On the influence of plant morphology in the extensive green roof cover: A case study in Mediterranean area

The success of green roofs in Mediterranean areas requires the selection of fast development plant species, able to grow in a shallow soilless substrate also under dry climatic conditions. In this context, the exploitation of native species from marginal sites characterized by limited availability of nutrients and water could be a strategy to select plants. In this work, various species, belonging to the Crassulaceae family from Central Mediterranean and sub-tropical regions, have been evaluated for their growth performance on extensive green roofs outdoor in Central Italy in mono and mixed culture. The morphology, i.e. branched or rosette type, resulted a fundamental trait for the coverage and species with prostrate branches, presented higher plant cover in the first year after transplant respect to the “rosette” type in monoculture. In particular, the Mediterranean native branched species S. album, S. rupestre, S. sediforme and the “rosette” Sempervivum tectorum presented a good performance under the seasonal variations, with high cover ability or biomass production after 15 months. In the mixed cultures, these species presented a different performance. S. album resulted highly competitive and “aggressive spreaders” respect to the other Sedums and may be suitable for monoculture or in combination with low covering shrub species. Among the “rosette” species, the allochthonous subtropical Aeonium castello paivae Bolle presented a good surface coverage and biomass production respect to the branched species while the native Sempervivum tectorum (Griseb. & Schenk) L. presented low covering ability in all the combinations. However, the introduction of “rosette” species in combination with “branched” ones (Sedums) could increase the diversity and aesthetic appearance of the roof.