The use of green roofs in the Mediterranean area is becoming more widespread. Use of the species which are most commonly used across the world can be limited by the specific conditions of the Mediterranean climate. An experimental green roof simulation tested the performance of 12 species divided into three groups according to growth form (ground cover, sub-shrub, rhizomatous-herbaceous). Different irrigation criteria were applied to each group: in two sections potential evapotranspiration (ET0) levels of 20% and 40% were applied, while the third section was rainfed. Mortality, green coverage, blooming and final biomass were measured. The amount of water received and the low winter temperatures affected plant mortality. Significant differences in coverage were observed when comparing the rainfed section with the two irrigated areas. Ground cover plants in the irrigated sections achieved the best results in terms of coverage. Significant differences in biomass were observed only when comparing the irrigated plots with the rainfed plot. The influence of different irrigation protocols on flowering was observed in five species only. The growth form groups responded in different ways to drought stress: rhizomatous-herbaceous developed slowly and consistently; ground cover developed quickly and relied more on the water supply; sub-shrubs showed lower resistance to drought stress under the present experimental conditions. Of the 12 plants tested for green-roof adaptation in a Mediterranean climate, those which performed best were: Centranthus ruber (L.

The green roofs sector is a relatively recent phenomenon in Mediterranean countries, meaning that as yet there is no complete understanding of which plants are suited to this particular environment. Such plants would need to be adaptable to the green roof itself as well as to the drought and intense lighting typical of a Mediterranean summer. Two simulated green roofs were planted with a variety of species and life forms and subjected to two minimal irrigation treatments and one treatment without irrigation. Mainly subshrub species were planted in one simulation, while the other contained a prevalence of groundcover species. The study looked at performance in terms of species, life forms, and simulation. We analysed flowering, mortality, frequency of planted species and of invasive species, and biomass. We took periodic measurements of cover and of the Shannon-Wiener vegetal diversity index. The groundcover species obtained a higher degree of cover, but suffered more from seasonal stresses. Both green roofs saw an increase in the abundance of a few species, leading to a simplification of the original design. Diversity and the ability to host colonising species were influenced mainly by differences in vegetational structure. Of the subshrub species, Centranthus ruber and Helichrysum stoechas performed best, while Frankenia laevis and Thymus serpyllum came top among groundcover species. In order to withstand severe drought stress, a minimal amount of irrigation was found to be necessary. Our study demonstrates that it is possible to obtain good results from a Mediterranean green roof by using an appropriate combination of vegetal species with different structures and development.

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.