Green roofs are being incorporated on buildings worldwide but there is a lack of information about plant species in terms of which are suitable for green roofs within a particular climate, how to use certain species in combination with other species, and which species are appropriate for specific production systems. Green roof regulations have reached the municipal level in cities such as Toronto, ON, Canada (City of Toronto, 2009) and the federal level in countries such as Germany (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau, 2008). Germany and much of Europe are advanced in green roof knowledge and implementation (Dvorak and Volder, 2010). However, in North America, much more needs to be learned about creating successful green roofs that are specifically designed for its unique northern climate.
Green roofs are currently in high demand in North America (Carter and Fowler, 2008; Green Roofs for Healthy Cities, 2013). To both meet these demands and maintain the attention and satisfaction of the green roof market, various green roof research has been, and is still being, performed to improve the green roof industry. Such research includes studying the overwintering survival of stonecrop species treated with various rates of fertilizer (Clark and Zheng, 2012); determining the optimum growing substrate pH for commonly used stonecrop species (Zheng and Clark, 2013); testing the performance of native plants to meet legislative regulations for natural landmarks (Hollanda and Wrixon, 2012); surveying green roofs in Ontario, Canada, to identify successful plant species for northern climates (Vinson and Zheng, 2013a); and comparing different life-form groups of plant species native to eastern Canada (MacIvor and Lundholm, 2011). However, more research is needed, especially for different green roof production systems, in creating a reliable and accessible list of plant species and species combinations that green roof companies can incorporate into their product lines.
Currently, more information is required to provide growers, green roof companies, and green roof customers with a wider range of options for nonsucculent (e.g., nonstonecrop) plant species, especially perennials, that can be used on green roofs, specifically for extensive green roofs in northern climates. Determining which species do not, or may not, grow successfully on green roofs will help eliminate the weaker species and form a robust list of useful species. MacIvor and Lundholm (2011) studied the percent vegetative roof coverage of plant species for green roofs in the Canadian Maritimes. The data from that study may be combined with data from this study to build toward a comprehensive guideline for green roof plant species usage in northern climates. With the use of plants comes maintenance, and therefore, costs; however, if species are chosen correctly so that they are not only suitable for the environmental and growing conditions of a green roof, but also well-suited for the surrounding plant species, then maintenance and costs can be reduced. Therefore, plant interactions must be studied to ensure a healthy and stable coexistence among species. Also, if nonsucculent, perennial species can be used successfully from the beginning of the green roof production stages, specifically for mat production, as opposed to being planted postinstallation of the green roof, then green roof installations can continue to provide instant greening. Providing instant greening without being restricted to the use of stonecrop species would be advantageous for a mat production system because it would mean that green roof companies could offer a wider selection of products to their customers (e.g., a variety of planting designs could be offered to address different visual and or ecological interests). This common green roof system uses prevegetated mats that typically consist (from bottom up) of a root barrier membrane, filter fabric, a layer of interconnected stands, growing substrate, and vegetation, and which measure only a couple of inches or less in thickness. Therefore, there is a need to test a variety of species in a mat production system to determine which species can grow on such shallow formats and endure the production practices leading up to and including installation; with this information the true convenience of the mats can be revealed.
If the mats can sustain various growth forms of different species, the application of mats for green roofs will be increased. This will help encourage more green roof installations, especially for roofs that are limited by load-bearing restrictions (i.e., applies to existing roofs being retrofitted and new roof projects that have insufficient funds for building the stronger support structures necessary for heavier green roofs), and will create more sale opportunities for green roof production companies because the potential arises for green roofs to appeal to a larger audience. Therefore, there is an opportunity to use new plants on mats that, together, will satisfy the green roof market.
In this study, a variety of plant species and species combinations were grown in a green roof mat production system and installed on a rooftop for further evaluation. The objectives were to 1) determine the suitability of a diverse selection of plant species for use in extensive green roof projects located in northern climates, 2) determine compatibilities between species, and 3) determine the appropriateness of the species chosen for a mat production system.
City of Toronto2009Article II: Requirement for green roofs. Chapter 492. 1 Mar. 2012. <http://www.toronto.ca/legdocs/municode/1184_492.pdf>
ClarkM.J.ZhengY.2012Evaluating fertilizer influence on overwintering survival and growth of Sedum species in an autumn-installed green roofHortScience4717751781
Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau2008Guidelines for the planning construction and maintenance of green roofing: Green roofing guideline. Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau Bonn Germany
Green Roofs for Healthy Cities2013Green roof industry grows by 24 percent in 2012: Washington DC is #1 - installing over 1.3 million square feet in 2012. 12 June 2013. <http://www.greenroofs.org/index.php/resources/2012-green-roof-industry-survey>
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MacIvorJ.S.LundholmJ.2011Performance evaluation of native plants suited to extensive green roof conditions in a maritime climateEcol. Eng.37407417
VinsonK.ZhengY.2013aPlant species recommendations for green roofs in northern climates: Based on survey. 22 Jan. 2013. <http://www.ces.uoguelph.ca/greenroof/research/GREENROOFSURVEY-Jan2013.pdf>
VinsonK.ZhengY.2013bSelection of plant species and species combinations for northern climates. 22 Jan. 2013. <http://www.ces.uoguelph.ca/greenroof/research/SpeciesSelection_Vinson_Zheng_2013.pdf>