Having a garden on your roof isn't just nice for a garden party; it can make your city more environmentally friendly. Many American cities are beginning to incorporate greenroofs into their planning ordinances because they recognize that, planting a rooftop garden can offset heat, increase city biodiversity and decrease stormwater runoff. This runoff can be problematic in cities where rainwater is funneled by streets and parking lots directly into streams, carrying with it chemicals and debris and increasing the risk of flash floods.

But the plants on greenroofs can absorb some of this water – "like a sponge being saturated," says Olyssa Starry, a graduate student at the University of Maryland at Baltimore County. Starry studied a greenroof atop a Baltimore building in comparison to a similar building without a greenroof to determine how well the roof would absorb water from frequent storms. By measuring water flowing out of building downspouts, she found that the greenroof retained from 30 to 75 percent of water from storms, compared to a negligible amount retained by the building with no greenroof.

Although her results are preliminary, Starry thinks that cities can reap benefits from making greenroofs a part of their building requirements, as cities like Toronto and Berlin have recently done. Using GIS satellite imagery, she estimated the number and area of buildings that could hold greenroofs within one watershed in the Baltimore area. If all these roofs were greened, she says, the city could save the watershed 8 million gallons of water per year, or about 10 percent of its yearly water loss.

"We need to understand what implementing these greenroofs at the whole watershed scale can do," she says. "Getting people to learn about this technology and providing incentives is the first step." This research was presented at the Ecological Society of America's Annual Meeting on August 3, 2009.

(Science Daily, 03/08/2009)