Extensive green roofs: Plant evaluations and the effect of slope on stormwater retention

Green roof technologies, which entail growing plants on rooftops, are increasingly being used primarily to alleviate stormwater runoff problems and to save on energy costs. Proper plant selection for green roofs is of the utmost importance, for if the plants fail, stormwater benefits will be reduced and future green roof sales will be affected. Therefore, three experiments were performed which quantified stormwater retention and evaluated plant performance. The stormwater study quantified the effect of roof slope on green roof stormwater retention and found that roof slope does affect retention quantities. Curve numbers, a common method used by engineers to estimate stormwater runoff for an area, ranged from 84 to 90, all of which are lower than a conventional roof curve number of 98, indicating that all of these greened slopes reduced runoff compared to traditional black roofs. A season of planting study demonstrated the need for plants to have proper establishment time before a killing frost in order to overwinter successfully. Plugs of nine species of Sedum were planted in autumn and spring and then evaluated for survival the following spring. Overall, spring planting exhibited superior survival rates (81%) compared to autumn planting (23%). An initial growth study demonstrated the importance of substrate depth on plant performance. Of the species and substrate depths evaluated in this paper, a substrate depth of at least 7.0 cm is highly recommended.