Project Overview

In the fall of 2008, a pilot project was launched along the Queensway thoroughfare in Toronto: four trees were planted in Silva Cells — all of them on the north side of the road, two each separated by 10 meters of distance and a preexisting street tree. One of the primary objectives was to determine the feasibility of adding stormwater management to a suspended pavement system, in addition to monitoring the performance of the subsequent tree health.

The results? The planting soil in Silva Cells proved to be an amazingly successful source control, removing pollutants, reducing peak-flow surges, and significantly limiting the ultimate volume output. The trees also grew large and healthy, particularly the two that received stormwater from both the road and sidewalk as opposed to just the sidewalk (see more on the testing strategy below). This first-ever Silva Cell stormwater project proved the effectiveness of using the system for underground bioretention, leading to hundreds more successful such projects around the world.

Testing Strategy

This pilot project was launched in tandem with the City of Toronto and Ryerson University with the intent of tracking both tree and stormwater performance of Silva Cells.

All four trees were setup to receive stormwater from both the sidewalk and the street; however, as a way of testing two different management volumes, the two trees on the eastern edge of the project area were fed both sidewalk and street runoff, while the two trees on the west side had their catch basin closed off, thus receiving only sidewalk water. Data was monitored over the next decade — what became clear quickly was that the planting soil in Silva Cells served as an efficient stormwater control for all four trees (see below for specific data) for pollutant removal and both water quantity and quality performance. The east-side trees by 2018 were noticeably larger than their west-side counterparts, (though all four trees were big and healthy), underscoring the importance of irrigation. In 2018, after a decade of studying, the catch basin was opened to the west-side trees, which have responded in kind with even larger growth in the years since.

What did we learn? Firstly, that tree planting soil can be an extremely effective tool in managing stormwater. Secondly, irrigation matters: more water entering the soil means a greater chance of robust growth. Thirdly, road salt (which was a worrisome component of allowing road runoff into the planting soil) was not a hindrance to the health of the soil or tree — the salt simply gets diluted and dissolves its way out of the system.

Key Takeaways with Mark Adamaley

Mark Adamaley, DeepRoot Canada Account Manager in Ontario, identifies a number of important takeaways from this pilot project.

“By utilising well-designed tree planting as a stormwater management strategy, you gain appreciable storage capacity and impressive pollutant removal.”

“With minor modifications to our standard design practices, every single tree in a Silva Cell trench within the public realm can serve as a stormwater management and treatment solution. The modification would include introduction of sump space, catchment, distribution pipes, and underdrains. It only takes a few hundred dollars per tree trench to achieve this.”

“The stormwater engineer refers to the constituents of stormwater runoff as pollutants, while the landscape architect and urban forester refer to these same constituents as nutrients. It is incumbent on all stakeholders to embrace design that harnesses these nutrients to enhance urban tree health.”

The Power of Trees: Continued Success in Toronto

This pilot initiative has served as a catalyst for the deployment of Silva Cells across the GTA where hundreds of projects are now supporting tree growth and serving as an LID stormwater solution. The results, while unsurprising to city urban foresters, provided the demonstrable data needed to prove the stormwater-management effectiveness of street trees.