Weeds and gaps on extensive green roofs: Ecological insights and recommendations for design and maintenance

Abstract

While extensive green roofs (EGRs) are seeing increasing use around the world, the construction and maintenance "one size fits all" EGR solutions is under scrutiny. Apart from the search for local and adapted plant species that could complement or replace the widely used Sedum vegetation, practical problems such as weeds and gaps in the vegetation also still occur. Both weeds and gaps have been shown to impact EGR ecosystem service provisioning and preferences but have only received limited scientific attention. We therefore performed an in depth analysis of vegetation and local and regional environmental data gathered on 129 EGRs in Northern Belgium with a focus on understanding weeds and gaps. We found that gaps in the vegetation form a considerable issue on the studied EGRs, while weed cover is often relatively limited. Furthermore, we find that spontaneously colonizing plant cover is heavily impacted by substrate productivity and roof exposition, but is also influenced by planted species cover and the proximity of potential source habitats. For tree species cover, dispersal limitation plays an even more important role, while annual species are found to thrive on exposed roofs with a low planted functional diversity. Gap cover is found to decrease over age, and reduces with increasing substrate productivity. Finally, we provide design, construction and maintenance recommendations to prevent these problems based on our results and also discuss whether these issues should be considered a failure of EGRs as a system or as an opportunity for improved green roof functioning or the development of more 'natural' EGRs.