Functional green roofs: Importance of plant choice in maximising summertime environmental cooling and substrate insulation potential

Green roof plants can reduce local air temperatures and heat load to a building in the summer, improving thermal comfort of residents. Little is known, however, about how different plants compare in their potential to provide these two ecosystem services. Consequently, this study investigated whether some plants can offer more potential summertime environmental cooling and substrate insulation than others. Over two summers (2012/2013), canopies of two succulent and four broad-leaved plant genotypes, with contrasting plant traits, were monitored alongside bare substrate in Reading, UK. Plants were studied outdoors within small plots (1.5 × 1.5 × 0.1 m). Continuous monitoring took place during warm days and nights and included variables (leaf surface temperatures) and fluxes (long-wave radiation, sensible heat flux and transpiration) that are indicative of cooling potential. The strength of substrate insulation was estimated by comparing the ground heat flux below the canopies to that of the bare substrate. Plant traits (leaf colour or thickness), structural parameters (height and leaf area index, LAI), radiative properties (albedo and emissivity), and stomatal conductance were also measured to help explain the differences in cooling potential among the species. Non-succulent canopies, in particular light-coloured ones, with high leaf stomatal conductance and high LAI provided maximum potential for substrate insulation and environmental cooling in hot periods, particularly compared to bare substrate and thick-leaved succulents. These results suggest that succulent plants are not best suited to provide significant summertime environmental cooling and substrate insulation and that others (e.g. Salvia and Stachys) might be preferable where the delivery of these benefits is a priority. Our findings highlight that, in addition to survival, aesthetics and cost, the plants’ ability to deliver a range of ecosystem services should be considered in the plant selection/green roof planning process.