Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs

The growing desire to make the urban environment more sustainable from an ecological point of view has stimulated research on the architectural and agronomic aspects of green roofs. The practical realisation of green roofs, is however limited by economic and ecological issues. More specifically, water availability is the most limiting factor, and is likely to be ever more so in the future in the light of climate change. For this reason, we evaluated the agronomic performance of several xerophytes in a simulated dry green roof. Seeds of 20 species were collected in typically dry habitats (abandoned quarries, rocky soils, dunes, etc.) and studied in the laboratory for germination ecology. In cases of strong dormancy, methods were tested to stimulate germination and their germination ecology was studied. The resulting seedlings were transplanted in spring 2008 in two green roof types that differ in substrate depth (150 and 200 mm) made up of lapil, pumice, zeolites and peat, resting on a drainage layer of hydroperlite. Temperature and humidity in the substrate and drainage layer were measured during the whole test period. Survival of the seedlings in both substrate depths was almost 100%, favoured by a rainy spring. Most of the tested species showed an excellent performance during the hot and dry summer months in terms of survival rates, growth, and vegetation cover dynamics, notwithstanding the difficult ecological conditions (temperatures around 50°C; hydric potential Ψ -15 bars). Furthermore, most of the species had a long flowering stage in the first year of growth. Plants in the green roof with the deeper substrate depth produced, for most of the tested taxa, a significantly higher vegetation cover and growth compared to when they were placed in the 150 mm substrate. The results of this study show that some Mediterranean xerophytes have biological characteristics suitable for their use in dry green roofs, although an irrigation system for emergency use seems advisable. To conclude, further research should focus on long term evaluation of green roof vegetation in terms of plant survival and flowering dynamics.