Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Key Factors in Plant Selection for Green Roofs Nigel Dunnett Department of Landscape, University of Sheffield The Green Roof Centre, University of Sheffield n.dunnett@sheffield.ac.uk Introduction Academic research in the green roof field is a relatively young discipline, and within that, studies on green roof plants and vegetation selection are one of the more recent research topics to develop a substantial literature. Of course, research and trials into suitable green roof plants have been carried out for many decades, but the output and publication of that research was largely confined to technical reports, internal company publications, and technical and ‘popular’ journals and books. Crucially it was published only in the German language. Such research was most often based upon observation and description, and experiments were not designed with sufficient rigour to be publishable in international peerreviewed academic journals. The first peer-reviewed scientiflc journal article to be published on green roof plants was by Boivin et al in 2001. This was a very focused piece of work that considered the sensitivity of a number of a number of extensive green roof species to frost injury in a harsh winter climate at different substrate depths. The next published journal paper did not appear until 2004, and was a Sheffield-based piece of work that investigated the influence of irrigation and substrate depth on around 30 plant species (Dunnett & Nolan 2004). This study introduced the concept of competitive interactions as a mediating factor in green roof plant survival. While much early published work concentrated on Sedums and other succulents, other ground-breaking papers included Monterusso, Rowe and Rugh (2006) who, for the first time, published a specific assessment of native vs non-native plant species for green roofs, and Lundholm (2006) who introduced the concept of habitat templates and reference plant communities as a basis for plant selection. While these research strands continue, the early focus on individual species and genera has evolved into a more complex viewpoint that considers green roof vegetation as a functioning system, with different life-forms and components contributing to the longevity of the vegetation on all but the most extensive of thin substrate layer green roofs. Dunnett (2008) and Kohler (2006) were among the first to highlight the importance of a long-term view of the dynamics of green roof systems.
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield Without doubt, the key factor in plant selection is tolerance of the prevailing moisture regime within the green roof (which in turn is a combination of climatic variables, substrate and system moisture holding capacity and water availability characteristics, and the depth of available substrate or the system buildup). Beyond these issues of survivability come other factors such as biodiversity value, origin and nativeness, functional characteristics, aesthetic quality and flowering phenology. But the increasing availability of long-term studies points to other emerging factors that are likely to play a more prominent role in the consideration of green roof vegetation as a functioning ‘ecosystem’: longevity, ecological strategy, competitiveness, regeneration characteristics, life-form, and invasibility, are all part of a more sophisticated view of the living elements of a green roof as a functioning whole, rather seeing successful plant selection as the sum of the individual species that are tough enough to survive the hostile growing environment. Previous plant selection research at the University of Sheffield (which looked at a wide range of different plant functional groups that can contribute to the overall green roof ‘plant community’) informed the choice of species used in a number of real green roofs established in the last 10 years in the Sheffield region. In turn, monitoring of these roofs over the last ten years has indicated how the ecological and functional characteristics of those component species contribute to the long-term sustainability of green roof vegetation. Finally, those insights informed the experimental design and the selection of the ‘long lists’ of plant species for inclusion in the Marie-Curie green roof systems research programme.
Further Reading Boivin, M, Lamy, M, Gosselin, A. & Dansereau, B. (2001) Effect of artificial substrate depth of the freezing injury of six herbaceous perennials grown in a green roof system. HortTechnology 11(3) 409-412. Dunnett, N. & Nolan. A (2004) The effect of substrate depth and supplementary watering on the growth of nine herbaceous perennials in a semi-extensive green roof. Acta Hort 643: 305-309 Monterusso, M., Rowe, B., & Rugh, C. (2005) Establishment and persistence of sedum species and native taxa for green roof applications. HortScience 40 (2): 391-396 Lundholm, J. (2006) Green Roofs and Facades: a habitat template approach. Urban Habitats, Dec 2006, http://urbanhabitats.org/v04n01/habitat_full.html Kohler, M. (2006) Long-term vegetation research on two extensive green roofs in Berlin. Urban Habitats Dec 2006, http://urbanhabitats.org/v04n01/berlin_full.html Dunnett, N, Nolan, A. & Nagase, A. (2008) The dynamcs of planted and colonising species on a green roof over six growing seasons. 2001: 2006: influence of substrate depth. Urban Ecosystems 11 (4): 373-384.