The promotion of green roofs for their wider value for biodiversity has received great attention over the past five to ten years. Indeed, in a number of European cities, the main driver for implementation of green roofs is their habitat value, particularly where development on the ground is destroying or threatening protected or rare wildlife sites. In many ways, consideration of biodiversity has revolutionised green roof design. A particular approach has developed that has lead to certain types of green roof being referred to as ‘biodiversity roofs’, and a set of rules has emerged (closely related to concepts of restoration ecology) that typify an approach that is good for biodiversity. Whilst this may be a positive development, many such roofs are dominated by ecological ideas, and there is little consideration of aesthetic factors, or of human preference. This paper reviews some of these rules or assumptions, using the latest scientific evidence, and evaluates their direct application to the green roof context. Specific aspects covered include: the use of native plant species only (assumed to be better fitted than non-natives to regional climates, and to support a wider range of other life than non-native plants); the use of locally-characteristic plant communities (assumed to provide local distinctiveness, and again because such communities are better fitted to local conditions); the use of local-provenance material and local ecotypes (assumed to preserve local genetic diversity and locally adapted populations); and the use of local soils and substrates (assumed to be better fitted to local conditions, to best support local vegetation, and to have lower ‘embodied energy’ than other substrates). Many of these assumptions do not hold true in pure ecological terms, but they are highly beneficial in creating distinctive green roofs that reflect local conditions. Finally, a framework is set out that considers aesthetic as well as scientific criteria when designing green roofs for biodiversity.
Keywords: green roofs, biodiversity, habitat, local species, local communities, substrates