Green Roofs: Developing a Native Plant Palette for Southern California

Green roofs offer a variety of ecological benefits that include decreased storm water runoff, reduced pollutant loading of storm water, and reduced urban heat island effects. This study investigated suitable plant species for green roofs in southern California. Four treatments were selected: plant species native to southern California, with and without irrigation; and plant species typical of green roof industries, with and without irrigation. A database of plant species native to the Southwest California region of the California Floristic Province was compiled and used in the selection of the native species. Mesocosms for treatments were constructed using large plastic containers. Each mesocosm was lined with a drainage layer and a moisture layer (Green Roof Solutions, Inc.), and then filled with a soil mixture consisting of perlite, potting soil, and lava rock (4:5:1 by volume). Industry-standard mesocosms were planted with sedums (Sedum angelica, Sedum brevifolium, Sedum spurium). The native species used consisted of a mix of succulents, low-growing perennials, and a bunchgrass (Achillea millefolium, Artemisia californica, Baccharis pilularis, Dudleya edulis, Dudleya lanceolata, Dudleya viscida, Erysimum menziesii, Nasella pulchra). Succulent species showed greatest survival across all treatments. In the absence of irrigation native succulent species had greater survival than the non-native sedums as the latter were adversely affected by heat-stress.