Mediterranean Green Roof Simulation in Caldes de Montbui (Barcelona): Thermal and Hydrological Performance Test of Frankenia laevis L., Dymondia margaretae Compton and Iris lutescens Lam

Green roofs provide a number of environmental advantages like increasing urban biodiversity, reducing pollution, easing burdens on drainage systems, and lowering energy costs thanks to thermal insulation. Frankenia laevis, Dymondia margaretae and Iris lutescens were tested in a green roof installation. For all three species, we assessed two minimal irrigation treatments and one rain-fed treatment to resemble Mediterranean climate conditions analyzing the thermal and hydrological performance of all three species and their substrates through an evaluation of green cover, mortality, and biomass. The most influential factors registered for all three species are the relationship between air and water in the substrate and the interaction between green cover and substrate, respectively, for summer and winter seasons. In particular, D. margaretae preserved more water in its substrate than the other species both in summer and winter and after each rainfall event. F. laevis registered the highest level of variation in terms of substrate water content and of rainwater retention. I. lutescens achieved low hydrological performance, a limited amount of green cover, and slow growth. Our results suggest the absolute need of additional irrigation, managed in accordance with specific functional objectives, for all three species analyzed under Mediterranean conditions and different water regime.