Green roofs mitigate many negative environmental effects of urbanization, especially stormwater runoff and the urban heat island effect. There is also potential for green roofs to function as islands of biodiversity within urban and suburban environments. Historically most roofs have been planted with Sedum, a very stress-tolerant plant, but many people are promoting the planting of a more diverse set of plants, especially native plants. The performance of other species has been mixed and this necessitates greater focus on both patterns and mechanisms of plant growth and survival. In this dissertation, I began by reviewing and analyzing rationales for preferring native plants on green roofs. I identified 113 green roofs planted with native plants and 89 scholarly papers that promoted this practice. Scientific arguments were commonly used, but rarely tested experimentally. I then conducted a rooftop experiment to assess suitability of 19 native and non-native plant species. Summer water deficit resulted in high mortality of all but the most popular green roof species: Sedum (Crassulaceae). To determine if Sedum's high performance was due to photosynthetic plasticity, I grew Sedum under wet and dry conditions in a greenhouse. There was variation in photosynthetic pathway among the eight species tested, including examples of C₃, CAM-cycling, and CAM-idling. Furthermore, several species exhibited rapid switching in photosynthetic pathway in response to short-term changes in water availability. Finally, I tested the hypothesis that Sedum species would reduce peak soil temperature and increase performance of neighboring plants during summer water deficit. During a three-year experiment on the Tisch Library Green Roof, Sedum species decreased peak soil temperature by 5–7 °C. Overall, Sedum reduced neighbor growth during wet periods, but increased neighbor performance during summer water deficit. The results of this dissertation suggest that plant diversity on green roofs is constrained by abiotic stress, especially summertime water deficit and heat. Many Sedum species used on green roofs have high photosynthetic plasticity, which may explain their success as green roof plants. The palette of green roof plants can be expanded by using Sedum species as nurse plants.