cityscape City Failing to Harvest Potential of Green Roofs

Toronto is poised for more green roofs, but the City's bylaw largely rules out growing fruits and veg.

Bees buzz around nearby zucchini flowers as Arlene Throness sinks her fingers into the rich, black soil to fish out one gem-like purple potato after another. If Throness—Ryerson University’s agricultural coordinator—digs too deeply, her fingertips will hit the roof of the building on which the garden sits.

Urban hunger for fresh, local produce grown without pesticides is skyrocketing, just as roughly 73,000 square metres of green roofs are poised for construction under Toronto’s 2010 Green Roof Bylaw. And while almost 200,000 square meters more of green roofs are expected to revolutionize the top of the city over the next few years, more green roofs won’t necessarily mean more local produce. That’s because the bylaw rules out the growing of most food plants.

When Toronto first started formulating its green roof bylaw in 2006, the idea of urban agriculture had barely arrived in the city. Bylaw writers focussed their regulations on soaking up storm water and softening the urban heat island effect. Side benefits included conserving energy, cleaning the air, and increasing biodiversity. But it hardly crossed the minds of bylaw writers that green roofs might do double duty as urban farms.

Other than Port Coquitlam, B.C., Toronto is the only city in North America with a green roof bylaw. It applies to most types of new buildings larger than 2,000 square metres and requires that up to 60 per cent of the roof surface be green. It also stipulates how green roofs must be constructed.

The bylaw doesn’t contain a list of prohibited plants—except for noxious weeds. “There is nothing to prevent someone from growing any type of fruit or vegetable on their green roof,” says Dylan Aster, a technical advisor in Toronto’s office of the chief building official.

But the roof still has to meet the bylaw’s construction rules. The one requiring that plants cover 80 per cent of a green roof by the third year effectively prohibits most food plants, the majority of which live only one season. Lettuces, for example, are harvested and replanted throughout the summer and finally die off in fall. The idea behind the rule is plant survivability because, “if the green roof was left to fallow and die, it wouldn’t be a functioning green roof,” says Aster.

But some of the most popular green roof plants that can survive winter, such as sedums, aren’t great green roof performers. Because sedums require little soil depth, water, and nutrients, a shallow sedum roof is one of the most cost-effective green roof options for both construction and maintenance. At the same time, a typical 100-millimetre-deep sedum roof absorbs only 9.8 litres of water per square metre, while a 50 per cent deeper meadow roof absorbs 15 times as much.

Like meadow grasses, food plants are better green roof performers than sedums. “If you grow vegetables, normally you have to have much deeper soil, so you can hold more water and can shield the building from heating up much better,” says Youbin Zheng, an associate professor at the University of Guelph.

The 100-millimetre-deep green roof on Ryerson’s Engineering Building on Church Street was built before the bylaw. It was planted with lilies but was soon overcome with weeds. Then, in 2013, an urban agriculture student group, Rye’s Homegrown, won permission to create a pilot vegetable plot on 93 square metres of the roof. With some added soil, the garden produced 350 kilograms of food that year. Then this year, permission was granted to expand the garden to eight times its original size. Throness expects to produce more than 2,268 kilograms of food by the end of the season, dividing the harvest between Ryerson Food Services and the Ryerson Farmers’ Market. The plant list includes everything from kale, eggplants, broccolis, and squashes through to watermelons, ground cherries, and edible flowers, including nasturtiums.

While there are plenty of examples of productive green roofs across the city—including the Royal York Hotel, the Carrot Common, and Eastdale Collegiate Institute—none of these were mandated under the bylaw.

And as productive as Ryerson’s green roof has proven itself, it wouldn’t be allowed if required to be built today. That rankles some boosters of green roof agriculture. “I think there’s a way to meet the goals of the city in terms of storm water reduction and reduction of urban heat island effect and still have functioning planters that support food production,” says Scott Torrance, landscape architect and green roof designer.

City officials do acknowledge a tension between the bylaw and the urban agriculture movement. “It’s something we’re going to have to look at in the future,” said Joe D’Abramo, director of zoning and environmental planning at the City of Toronto, speaking at a public lecture in March.

But even if that comment signals a coming bylaw change, it may be too late for the green roofs already lined up for construction. Unlike the building that hosts the Ryerson agricultural green roof, most new buildings have spare capacity engineered out of them to minimize infrastructure costs, says Mark Gorgolewski, professor of architecture at Ryerson University.

There’s also been no indication that rooftop farming is a particular desire of building developers. In the four years since Toronto’s bylaw came into force, there have been no green roof applications that have sought to include agriculture. It’s uncertain whether leasing roofs to farmers even makes business sense to developers, given the added costs of roof elevator access and the sturdier building infrastructure needed. Liability issues could also complicate matters.

But urban agriculture boosters still hope for a change in the bylaw. “It’s just an incredible resource, all these roofs that we have,” says Torrance. “And to be able to promote local, healthy, organic food production is fantastic.”

Photos by Michelle Adelman.

This post originally stated that a typical 100-millimetre-deep sedum roof absorbs 8 litres of water per square metre—in fact, it absorbs 9.8 litres.