Stormwater Tree Trenches
Trees can be incorporated into bioretention cells with other plant types, or otherwise into their own planting pits.
Take a look at the downloadable Stormwater Tree Trenches Fact Sheet below for a .pdf overview of this LID Best Management Practice:
A commonly held view is that a tree's root system will be similar to it's visible crown. For many trees, this is not the case, as roots will more often spread much more widely, but to a shallower depth . For more detailed information on planning (site) considerations see Bioretention
Planting in slopes
Smooth slopes should be amended into localised terraces by the Landscape Architect when planting large trees into slopes > 5 %. 
There are many design configuration options for including trees into stormwater management plans.
These are (usually plastic) supporting structures placed around the trees and beneath adjacent paved areas. They prevent compaction to the roots of the tree and prevent root damage to the paving. They are sometimes configured to receive stormwater and to enclose ponded water which can then infiltrate the soil surrounding the tree. Things to consider in design:
- If the system is unlined it is hydraulically equivalent to a bioretention cell and provides similar water quality benefits.
- If the system is lined and underdrained it is hydraulically similar to a large stormwater planter. Depending on the design detail it may retain significant stormwater within the planting soil volume and will provide water quality benefits.
Multiple methods for distribution and conveyance of runoff into the system are recommended for redundancy and conservative designs. Combinations may be made of:
- tree well flow,
- catchbasins and distribution pipes, and
- direct infiltration from permeable paving.
For a detailed overview with in-depth information on species selection please visit our Plant lists wiki page and you can continue on to our further detailed tables showcasing how to develop planting plans that include selection of species for specific functions in several stormwater tree trenches and other LID practices.
Planting pit sizing
Tree canopies intercept and store rainfall, thereby modifying stormwater runoff and reducing demands on urban stormwater infrastructure (Xiao et al., 1998; Xiao et al., 2000; Xiao and McPherson, 2002; Xiao et al., 2006). Canopy interception reduces both the actual runoff volumes, and delays the onset of peak flows (Davey Resource Group, 2008).
The extent of interception is influenced by a number of factors including tree architecture and it has been estimated that a typical medium-sized canopy tree can intercept as much as 9000 litres of rainfall year. (Crockford and Richardson, 2000).
A study of rainfall interception by street and park trees in Santa Monica, California found that interception rates varied by tree species and size, with broadleaf evergreen trees provided the most rainfall interception (Xiao and McPherson, 2002). Rainfall interception was found to range from 15.3% for a small jacaranda (Jacaranda mimosifolia) to 66.5% for a mature brush box (Tristania conferta now known as Lophostemon confertus). Over the city as a whole the trees intercepted 1.6% of annual precipitation and the researchers calculated that the annual value of avoided stormwater treatment and flood control costs associated with this reduced runoff was US$110,890 (US$3.60 per tree).
Birch trees in parking lot bioretention 'IMAX site' Mississauga, ON
Trees in parking lot bioretention 'IMAX site' Mississauga, ON
Open tree pits
Extended tree pit planting in USA
Photo credit: USEPA
Soil cells under construction.
Soil Cell installation along the Moynes and Berl Avenues on north side of The Queensway in Toronto. The picture depicts stormwater distribution pipes through the system, used to help provide water to the trees that will be later planted in the BMP feature.
Photo credit: City of Toronto
Silva Cell “top” deck prior to organic layer and screenings for pavers. Photo credit: City of Toronto
Soil Cells being installed along Edgewater Drive for Waterfront Toronto.
Photo credit: DeepRoot
Stormwater Tree Trenches at East Bayfront Promenade, Toronto.
Photo credit: DeepRoot
Seawall Soil Cells (Tree Trenches) located in Vancouver.
Photo credit:City of Vancouver
Stormwater tree trenches designed with a structural concrete pad over top of the installation.
- standard details and specifications from James Urban
In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.
- Crow, P. (2005). The Influence of Soils and Species on Tree Root Depth. Edinburgh. Retrieved from https://www.forestry.gov.uk/pdf/FCIN078.pdf/$FILE/FCIN078.pdf
- Wilkus A., 'Slope Style', Landscape Architecture Magazine, April 2018, accessed 21 December 2018, https://landscapearchitecturemagazine.org/2018/04/24/slope-style/