Photo: Abundant vegetation by the roadside. (Photo: Tony Atkin)

Plants and trees make the world a greener place, and a new study shows one of the best places to grow them may be alongside highways.

Greg McPherson, one of the study’s authors and a researcher with the United States Forest Services, says that growing trees, shrubs and other kinds of greenery alongside highways can benefit the surrounding communities by cleaning up the air quality.

“There’s this kind of filtering that reduces the number of air pollutants,” he says.

McPherson says roadside vegetation can also reduce energy costs. Shrubs provide shade, which mitigates the urban heat island effect that often results in cities being warmer than the surrounding areas. In the winter, McPherson says these walls of vegetation can reduce wind speeds, thereby lowering heating costs.

But it isn’t as easy as planting trees and shrubs by highways. If these vegetation corridors are improperly designed, tunnels of vegetation can trap pollutants in an area, whereas a more open landscape would allow pollutants to disperse.

Another negative effect would be that some tree or plant species emit more ozone than others, creating concentrations that exceed human health standards. They can also produce a lot of pollen, which can be a problem for asthmatics.

As a result, the report sets out strategic guidelines for species selection and planting strategies. Some of the latter include planting rows of trees to effectively stop pollutants from reaching surrounding communities. McPherson says that five metres of vegetation is generally the minimum.

Other benefits include reducing storm water run-off, improving biodiversity by creating wildlife habitat and reducing stress through the aesthetic effect of having greenery around. “It’s healthy and connects us with the natural world,” he says.

The study didn’t examine the overall potential benefit to the environment of placing vegetation buffer zones around pollution-generating areas like highways. But plants and trees can help filter gases like carbon dioxide out of the air.

To learn more about energy and the environment in Canada, check out Let's Talk Energy Week, running from Feb. 21 to 28.