"By literally replacing the land that the building displaces," says David, "we can help mitigate a lot of those problems that development causes." David says green roof materials can absorb up to 40 percent of the water from a regular rain event, and then slowly release it.

At the Flower Show, Temple's exhibit features a small wetland area created below one end of the roof, which can treat water with natural process, and an infiltration trench on the other side, where water can slowly filter into the ground.

David says preventing stormwater runoff and sewer overflows is just one of the benefits of green roofs. They can also improve air quality, provide natural habitat for wildlife, and even wind up saving money in the long run, although they are more expensive to install than traditional roofs.

"Basically it's the ultraviolet radiation of the sun that breaks down conventional roof systems," says David. "And with this vegetative roof layering, you're basically absorbing all the sun's energy through that and protecting the underlying drainage membrane. So it's not uncommon that a green roof will last decades longer than a conventional roof system."

A green roof actually consists of a special growing medium, made of organic and inorganic materials, but especially formulated for roofing. It is not the same thing as soil. A proper system also requires a strong structural base to support the weight of the roof. But once in place, a green roof requires very little maintenance. Regular rainwater is all that's needed to keep an established roof healthy.

"The growing medium can actually absorb up to 40 percent of its volume in rainwater," says David, "so the smaller, more regular, frequent events which contribute to the large volume of precipitation will actually be held in the roof system. And it's only during some of the larger storm events that you actually get some of the water draining through and running off."

Because of the roof's ability to reduce stormwater overflow, the Philadelphia Water Department has also taken an interest in the technology. Water Department Spokesperson Ed Grusheski says runoff can have dramatic effects on Philadelphia's drinking water, supplied by the Schuylkill and Delaware Rivers.

"Some of the sewers go directly into a local stream," he says, "and 80 to 90 percent of the pollutants in the rivers today are from stormwater runoff. So we're making great efforts throughout the city to control that, and gardeners can be a great help in that respect."

Grusheski says when water falls off a typical rooftop or paved surface, it can rush along the ground, picking up pollutants as it goes, before entering the sewer system or nearby streams. Green roofs provide an attractive way to slow the water down.