New soil cell technology aims to grow big trees in Edmonton’s tight spaces

August 2, 2016

By: Edmonton Journal

Life can be nasty, brutish and short for Edmonton’s street trees, stuck growing in small tree pits between a sidewalk and a heavily-compacted road.

Starved of space to spread their roots, 85 trees on 124 Street died in the past three years alone, 40 per cent of the tree canopy. The others are scrawny. Planted years ago, they still give almost no shade or any of the other benefits that come with a mature canopy such as cleaner air, increased oxygen and reduced stormwater runoff.

Fortunately, new technology is starting to make a difference elsewhere. Small plastic tables stacked underground and filled with good, un-compacted soil can give the trees the space they need, even under the sidewalk or parking space.

Edmonton made a new commitment this spring to at least triple the amount of soil it requires for street trees in city projects.

All that’s needed now is the will to make that upfront investment.

“To me, this is a slam dunk,” said Coun. Scott McKeen, who asked for an update on the soil cells for council. Trees buffer pedestrians from the street traffic, he said. “They add a certain majesty to the street. We need some nature around us.”

But these alternative technologies can cost up to $16,000 per tree. That has to be found in each road project budget.

“They plant trees (in small tree pits) and you’re putting them in a position to fail,” said Joel Beatson, executive director for the industry group Landscape Alberta.

Trees compete for budget dollars against drainage and transportation needs. But a well-planted tree and mature canopy brings many surprising benefits.

A mature tree increases the life expectancy of the pavement in its shade by at least 10 years by reducing the surface temperature, he said. It absorbs 54 to 110 kilograms of small particles and air pollution each year, which is particularly useful next to roads. A good canopy reduces stormwater runoff dramatically and two mature trees produce enough oxygen for a family of four.

But they need to reach a certain size before they give those benefits, he said. “The city is a dangerous place for trees but when they fail just for lack of planning, that’s sad.”

Edmonton has experimented with new soil cell technologies along several streets, including Stony Plain Road, 112 Avenue in the Highlands and 96 Street downtown. There’s even a system to use run-off from 96 Street and its sidewalk to water the trees like an underground rain garden.

This spring, the city formalized that commitment, rewriting the rules to give 17 cubic metres of soil to each elm or oak-sized tree. Smaller species should get at least 10 cubic metres. It’s been written into the 2016 Design and Construction Standards, the rule book that governs each engineer as they design city projects.

Engineers used to believe trees would find their own room to grow, said Crispin Wood, head of Edmonton’s urban foresters. With new road designs, that’s no longer true. Now construction crews compact soil as much as possible to create a good surface for the pavement and tree roots can do nothing with it.

The new soil cells that look like plastic tables work by giving a compacted surface for parking or sidewalks, while protecting uncompacted soil underneath. It can create long soil vaults between trees, or soil bridges underneath the sidewalk to let roots reach into front yards.

What benefits can mature trees bring?

• Strategically planting trees around a building can reduce the heating bill by 10 to 50 per cent, and air conditioning by 25 to 30 per cent.

• Two mature trees can produce enough oxygen for a family of four

• A mature tree absorbs 54 to 110 kilograms of small particles and air pollution each year

• Shade from a mature tree canopy can reduce the surface temperature of pavement by up to 20 C, increasing its life expectancy by at least 10 years and cutting maintenance costs by half.

• For every five per cent of tree cover added to a community, run-off is reduced by seven per cent.

• A well-landscaped property can add up to 20 per cent to the value of a house.

By: Edmonton Journal

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