Getting the dirt on Vancouver’s tree roots

It’s not just people fighting for space to live in Vancouver. Trees are competing with sidewalks, asphalt and other urban development. But over 100 new trees planted downtown last summer may be an example of how to support future tree health. 

The Richards Street project downtown brought city arborists and engineers together to collaborate on a new initiative. It mimics nature in an urban setting.  This includes a rainwater tree trench to water trees and soil technologies that allow for root growth underneath roads and sidewalks.

“The kind of growing environment we want for trees [is] where the roots can spread out,” said Robb Lukes, a civil engineer who worked on the project and is also branch manager of green infrastructure for Vancouver.

But soil volume is limited in highly developed urban areas. Susan Day, a University of British Columbia urban forestry professor, said that tree roots -– if given adequate space -– usually grow outwards 38 times their diameter in every direction. 

Soil, she explained, if often too compact in urban areas for roots to grow through. With restricted soil volumes, root systems have less available water and nutrients, which can impede tree growth.

To successfully grow trees in the highly urbanized Richards Street green infrastructure was needed.

“[Richards Street is] a crowded utility corridor,” said Cherie Xiao, senior project manager.

The technologies used were soil cells and structural soil. Soil cells were used for three blocks and structural soil for five blocks.

Soil cells are frames with vertical columns that have the strength to support urban development, such as sidewalks and roads. Between the columns is growing soil and enough space for roots to grow freely. 

Structural soil is a mix of gravel and planting soil. The gravel has the strength to support the heavy city infrastructure while the roots grow in the soil.

Additionally, the project includes a rainwater tree trench, which is an underground trench that collects rainwater and distributes it to tree roots. If there is excess stormwater it feeds directly into the storm sewer.

The best approach for growing trees downtown combines these green infrastructure technologies. Structural soil is more flexible and easier to work with while soil cells allow for more soil growth, according to Xiao.

This green infrastructure project will likely be duplicated in new developments. 

“It’s becoming a city standard, what we did on Richards,” said Lukes. In a few months, he expects to see it added to the city’s design manual.

Many trees in Vancouver are not planted with such technologies and suffer as a result. 

For example, trees planted conventionally in Yaletown are performing poorly when compared to those planted with soil cells in Olympic Village, according to Lukes.

Often, competing interests might prevent developers and engineers from ensuring they provide adequate space for tree roots. Instead, roots are viewed as a public safety hazard or hindrance to building, according to Joe McLeod, Vancouver’s city arborist and supervisor of urban forestry.

“From our standpoint, roots are critical. From other practitioners’ standpoints, roots are a problem,” he said, explaining that tree roots can create tripping hazards if they infringe on sidewalks. 

But the importance of trees –- especially with events such as last summer’s heat dome –- is becoming more apparent to others in the city. 

“We need proper soil volumes, we can’t just get away with cutting holes in the asphalt or concrete and dropping a tree in the ground,” said McLeod. “Then we’ll end up having to plant a new tree every year for eternity.”