate for the site
  • Over or underwatering
  • Disease or other problems
Consult with an arborist. Check that plants are suited to the local conditions.
Erosion or scouring around the inlet Flow is obstructed by debris or improper grading Correct for drainage and flow path issues to make sure flows are evenly distributed. Make sure the flow paths are unobstructed


Maintenance Costs

Maintenance costs vary depending on a number of factors, including but not limited to the following.

  • Size of the practice and its contributing drainage area
  • Type of plantings used
  • Site visit frequency
  • Level of maintenance needed
  • Local weather conditions
  • Staffing needs (number of staff, external vs. internal staff, etc)
  • Travel time between sites
  • Efficiencies of scale (single GI (green infrastructure) vs. a cluster of GI)
  • Equipment needed

Maintenance costs for trees and tree boxes should be relatively low compared to other green infrastructure practices, partially due to their size. Routine maintenance can take as little as 30 minutes per unit and does not require any special training, tools, or machinery. Because routine maintenance does not require any specialized training, the range of annual maintenance cost is around $100-$500 (CRWA). A study published in 2017 by ASCE describes the annual maintenance cost for a tree planter to be $260 per year, based on 2015 data from Fort Collins, CO (Clary, 2017).

Maintenance costs for stormwater and rainwater harvest and use/reuse practices
Link to this table

Example Maintenance Costs
Activity Frequency Annual Cost Reference
Inspection, Reporting & Information Management Semi-Annually $260 2009 WERF
Roof Washing, Cleaning Inflow Filters Semi-Annually $480 2009 WERF
Intermittent System Maintenance (System flush, debris/sediment removal from tank) Every 3 years $130 2009 WERF
Pump Replacement Every 3 years $198 2009 WERF
Cartridge Filter Annually $20-60 MPCA Cost-Benefit Considerations
Reverse Osmosis Filter Annually $400-1,500 MPCA Cost-Benefit Considerations
UV Light Disinfection Every 10,000 hours or 14 months $350-1,000; $80 to replace UV bulb MPCA Cost-Benefit Considerations
Ozone Disinfection As needed $700-2,600; $1,200+ for in-line monitor to test effectiveness MPCA Cost-Benefit Considerations
Chlorine Disinfection Monthly $1/month manual dose or a $600-3,000 automatic self-dosing system MPCA Cost-Benefit Considerations

Useful Resources

Additional Detailed O&M Information

Case Studies

Maintenance Training Documents and Videos

Example O&M Plans, Checklists, Reports, and Maintenance Agreements for tree trenches and tree boxes
Link to this table

Symptom Possible causes
Operation & Maintenance Plan City of Redding, CA: Instructions to Inspect and Maintain Tree Box
Construction phase inspection checklist District of Columbia Department of Energy and Environment, Construction Inspection Checklists
O&M inspection checklist
Maintenance Agreements


References


Related pages

Green Infrastructure

General information on trees

Tree trenches and tree boxes

Urban Forestry

For more information on urban forestry, we suggest visiting the following websites.

This page was last edited on 29 September 2021, at 12:53.

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