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Silva, cells made up of stacked frames two high, are placed beside the promenade at the Olympic Village. The cells are filled with soil. Then a deck and textile and sidewalk are placed on top.
Silva, cells made up of stacked frames two high, are placed beside the promenade at the Olympic Village. The cells are filled with soil. Then a deck and textile and sidewalk are placed on top.

Things that work

An urban canopy to nurture a city's growth Add to ...

Mike James sees them all the time: spindly city trees, their roots crammed into shallow holes that might as well be flower pots.

It’s too late for those arboreal prisoners, which sooner or later will stop thriving and die, after a stunted life span that’s typical for many trees planted in cities across the country.

But for trees yet to be planted, Mr. James – general manager of DeepRoot Canada Corp., which makes products designed to help trees thrive in urban environments – has hopes of life spans in which trees get broad and tall enough to provide the full benefits of what scientists call the urban canopy.

Those benefits, including reduced energy costs and better storm-water control, have been shown to march in lockstep with the size of trees.

“We tell municipalities that trees are as much a part of their infrastructure as the gas line, the sidewalk and the light pole,” says Mr. James, who is based in Vancouver. “And as such they belong not just to the arborist, but to the roads engineer, the streets engineer and the storm-water engineer.”

DeepRoot was founded in the United States in the 1970s with one product, a root barrier system that forces roots to grow down, not out, lessening the possibility of tree roots buckling pavement and sidewalks.

Over time, working with American landscape architect James Urban, the company developed the Silva Cell, a system designed to help nurture big trees in urban environments based on research that shows larger trees provide exponentially greater benefits than smaller ones. A 2010 report for the City of Toronto, for example, found a tree that’s 75 centimetres in diameter intercepts 10 times more air pollution, stores up to 90 times more carbon and contributes up to 100 times more leaf area to the city’s canopy than a 15-centimetre tree.

Big, mature trees define established neighbourhoods in cities such as Toronto, Winnipeg and Vancouver. But such trees can be scarce in new suburbs, on downtown streets and in neighbourhoods where construction and excavation have left little soil that isn’t compacted.

Often, cities plant new trees in a few cubic metres of poor-quality soil, with predictable results.

“These trees don’t grow – they just sort of stall – and they might put out a little bit of green every year or they get sick and die,” Mr. James says, adding that the average life of a tree in a Canadian city is 13 years.

Silva Cells are a modular system that puts tree roots under buried decks, with a layer of aggregate and pavement over top. The system provides drainage and irrigation and puts roots below the decks, so they don’t push up and buckle pavement. It also ensures plentiful volumes of unpacked soil – about 15 to 20 cubic metres in a typical installation, compared with 1.5 to 3 cubic metres for a tree planted in a conventional pit.

The system has been used in several recent urban showcases, including Vancouver’s Olympic Village and Toronto’s Sugar Beach. With 20 to 25 cubic metres of soil, trees can be expected to live for 50 years or longer.

Planting a tree in a conventional pit might cost between $600 to $1,000, with the grates pushing that total to $3,000 or more, Mr. James says. Using a Silva Cell, by contrast, costs between $8,000 and $10,000, with the cost of a tree and grates on top of that.

“So we’re a big multiple more expensive – but we provide value,” he maintains.

The Resort Municipality of Whistler, looking for a way to grow a big tree in a plaza that also had to be able to support the weight of a fire truck, in 2007 used Silva Cells to plant a Red Oak in front of its public library.

The tree is thriving, says municipal horticulturist Paul Beswetherick. He’d like to use the system for other installations.

“The big problem is persuading the powers-that-be that investment in green infrastructure carry the benefits they do,” Mr. Beswetherick says. “They’re not always as visible as the bricks-and-mortar investment.”

That may change as cities pay more attention to urban forests and their potential role in areas such as storm-water management and energy consumption.

As part of its Green 2020 plan, Vancouver specifies a 40-per-cent canopy and is developing an urban forest management plan, an initiative already underway in cities such as Seattle, Portland and Toronto. Vancouver’s plan calls for planting 150,000 trees – one for every four residents – by 2020.

Planners such as Mr. Beswetherick are already on board with new ways of planting city trees, saying the conventional, cheaper methods don’t pay off in the long run.

“They are struggling,” Mr. Beswetherick says of trees planted in conventional volumes of soil. “They are absolutely puny and dying. They are in horrible shape. You have a $150 tree in a five-cent hole. And 15 years later, you’ve got a five-cent tree.”

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Silva Cell

What it is: a system to provide enough high-quality, unpacked soil to allow city trees to grow big enough to offer the maximum benefits of what’s called the urban canopy, including reduced energy consumption and better storm-water control, while still allowing for amenities such as paved sidewalks and plazas.

How it works: a modular system that puts tree roots under a deck that’s topped with aggregate and pavement and also includes irrigation and drainage.

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