While the idea of green roofs sounds lovely and eco-friendly, keeping those plants alive on your rooftop is easier said than done. That’s the message given the ecology meeting today by Colleen Butler, who’s been growing experimental green-roof plots at Tufts University in Boston for the past few years.

Atop the campus library she’s been planting various species to see which ones do best. “People assume that if it grows in your garden, it’ll grow on your roof,” she says. But the high temperatures and variable rainfall can often do in even the best-meant plants. One that seems to do well is the Sedum species of low-growing flowering plants. In fact, Sedum might even be bullying out its neighbors in certain circumstances.

Butler’s test plots suggest that when drought comes, Sedum might help its neighbouring plants survive. But in normal rainfall years, it seems to out-compete its neighbours and hog the green roof all to itself. The lesson, she says? Be sure you know what you’re planting up there.

Image: The California Academy of Sciences’ green roof, in San Francisco