University of Cincinnati pilot study revealed plants that performed well on free roofs without supplemental irrigation.
(2012) University of Cincinnati pilot study to determine suitable extensive green roof plants. Two plants survived without supplemental irrigation: Allium cernuum, a native, and Sedum acre, native to Europe. Others, Aster ericoides, Euphorbia corolla and Lysimachia lanceolata did not survive on rainfall alone. Researchers selected the plants because they are native to prairies and meadows where sunny, dry conditions are the norm. Reasons for survival of the Allium and Sedum are their shallow root systems and the ability to efficiently use water in hot, dry conditions. Allium stores water in its fleshy, bulb while Sedum has thick foliage and the ability to close stomata during the day to retain moisture. Stomata open at night so the plant can complete part of the photosynthesis process. Stormwater runoff is an important aspect of green roofs. The pilot study also looked at which plants absorbed more water. The test showed no significant differences between species, however plants receiving water only from rainfall retained 51% of the rainfall, while plants supplemental irrigation retained 44%.