But the city says it has had no applications so far for the project, which was announced in February.

"The goal of it as a pilot project is to see what it would take to encourage more green roofs in the city," said Liora Zion Burton, a Toronto environmental planner. "But we can't really penalize developers for not putting green roofs on the buildings because of practical difficulties."

Environment Canada projects that the greening of 5 percent of Toronto roofs would cut the city's average temperatures by up to two degrees Celsius, or up to 2.6 degrees Fahrenheit. The roofs are cooler, and the rooms under them are cooler as well.

Buildings are responsible for up to 27 percent of Canada's greenhouse gas emissions and up to 30 percent of its energy use, so a grass-planted roof can help.

Green rooftops lower the "heat island effect," where city temperatures are above those of the surrounding countryside. They help eliminate toxins from the air, lower the heavy metals in rainwater, reduce water runoff by up to 90 percent and reduce levels of smog and ground ozone in the atmosphere.

One of Toronto's green-on-top buildings is the luxury Fairmont Royal York Hotel, which grows herbs in a 3,600 sq. ft. rooftop garden and offers patrons a C$130 tour of the facility, coupled with a three-course lunch.

"There's a lot of different reasons that we do it. It's a pleasure growing herbs, but really we could buy herbs for a lot cheaper than we can grow them. That's not really the motivating factor," said Chef David Garcelon, standing among beds of organic basil, chocolate mint and lemon balm, a single apple tree and just-ripening cayenne pepper plants.

"Certainly because it's good for the environment. Fairmont is a strong supporter of green programs. This has been here for many years and we're actually considering expanding and adding some more green space up here."

The herbarium, which was started in 1998, is one of a growing number of urban gardens high above Toronto's bustling downtown sidewalks.

The city has no precise figures, if only because the gardens are high out of sight. But a recent city study counted 59 existing public and private green roofs and 17 more planned or under construction.

The city lags places like Germany, which levies penalties on those who build rooftops that cannot be green-adapted or Chicago, where a green roof speeds the permissioning process.

Environmental planner Burton said problems include the suitability of buildings for a heavy garden, as well as the ability of building managers to install and maintain the plants.