Fly into LaGuardia Airport any summer afternoon and, depending on air traffic that day, your plane may traverse the skies directly over the island of Manhattan. It's an impressive sight, those thousands of buildings rising up from a slender finger of land. Now imagine looking down on a vast tapestry of verdant rooftops: acres of grass and flowering alpine plants.

According to Colin Cheney, director of the green roofs initiative at the nonprofit Earth Pledge Foundation, New York's architecture makes it an ideal place to realize this vision. "There are more than 170 million square feet of flat roofs in Manhattan alone," he says. If the densely packed city turned those asphalt squares into fields overhead, it would add nearly five Central Parks' worth of open space to the urban landscape.

The green roofs that Earth Pledge champions bear little resemblance to your average rooftop garden -- say, a few geraniums poking up from a flower box. Each of these lusher, more elaborate gardens requires the addition of up to seven layers of material, including waterproofing, a root barrier, and a drainage layer. All this is a little pricey: A green roof costs $15 to $35 per square foot, compared to $5 to $10 per square foot for typical roof construction, but Cheney points out that green roofs last twice as long as the nonsprouting variety, and their environmental benefits are well documented. Since rain soaks into the soil, green roofs can reduce storm runoff by 50 to 90 percent, thus preventing all that water from carrying oil, gasoline, and toxic chemicals into the sewer system. Green roofs also ameliorate the heat island effect. (On summer days, asphalt roofs can hit 170 degrees, and Manhattan can climb 8 degrees higher on the mercury than surrounding areas.)

These grand plans remain largely unrealized -- so far. Mayor Michael Bloomberg has spoken publicly in favor of green roofs (Seattle, Chicago, and Atlanta have already installed them atop their city halls) but stopped short of committing New York City dollars to fund a green roof program. Still, an 8,000-square-foot green roof is being installed atop a South Bronx housing complex, and Pace University is planning one for its campus in lower Manhattan. Even the Environmental Protection Agency is a fan of the idea. Last year, the agency gave Earth Pledge a $10,000 grant to help the group publicize its vision. "Our goal is to make a Big Green Apple," says Jane Kenny, EPA regional administrator. "We want it to catch on like the hula hoop."
-- Joel Gershon