IFLA Advisory Circle Article
The IFLA Advisory Circle is integrated by a group that provides expert insight and thought leadership on national, regional and international developments which directly or indirectly concern landscape architecture. The Circle also monitors and co-ordinates the dissemination of key position statements and advice from our members, friends and partners to landscape architects around the world.
Street trees and salty water
Dr Jen Hill is a Research Scientist at Toronto Region Conservation Authority where she is primary author to the current guide for the design of low impact development (LID) stormwater management strategies. She is a member of their professional training team, an expert panelist for Sustainable Buildings Canada, and an Adjunct Professor at University of Toronto. She wrote her PhD in water resources engineering at the University of Toronto, following a Master’s degree and work experience in Landscape Architecture.
Soil cells are increasingly popular in cities across Canada and elsewhere in the world. These rigid plastic supports permit urban trees to flourish with their roots in engineered soils beneath sidewalks or other paved surfaces. The soil cells support the surface, preventing compaction from damaging the tree roots, and subsidence or heaving from damaging the overlying surface. As many cities are now directing resources on green infrastructure to manage stormwater, the soil cells can take another role in capturing excess runoff water, like an underground bioretention facility.
In Canada, throughout many months of the year, city roads are ploughed and salted to keep them safe and clear from snow. This may create a tension between the needs of the transportation department and the urban forestry teams. But research conducted on soil cells nearly a decade after planting demonstrates that year round irrigation with road runoff can improve the size and health of urban street trees, even when that water is salty through the winter months.
The trees, monitored in the City of Toronto, were American Liberty elms (Ulmus americana ‘libertas’) and Freeman hybrid maples (Acer x fremanii). All of the trees in the supported soil cells are larger and healthier than nearby specimens in standard urban tree pit plantings despite regular salt application on the adjacent road. In a nearby city, Mississauga, a similar installation constructed in 2014 contains Chantcleer Pear (Pyrus calleryana) and Fragrant Sumac (Rhus aromatica) also helps to manage stormwater, whilst supporting excellent plant health. The important feature appears to be the freely draining engineered soil which washes clean of the salty water as the snow melts in springtime.