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Plants, grass on the rooftop? No longer an oddity.

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Green roofs may be surprising in a city still more known for manufacturing than composting, but they are relatively common in Europe. Germany – the country that gave Mayor Daley the idea – has green roofs on about 20 percent of all flat roofs, according to one estimate. With a history dating back to the hanging gardens of Babylon, green roofs range from simple trays filled with hardy plants like sedum, to complex systems like the City Hall roof, which features some 150 plant species, a small apiary, and two trees.

In Chicago, they sit atop the Apple store, a Target, and a McDonald's. Even Chicago's soon-to-open Wal-Mart will have one – the company's first.

Nationally, green roofs grace the Gap headquarters in San Bruno, Calif.; a Ford Motor plant in Dearborn, Mich.; and the American Society of Landscape Architects building in Washington.

The idea is simple: bring back some of the organic material displaced by buildings, streets, and parking lots. Advocates tout benefits that range from reducing the urban "heat island" effect – which makes cities several degrees warmer than surrounding areas and can translate into millions of dollars in energy costs – to lengthening the life span of a roof, providing community garden or recreation space, and contributing to a building's energy efficiency.

"Cities are just going to keep getting hotter," says Steven Peck, president of Green Roofs for Healthy Cities. "So you take away hot surfaces and turn them into air conditioners. Green roofs do that very, very well."

On City Hall, for instance, the ambient temperature on the planted, city side of the roof is often 50 to 70 degrees cooler than that on the county side, still traditional black tar. Commissioner Johnston acknowledges that it's hard to know how many such roofs are needed before the effects become felt throughout the city, but he's determined to keep encouraging them until Chicago gets there.

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