Fact sheet for tree trenches and tree boxes

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Urban tree and soil systems provide water quantity and water quality benefits through stormwater infiltration, filtration, interception, evaporation, and uptake of pollutants by trees and associated microbes. Trees are already part of virtually all development and can be integrated even into the densest urban areas. Many cities already have tree requirement ordinances. However, the potential of these trees to provide significant stormwater benefits is largely untapped to date. Integrating urban trees into stormwater management systems provides opportunity to provide significant stormwater benefits using elements (trees and soils) that are already part of most sites and developments.

Summary of design criteria

• Infiltration requires suitable in situ soils; if soils are not suitable for infiltration, filtration is still possible.
• To provide the maximum benefits and lifespan possible, trees need adequate rootable soil volume (2 cubic feet of soil for every square foot of tree canopy)
• Use impermeable liner as needed to separate tree BMP from road, parking lot, sidewalk or adjacent walls or building foundation.
• A large enough media infiltration rate should be provided to allow for adequate filtration (and perhaps infiltration) of stormwater. Infiltration rate shall not be so high as to limit tree growth or water quality treatment, however. Media shall have infiltration rates between 1 and 4 inches per hour.
• A typical bioretention soil media may be used in tree filters. If a non-standard soil mix is utilized, a maximum of 15 percent silt and clay and 10 percent organic matter by volume should be specified. If an underdrain is used and the media has a high organic matter content, phosphorus may be leached from the media.

Benefits

The soil, trees, and microbes in a bioretention system with trees work together as a system to improve water quality of stormwater that falls on the tree and/or is filtered through the soil volume. Some pollutants are adsorbed or filtered by soil, others are taken up or transformed by plants or microbes, and still others are first held by soil and then taken up by vegetation or degraded by bacteria, “recharging” the soil’s sorption capacity in between rain events.

Trees have a wide range of non-stormwater benefits, such as cleaner air, reduction of heat island effect, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, cooler cars in shaded parking lots, windbreaks, reduced annual air conditioning costs, and social benefits (e.g. reduced stress, improved outdoor leisure and recreation, reduced crime).

Limitations or constraints

Potential constraints on the use of urban trees for stormwater management include:

• above ground space limitations such as utilities, lighting, signs, structures;
• below ground space limitations such as structures, pavement, existing trees, and utilities;
• regulations regarding types and locations of trees planted along public streets and right of ways, such as, for example, minimum sight distances and setbacks from street corners; and
• the need to locate trees outside of snow plow paths and snow storage areas.

General description

Many different types of urban tree stormwater BMP’s exist. Where existing trees exist, tree preservation is highly recommended, as existing trees are typically bigger than newly planted trees, and bigger trees provide significantly more benefits than smaller trees. Incorporating trees into traditional bioretention practices is also highly recommended. Street trees, trees in parking lots, trees in urban plazas, as well as any other trees also provide stormwater benefits.

While trees have tremendous potential to provide stormwater benefits, most urban trees do not provide nearly the magnitude of stormwater benefits they are capable of providing given adequate growing conditions. Large trees provide orders of magnitude greater stormwater benefits than small trees, but the average lifespan for urban trees is only 13 years (Skiera and Moll, 1992), so most urban trees do not survive nearly long enough to reach their mature size and provide the magnitude of stormwater benefits they are capable of at maturity. By far the most important factor to grow healthy trees is to provide an adequate volume of rootable soil, to allow for adequate air, water and drainage (e.g. Coder 2007). Research has shown that trees need 2 cubic feet of rootable soil volume per square foot of tree canopy area to thrive (e.g. Lindsey and Bassuk 1991). To provide 2 cubic feet of rootable soil to allow a tree with a 30 foot canopy to thrive would require 1413 cubic feet of rootable soil, 22 times more than the typical 64 cubic feet an urban tree box has.

Where there is not enough open space to grow large, healthy urban trees, 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; and
• soil boxes.

Management suitability

Same as bioretention, can range greatly in size, and can be sized to meet desired goals.

Mechanisms

• Infiltration (with appropriate in situ soils)
• Filtration
• Temperature Control
• Settling
• Evaporation
• Interception
• Transpiration
• Soil Adsorption
• Biological Uptake

Pollutant removal (phosphorus, and total suspended solids)

Pollutants are removed through infiltration and filtration of stormwater. See Calculating credits for tree trenches and tree boxes.

Site factors

• Drainage Area – same as for bioretention. See Manufacturer’s recommendations for proprietary systems.
• Max. Slope – same as for bioretention. See Manufacturer’s recommendations for proprietary systems.
• Min. Depth to Bedrock and Seasonally High Water Table- same as forbioretention
• SCS Soil Type – Hydrologic Soil Groups A and B (can be used in C and D soil types with modifications (e.g. underdrains)
• Freeze/ Thaw Suitability - Good
• Potential Hotspot Runoff (requires impermeable liner) - Suitable

References

• Coder, Kim. 2007. Soil Compaction Stress and Trees: Symptoms, Measures, and Treatments. Warnell School Outreach Monograph WSFNR07-9, available August 2013.
• Lindsey, P; Bassuk, N. (1991). Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers. J. Arboriculture. 17(6), 141-149.
• Skiera, B.; Moll, G. (1992). The Sad State of City Trees. Am. Forests. March/April, 61-64.