Enhancing biodiversity and functionality of extensive green roof: A comparative study of five native Mediterranean perennial species in Rome

Research widely demonstrates the usefulness of green roofs for energy saving and climate mitigation, such as their role in favoring biodiversity in urban ecosystems. However, challenging conditions for plant growth, particularly in Mediterranean regions with dry summers, require careful species selection. Through an in-situ experiment on a pre-existing extensive green roof in Rome, this research evaluated five native Mediterranean species (Helichrysum italicum, Convolvulus cneorum, Anthemis maritima, and Thymus vulgaris) in comparison with the commonly used Sedum album as a control. The 14-months experiment monitored plant health, reproductive stages, and coverage across nine experimental modules. Results revealed a superior performance of the Mediterranean species compared to Sedum album, with particularly good health and coverage rates of Convolvulus cneorum and Helichrysum italicum, that together Anthemis maritima displayed extended flowering periods, contributing to both aesthetic values. Thermal imaging analysis showed temperature reductions of 2.9–4.4 °C in green roof areas with respect to adjacent concrete surfaces and all planted areas contributed effectively to thermal regulation, with daily temperature fluctuations significantly reduced compared to conventional roofing. The poor performance of the Sedum album challenges its widespread use in Mediterranean climates and highlights the importance of selecting regionally adapted species. This research provides valuable insights for sustainable urban development, demonstrating that Mediterranean perennials can effectively enhance green roof performance while supporting biodiversity and improving thermal regulation in challenging climatic conditions.