| Summary: | Extensive green roofs (roofs with low diversity surface vegetation and a thin layer of porous substrate) generally utilize a very narrow range of available plants in their designs; namely Sedum species. These plants share critical characteristics of drought resistant, low nutrient demand, vegetative propagation, and simple shallow root systems- making them highly tolerant to the extreme environment of the extensive roof. Sedum species dominate the extreme green roof plant palette as these are low maintenance, but their monotonous colour, texture and structure, combined with their minimal ecosystem services, result in the creation of green roofs of low aesthetic value and few environmental benefits. Sedum species belong to the Crassulaceae family of mainly drought tolerant succulent plans. Mexico is a bio-diverse hotspot of Crassulaceae. By developing a plant selection methodology based on a combination of climatic classification systems with temperature and precipitation information, a broad range of Mexican Crassulaceae species are identified as potential candidate species to improve and diversify the extensive green roof palette. This selection methodology is applied to Mexican Crassulaceae for two specific study sites of highly contrasting climates (Cwb and Cfb according to the Köppen map of climate classification) in Mexico City, Mexico and Sheffield, UK respectively. Candidate species performance and survival are investigated in screening and competition experiments over several growing seasons comparing plant responses to substrate depth and planting season. These species response are then used to determine the efficacy of the plant selection methodology in the identification of possible candidate species for expanding the extensive green roof plant palette.
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