How Green Is My (Rooftop) Garden? by Jerilyn Watson

Adapted from the VOA Special English Development Report, Broadcast: 18 October 2008

Green roofs are designed to save energy and capture rainwater. Rooftops covered with plants help keep buildings cool. They can extend the life of a roof. And they can reduce water runoff into streets and storm water systems. Experts say green roofs usually do not get much above 27°C. So temperatures might not be a problem for growing. But rooftop gardeners have to be careful not to put too much weight on the roof. This is true whether plants are grown in soil or water.

Earlier this year, researchers in Texas reported that green roofs can reduce a building's air conditioning costs by about one-fifth compared to black-top roofs. But they also reported that not all green roofs perform equally well. They found that designs differ in their ability to keep buildings cool and to capture rainwater.

Mark Simmons led the study for the Lady Bird Johnson Wildflower Center at the University of Texas at Austin. "Green roofs have to be done right," he says. For example, he says, "Just having a green roof may not mean anything in terms of preventing water from reaching the street level."

The team put experimental roofs on top of metal insulated boxes, to recreate green roof conditions. The study compared products from six companies. The researchers say they want to help businesses understand how to improve their designs. Each rooftop had sixteen different kinds of plants native to Texas. The researchers say the presence of native plants likely helped all the green roofs capture water better compared to sedums. Sedums are plants that need little water and often are used on green roofs. The native plants could take in more water and release more of it to the atmosphere.

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