By John Lorinc
Green roofs are sprouting up in urban centres around the province, creating mini-ecosystems where you least expect them.
W hen Beth Ann Currie recently dug up the taproots of a cluster of coneflowers growing on the roof of an old industrial building in downtown Toronto, she realized this tall meadow species seemed to have an affinity for city living.The JAS Robertson Building, in Toronto’s garment district, was fitted with a 372-square-metre green roof in June 2004. Its owners, Urbanspace Property Group,wanted to create a natural meadow on a 20-centimetre-deep base of compost, using a wide array of indigenous species, including columbine, switchgrass, swamp milkweed and New England aster.
Although the seedlings were planted in large clusters, Currie, the environment, health and safety coordinator for Urbanspace, reports that not all the species took to the gusty environment. The coneflowers, however, adapted quickly. In ordinary soil, their roots go straight down. But the roots of those growing on the roof of the Robertson Building extended laterally and thus at a right angle to the prevailing northwest wind. “It’s a wonderful story,” says Currie. “Basically, they’re bracing themselves against the wind. The taproots aren’t going anywhere near the asphalt [beneath the compost], which debunks the myth that these plants dig holes in the roof.”
Over the last two years, green roofs have stormed onto the urban environmental agenda. Proponents say the green roofs —essentially green space on a rooftop — help reduce stormwater runoff, improve the energy efficiency of buildings by providing
better insulation, and counter the so-called heat island effect, the climatic anomaly that produces a column of warm air over large urban areas due to all the cars and heat-absorbing paved surfaces. In April, Green Roofs for Healthy Cities, a Toronto-based industry association, released a survey showing that the total area of installed green roofs across the United States and Canada increased by 72 percent between 2004 and 2005. U.S. cities such as Chicago and Washington, D.C., are leading the way, while in Ontario,Waterloo, Ottawa and Toronto have all taken steps in the past year to promote green roofs. Still, green roofs remain expensive and, in comparison to their presence in Europe, relatively rare. “The concept is basically in its infancy in North America,” says Bill DeLuca, president of Aldershot Landscape Contractors, the firm that installed the 10,684- square-metre green roof on the Canadian War Museum, which opened last year in Ottawa.
A significant challenge associated with installing green roofs is structural: the soil is heavy, so buildings with such facilities must be strong enough to support the load and be fitted with special roofing materials to protect against water or root damage.
But the biological issues are equally weighty. “The choice [of plants] is very limited because a green roof is a unique growing environment,” says Rick Buist, of Landsource Organix, the firm that worked on the Canadian War Museum and the Robertson Building, a joint venture with Gardens in the Sky, a Toronto-based company. Even two storeys up, the wind can be intense. Nor is it easy to predict what other species will blow in and take root alongside the plants put there intentionally.
Currie points out that many green roofs support only monocultures because the growing medium used is made of a porous mesh laced with a mineral mix. Urbanspace wanted biodiversity in its rooftop ecosystem, so the company chose a Swiss method that relies much more heavily on indigenous species and an organic growing medium.
In most of Europe, the growing medium is typically about 85 percent gravel and minerals, Buist told a recent gathering of the Ontario Association of Landscape Architects. His firm, in contrast, produces a growing medium made almost entirely of recycled and composted organics.“It’s natural,”says Currie,“not mined or quarried.”