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Waist-high field grasses bend gently in the breeze on a downtown building roof-top as street noises echo down below.

"We've got a green space in the middle of a concrete jungle," Peter Carr-Locke, an employee at Mountain Equipment Co-op on King Street, said proudly. "At the height of the summer, this can just be a mass of colour."

This unexpected bit of aerial beauty is what some experts would like to make a growing trend in cities across Canada.

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More than 40 city planners and industry experts from Canada and the United States met at University of Toronto's Faculty Club last week to discuss advantages of roof-top green spaces. The workshop presented by Green Roofs for Healthy Cities, a coalition that wants barren city roof-tops turned into environmentally friendly greenery.

Green-roof landscaping is already a booming industry in Europe, with as many as 77 municipalities in Germany boasting sky-high havens of green.

Experts hope municipal politicians will follow the lead of other North American cities, such as Chicago, and finance green roofs to improve air quality and overall city aesthetics.

By this fall, Chicago will have 21,000 square feet of garden on top of their 12-storey city hall building, said John Beaudry, the city's chief planner, featuring a topography of hills that conform to the building structure, and more than 20,000 herbacious plants, vines, stretches of green grass, and a crab apple and hawthorn tree. The cost of the project in Canadian dollars is about $1.5-million.

Chicago assistant-project manager Kevin Laberge said poor air quality was the prime motivator for greening the city, where levels can exceed federal ozone standards as often as four times a summer.

In Toronto's east end, the Green Roofs coalition is planting a test site this summer on the roof of the gymnasium at Eastview Neighbourhood Community Centre to collect data on storm water run-off, temperature modulation and air-quality improvements. Another site at City Hall may follow if a $60,000 grant from a Toronto environmental group is approved.

Mike Dixon, head of the horticultural science division at the University of Guelph, said green roofs can significantly improve air quality. "There's absolutely no question. . .. This isn't revolutionary. Good heavens, this is how the planet works," he said.

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Cleaning air of toxins relies on a complex relationship between plants and microorganisms that coexist with vegetation. Together, they create a "machine that cleans the air," Mr. Dixon said, with plants breathing out oxygen and microbes removing harmful toxins.

Marie-Anne Boivin, a Quebec consultant who has installed 22 green roofs in urban centres across Canada -- mostly in condominiums, hospitals, and loft buildings -- sees the green-roof trend as "a general movement into the green mentality, with all the preoccupation with the environmental movement and climate change."

Steven Peck, Green Roofs' executive director, said in addition to improving air quality, green roofs absorb storm water runoff, provide recreational space, and modulate building temperatures by cooling in summer and warming in winter.

According to Mr. Carr-Locke, these were all reasons that persuaded Mountain Equipment Co-op, a company with an environmentally friendly, outdoor conscience, to jump on board.

"Just think of the benefits of putting all the oxygen back in to the air. Imagine if every roof had a green roof, what it would do to the city," Mr. Carr-Locke said.

Roofs are first reinforced to accommodate the weight of plants and a waterproof membrane ensures water does not leak through the ceiling below. A drainage layer, filter and a growing compound of absorbent red lava-type rock is spread over top and plants are introduced, naturally, by insects and birds, or artificially.

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Depending on the depth of the planting material, plants range from low-lying vegetation and shrubs, to deep-rooting trees.

Not every roof top is suited to the same type of landscaping, Mr. Peck said. They must be individually assessed for the right combination of plants.

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