"Why Green Roofs Don't Work - and How to Fix Them"

Link to Scientific American

"Researchers are trying to identify the best plant species suitable to green roofs, with an eye to designing ones that fulfill their promise. 

"A 2007 study in the journal BioScience found that green roofs can potentially help manage stormwater runoff, reduce urban heat-island effects and regulate building temperature. To deliver these benefits, rooftop vegetation has to be able to survive the high winds, prolonged UV radiation and unpredictable fluctuations in water availability.

"To resist these harsh environments, a majority of green roofs are planted with sedum, a non-native species that can survive wind and long periods withoutrainfall.

"A roof planted with sedum, however, is no greener, from the standpoint of sustainability, than is ordinary tar or asphalt."