What Lies Beneath: Subterranean Infrastructure for Street Trees
Wondering what’s under streets has always been a favourite daydream of mine. The juxtaposition of green cities, with vibrant trees emerging from endless paving evokes images of intrepid root networks spreading out beneath the smooth exterior.
Silva Cells, a “modular subsurface integrated tree and stormwater system that holds unlimited amounts of soil while supporting traffic loads beneath paving and hardscape,” is a technology that makes it easier for trees to survive and thrive in cities. The system provides an area for roots to spread in uncompacted soil and to share resources. The modular units can be laid out as bridges between soil volumes, connecting street tree roots to parks and lawns. The amount of soil that vegetation can access affects plan size, health, and longevity; therefore the Silva Cell system enhances the viability of urban plant life. When plants can achieve maximum root growth, trees can grow to full maturity.
Silva Cell technology also has financial and environmental benefits. The enclosed system prevents damage to pavement and utilities from growing roots, eliminating unplanned work and repairs. Further, Silva Cells are permeable, allowing for onsite stormwater treatment, which increases air and water quality while reducing flooding and erosion.
In Minneapolis, monitoring revealed that Silva Cells captured and treated storm water from 90% of local rain events (about 1 inch/per day, the same as Patrick Condon’s much espoused guideline). The city’s models now reflect a 10% reduction in peak flows (during peak events) as a result of installing the technology. Maryland studies found that Silva Cell filtration removed more than 80% of phosphorous, 60% of nitrogen, and more than 90% of lead, copper, zinc, and iron from stormwater. There is potential to bolster these numbers. The expanded canopies and root systems of mature trees increase filtration capability. Combining Silva Cells with permeable paving would yield greater capturing and filtration capacity.
Street trees are integral to the comfort, character, and ecological functioning of cities. With a little help from subterranean technology, trees and cities can be healthier. Next time you’re walking down Fourth Avenue in Seattle, Marquette Avenue in Minneapolis, or in Vancouver’s Olympic Village, think about the roots beneath the pavement you’re walking or biking on. They are silently growing to provide shade, fresh air, and clean water. As always, much of the most interesting city life is not visible on the surface.