With more than 700 green roofs, Toronto is missing out on the opportunity to grow its own food

A group of researchers from the University of Toronto are looking at ways to transform Toronto’s many green roofs into places where people can grow food.

Canada’s largest city has one of the most progressive green roof policies in the world – and was the first city in North America to adopt a bylaws that made them mandatory for many new buildings.

“Urban agriculture is emerging as a key component in bringing resilience to our food systems,” said Marney Isaac, professor of the Department of Physical and Environmental Sciences at U of T Scarborough, who oversees the project with Scott MacIvor, an assistant professor at U of T Scarborough’s Department of Biological Sciences.

“At the same time, new green roofs are appearing all over the city, but they are not set up to grow food. This project merges these two trends. “

There are currently more than 700 scattered across the city of Toronto, but MacIvor says most are “extensive” green roofs with shallow substrates (a mixture of organic material, crushed bricks and minerals like sand and slate) with little or no irrigation . They are also planted almost exclusively with a type of succulent called sedum. The durable and drought tolerant plants are excellent for storing water and growing in harsh environments, but are functionally limited and only help to improve water drainage and cool buildings.

The U of T Scarborough project, on the other hand, is looking at ways to test the limits of what is possible with current green roofing technologies and approaches.

PhD student Adriano Roberto harvests crops and records data from one of the green roof modules at the top of Highland Hall (photo by Don Campbell)

“We are really interested in transforming these extensive sedum-green roofs into those that can perform more services such as growing food,” says Isaac, who specializes in plant-soil interactions and making agriculture more sustainable.

While most of the research on green roofs has been done on a large scale, the team at U of T Scarborough is looking at more detailed plant-soil interactions at a smaller level to see how it will be possible to grow food.

A project led by PhD student Adriano Roberto and master’s student Andrew Nichols currently involves 300 hundred modules – boxes measuring 60 centimeters by 40 centimeters – on the roof of Highland Hall and a further 100 modules on the roof of Science Wing. Each of them contains one crop – beans, parsley or kale – grown with and without sedum and exposed to various environmental pressures such as low water content and low availability of nutrients. The goal is to see how these plants interact with each other and whether sedum can help these crops grow and survive in harsh environments.

“If we just planted beans or parsley in these shallow, exposed substrates, not much would grow,” says MacIvor, an expert in pollination and biodiversity in urban environments.

“But if we mix these with plants that grow well under harsh conditions – which hold water in the substrate and provide shade – they can act as care plants to support these crops and increase their yield.”

Last summer, 300 modules were tested on the roof of Highland Hall (Photo by Don Campbell)

This blending process, known as intercropping, has been thoroughly researched in the past, but the U of T Scarborough team uses that knowledge and applies it to the harsh growing conditions found in green roofs. Isaac says they chose the crops based on how well they grow together and how compatible their root systems will be with sedum.

Since inorganic fertilizers are not ideal for use in roof gardens, the team is experimenting with using worm molds, a type of organic fertilizer, to provide the necessary nutrients for plant growth.

The project received funding as part of a major $ 1.6 million grant from Canada’s Scientific and Technical Research Council for the DesignLIFES (Design of Living Infrastructure for Ecosystem Services) network. The network includes the Green Roof Innovation Testing Laboratory at U of T’s John H. Daniels Faculty of Architecture, Landscape and Design, with which MacIvor has collaborated over the past decade.

The last of this year’s green roof crops was harvested in early October, and the next steps for the project will be to analyze crop growth, root traits and water consumption efficiency from the summer growing season.

Next year, the team plans to look at how different availability of organic matter and nutrients can help with long-term growth conditions in green roof systems.

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