Growth and spread of native perennial herbaceous species on a green roof

The European Communication on Green Infrastructures encourages the use of wildflower verges, green walls, green roofs, etc. to mitigate the artificiality of the urban and rural environment. Green roofs are engineered ecosystems occupying underutilized urban spaces that rely on the plant cover to provide services, such as the reduction of temperature, retention of storm-water and enhancement of urban biodiversity. In this framework we explored the ability of some wild perennial species of arid and semi-arid grasslands to survive and colonize the substrate of the extensive green roofs of the Agricultural Sciences School of the University of Bologna. In mid- June 2015 young plants of Festuca ovina L., Thymus serpyllum L., Hieracium pilosella L., Acinos alpinus (L.) Moench, Sanguisorba minor Scop. and Achillea millefolium L., coming from wild local populations, were transplanted into containers (54×54×9 cm depth) forming the upper cover of a green roof. The plants were fertilized and then irrigated throughout the summer of 2015. The growth of the species (coverage and space occupation) was monitored approximately once a month, from August 2015 to May 2016. F. ovina reached the highest values of coverage, without any significant seasonal variations. T. serpyllum and H. pilosella reached moderate coverage values depending however on the seasons. A. millefolium showed very effective dispersal abilities, but poor coverage potential. It can be used as a filler species in multi-species green roofs. Our results showed that the wild local flora can be an important, though still poorly explored, reserve of biodiversity for a new generation of extensive green roofs, designed, following a careful selection of species, for the best possible performances of the services they provide.