Green alleys - residential laneways redesigned to enhance residents' quality of life, boost social cohesion and improve biodiversity - are gaining traction in cities across North America. But as the concept spreads, it remains loosely defined: what exactly counts as a green alley, and what benefits can it provide?
According to a new Concordia-led study published in the journal Ecosystems and People, there is no single definition of a green alley, as these spaces can take many forms. While green alleys are generally linked to more biodiversity and stronger social cohesion, the study found they do not always achieve both at the same time; results depend on alley design and purpose.
The study was led by Isabella Richmond, PhD 25, and supervised by Carly Ziter, assistant professor of biology at Concordia. It was conducted in collaboration with Université de Montréal and Université de Montréal à Québec.
Photo Isabella RichmondDifferent cities, different approaches
The differences were especially clear when comparing Montreal's resident-led alleys with Trois-Rivières' standardized, city-run program.
"In Montreal, green alley programs result in a range of conditions," Richmond explains. "A green alley can mean setting up a play area for children, or adding murals, or ripping up all the concrete and planting trees."
In contrast, the Trois-Rivières program focuses on stormwater management and keeping alleys accessible for vehicles, including garbage collection. In these alleys, vegetation is more standardized, with more grass cover and concrete strips to support vehicle passage.
The researchers found this approach produced more consistent effects, including cooler nighttime temperatures. However, it was not always responsive to residents' needs, such as the desire for more and larger trees.
This finding helps to explain why results in Montreal were so varied. Tree numbers were sometimes higher in green alleys compared to other parts of the city, but the trees were often smaller, limiting shade and cooling. In some cases, canopy cover was even lower than in nearby streets. Temperature data also showed only modest and highly variable cooling effects, with some green alleys performing no better - or even worse - than their grey counterparts at certain times of day.
Readings and resident interviews
In total, the researchers analyzed 53 green alleys, alongside 23 conventional "grey" alleys and 76 nearby street segments in Montreal's Villeray-St-Michel-Park-Extension borough and in Trois-Rivières. The study was conducted over the summer of 2023 using on-the-ground data collection and with interviews with residents.
The researchers recorded tree species, vegetation layers, canopy cover and the presence of native or invasive plants. In Montreal, they also tracked fireflies as a sign of habitat quality.
To measure potential cooling effects, temperature sensors were installed in alleys and recorded conditions every 15 minutes over several weeks. At the same time, 30 residents were interviewed while walking through their neighbourhoods to identify valued features such as shade, greenery and visual appeal.
No benefits without consistent engagement
In both cities, the researchers observed that older green alleys tended to become less green due to lack of maintenance funding. The cost is often shifted to individual residents, which can lead to the breakdown of a project, the researchers say.
"There are advantages and drawbacks to both cities' approaches," Richmond says. "If your municipality has a goal of 30 per cent canopy cover, and you use planting trees in alleys to achieve that, the Trois-Rivières approach will get there faster. However, if the point is to benefit the people who live there, Montreal's approach allows residents to implement projects that benefit them in the way they want.
"Green alleys provide a rare opportunity to green dense cities," she adds. "I think there is a middle ground, where you can have community-led projects that are supported by experts and sustained financial investment and support for ongoing management."
Read the cited paper: "Green alleys in Quebec provide variable biodiversity support and ecosystem services"
