Seven new Brandon elms line the sidewalk on the west side of 90th Street in the neighbourhood of Alberta Avenue.
Keeping the trees healthy takes a commitment deeper than you might think.
Buried under the concrete are rows of what look like skeletal Lego pieces. They’re called soil cells, they’re made of recycled plastic, and they’re integral to the sustainability of the City of Edmonton’s urban forest and climate-change initiatives. How? By helping trees grow bigger and live decades longer in “hardscape” areas such as sidewalks and parking lots.
On average, soil cells cost between $8,000 and $12,000 per large tree, but they can lead to substantial savings.
“Without soil cells, trees have a harder time thriving in sidewalks and parking lots,” said Trevor Thistle, an urban forester with the City of Edmonton. “Forty percent of all trees removed in hardscaped areas are less than five years old. This year, we had to remove about 120 young trees.
“But with soil cells, more trees can reach their full potential and the City can save money because we’re not replacing trees every five years.”
Most trees like oaks and elms need about 20 to 50 years to reach maturity, but they usually live a lot longer. American lindens, for example, have a life expectancy of 200 years and can grow up to 37 metres in ideal conditions. The same goes for Burr oaks. Brandon elms usually live 80 years and can grow up to 12 metres.
The City uses soil cells to grow trees in urban hardscapes throughout Edmonton, including the Quarters, Churchill Square, Jasper Avenue, Kinsmen Sports Centre and Butler Memorial Park on Stony Plain Road. They were also recently installed under the sidewalk along 88th Avenue—from High Level Diner to the Sugar Bowl—where they will support four American lindens. (They’ll be planted next spring.)
How do soil cells work?
An estimated 12.8 million trees live in Edmonton, including 380,000 inventoried Boulevard and Open Space trees. The City has a goal of planting two million new trees by 2050 as part of its plan for a healthy, urban and climate-resilient city.
Until recently, most trees in urban settings were planted in small plots with compacted soil, surrounded by concrete. But trees need loose soil and ample room for their roots to grow if they have a shot at reaching maturity—between 20 and 50 years. Soil cells provide both. They’re installed under sidewalks or parking lots, giving large-canopy trees access to at least 17 cubic metres of loose soil. That’s the standard set by the City of Edmonton in 2016. That’s the size of about 100 bathtubs.
“Think of a soil cell like an old plastic milk crate turned upside down,” said Matt Sloan, a senior landscape architect with the City of Edmonton. “It’s an engineered crate or structure that can support the weight of a sidewalk but also stops the soil from compacting, so that tree roots are free to grow and seek water or oxygen.”
Soil cells were invented in the early 2000s. The City of Edmonton has used them for about a decade. You’ll find some of its first ones under the elm-lined sidewalk on the south side of 105th Avenue, just south of St. Joachim Catholic Cemetery.
“The growth on those elm trees in the last eight to 10 years is just phenomenal,” said Sloan. “You can see how happy they are.”
Happier, healthier trees lead to happier, healthier Edmontonians. Mature large-canopy trees provide shade and cooler temperatures as well as capture more carbon dioxide and reduce greenhouse gas emissions. In 2009, the City’s urban forest absorbed an estimated 531 tonnes of pollutants from the air.
“When trees live only three to five years, we’re not getting the benefits that we would if they were able to reach maturity,” said Sloan. “Being able to plant trees in soil cells helps the trees thrive and helps us with our climate-change targets.”
New energy transition strategy
The City plans to be carbon neutral by 2050 as part of its new energy transition strategy to reduce greenhouse gas emissions. Planting two million trees is one of the many ways to achieve that goal, along with solar and wind power projects, electric vehicles and charging networks, as well as more efficient buildings and homes.
One of the consequences of climate change is more extreme weather. Not only do soil cells benefit trees and Edmonton’s air quality, they can also reroute stormwater, minimize flooding and any potential damages, such as sidewalk heaving, thereby also reducing repair costs.
“The stormwater will filter into the soil instead of directly into a stormwater system,” said Sloan. “Because the soil isn’t compacted, it acts like a sponge so it can hold onto the water and get used by the trees. Whatever water is leftover is then sent to the stormwater system but it’s of a better quality because it’s been filtered by the soil and trees.”
Think of trees as our strong, silent friends who we can always count on. They offer us shade and relief. They nurture us by purifying the air and soaking up excess rain. They let us climb all over them. They do so much for us, yet we often take them for granted.
The City of Edmonton is responsible for making sure our friends get what they need to thrive underground, so we can continue to enjoy them above ground.
That’s something to cell-ebrate!
Editor’s note: the pic at the top of the post shows one of the new Brandon elms on 90th Street in the Alberta Avenue neighbourhood on October 3, 2021. They’re part of the City of Edmonton’s neighbourhood renewal project in the Alberta Avenue area, which includes new street lights, sidewalks and road re-construction. The pic below shows the elms on November 5, 2021.