Green roofs: Performance of native plant species in a multifunctional perspective for the mitigation of impacts in urban environments

Research in landscape planning highlights the benefits of urban green systems' ecosystem services, and the need for integrated approaches to urban green planning that can adapt to different local conditions worldwide. Green roofs can reduce urban heat, improve air quality, and manage stormwater: research indicates that plant selection and substrate management are crucial for optimizing these performances. However, relatively few works have studied the effects of combined factors on the growth of native medicinal plants suitable for low-maintenance, multi-purpose urban GRs. Filling this gap is essential to increase the knowledge for an integrated approach to urban planning. This study performed an interdisciplinary, multi-factorial evaluation of the performances of three native medicinal forbs suitable for urban GRs: Trifolium repens, Melissa officinalis, and Hypericum perforatum. They were grown on experimental GR modules, comparing single-species vs. mixed-species cultivation, and chemical fertilizers vs. mycorrhizal bio-stimulants. Biomass, cover ratio, chlorophyll content, and metabolite production were measured. Results showed that these species can be effective for sustainable management of GRs: T. repens and M. officinalis may enhance multi-purpose GR's functionality, due to their production of secondary metabolites in quantities comparable to those of plants grown using other cultivation methods, while the fast growth and uniform cover of T. repens make it suitable for extensive green roofs, if properly managed. Mycorrhizal bio-stimulation was also effective on M. officinalis and H. perforatum growth, although to a lesser extent than chemical fertilization. Overall, the results proved that low-maintenance, native medicinal plants can perform well on GRs, provide sustainable, multifunctional solutions, and produce useful quantities of secondary metabolites.