Evaluation of potential green roof plant species in a Mediterranean climate

Abstract

In Northern Europe green roofs have been well known for decades, but in Italy - especially in the central and southern parts of the country- they have only recently become an object of study. Because green roofs are frequently subject to stress factors such as wind speed, heat, drought etc., they can represent an extreme environment for a plant’s survival. It is arguable that a pivotal element for the success of a green roof is the selection of the plant species, especially in a Mediterranean climate where summer is generally hot and dry. Good results might be achieved utilizing native species, which are well adapted to the local environmental conditions and play a considerable ecological role. The aim of the present work consists in evaluating a range of native and commercial perennial species and investigating the thermal effect of green roofs in a Mediterranean climate. Six experimental plots with two different substrate depths (8 cm and 15 cm) both in the presence or absence of a vegetation layer were assessed. Growth parameters (growth index, cover percentage, appearance of plants, flowering) were taken; temperature data was collected on the substrate surface and beneath the experimental plots, in the air and on the roof surface. Preliminary results have shown interesting differences in the growth behaviour among the species. The native species seem to respond positively, with faster and better establishment compared to the commercial ones. Thermal data has strongly confirmed the mitigation effect produced by the green roof systems tested. In the summer season, the values recorded on the conventional roof surface (concrete) and beneath the experimental green roof plots differed by up to 20°C.