Use of evergreen grey-leaved shrubs from Mediterranean garrigue for greening extensive living roofs

Green roofs are a successful solution against the damages from the cementification of the territory, able to provide many well recorded tangible and intangible public benefits at environmental, economic and social level, particularly in cities. However, their performance is linked to plant growth, in turn related to species-technology-environment relationship. In the Mediterranean Regions the garrigue plants could successfully replace sedums in extensive green roofs where organic matter poor, well-draining and stony substrate ensures an environment closed to the garrigue pedo-climatic conditions in the wild. Moreover, the use of evergreen cushion-shaped shrubs, dormant in hot and dry seasons and growing through winter and spring can change the roof garden aesthetics and functionality. The trichomes, typical of plants with grey foliage, could also play a part in capturing pollutants and atmospheric dust deposition. The growth of 12 grey-leaved shrubs from garrigue landscape (Artemisia maritima, Ballota acetabulosa, Cistus crispus var. procumbent, Convolvolus cneorum, C. mauritanicus, Dorycnium hirsutum, Helicrysum italicum, Obione portulacoides, Pallenis maritima, Santolina chamaecyparissus, Stachys byzantina and Senecio leucostachys ‘Vira vira’) and the evolution of plant association (including Sedum album and S. reflexum) were tested on a experimental roof in the Bologna city, using modules with 9 cm deep volcanic tuff for extensive green, white gravel mulching and without irrigation. After 4 years of growing and 4 arid months per year on average, only 20% of Ballota and Convolvolus cneorum plants survived. Helicrysum, Santolina and especially Cistus resulted the most suitable species, with a survival around 60, 80 and 100% and a canopy diameter up to 61.0, 60.3 and 104.1 cm, respectively. No species self-seeded freely, except for Dorycnium and Helicrysum. At the end of the first year, the heavy metal content in leaves did not differ from that of plants grown outside the urban area with a similar green roof system.