Garden in the Sky

In Chicago and elsewhere, rooftops are coming alive with greenery

| March/April 2001

Gardeners and environmentalists in Chicago are moving into uncharted territory in their effort to moderate summertime heat: up on the roof. Rooftop gardens are sprouting on buildings throughout the city, redefining urban green space in the process. Chicago is only one of many places trying to capitalize on the fact that plants are natural air conditioners. Meanwhile, city dwellers are discovering that flowers, grasses, and trees also make great upstairs neighbors.

Discuss rooftop gardening at the Nature conference in Café Utne's:

'Rooftop gardening is going to become more popular as land becomes more precious,' says Liz Serritella, co-owner of Chicago’s Old Town Bed & Breakfast and a new rooftop gardener. 'I love to dig in the dirt. We had gardens before, and when we moved I couldn’t garden anymore,' says Serritella. Then she put in two rooftop gardens, to the delight of her guests.

Environmental designers would like to see the same thing happen on a much wider scale.

'Flying into O’Hare airport, you see acres and acres of bare roofs. They all could be green, and that could make an enormous difference,' says David Yocca, principal of Conservation Design Forum, a design group of landscape architects, ecologists, and botanists. The firm has been greening up a large area on the 38,800-square-foot roof at Chicago’s City Hall, building a garden that as soon as this spring will harbor as many as 150 different plants. Their efforts are part of the U.S. Environmental Protection Agency’s Urban Heat Island Reduction Initiative, which aims to reduce smog by lowering the temperature in five congested cities.

Like dark clothes, dark rooftops absorb the sun’s rays, heating city buildings and streets by as much as eight degrees. Hotter buildings need more air-conditioning, which leads to higher energy use and more fossil-fuel pollution. The heat also cooks the pollution, creating ozone-heavy smog. Other cities in the urban heat island project—Baton Rouge, Houston, Sacramento, and Salt Lake City—plan to use reflective roof surfaces to control the heat-island effect. Chicago is the only city testing whether green rooftops can accomplish the same thing.

Plants cool the air by releasing water vapors through their leaves, a process called evapotranspiration. According to computer models conducted by the Lawrence Berkeley National Lab, which is participating in the heat island project, widespread use of reflective surfaces and green roofs could reduce summer temperatures in cities by several degrees.