Green roofs are building surface treatments where plants are grown in medium on a rooftop (DeNardo et al., 2005). Green rooftops can cool dwellings in warm climates and insulate buildings in cold climates (Shafique et al., 2018). The earliest examples of green roofs, between the seventh and eighth century bce in present-day Syria, were installed to cool dwellings (Oberndorfer et al., 2007). The Greeks, Romans, and Persians also installed green roofs to cool buildings (Dunnett and Kingsbury, 2004). In contrast, Scandinavian “turf-roofs” were installed until the last half of the 19th century to insulate homes during the winter from the cold (Jenkins 1977; Snodgrass and Snodgrass, 2006). Modern green roofs gained popularity in the 1960s in Germany in response to the energy crisis as a tool to reduce building energy consumption (Shafique et al., 2018).
Modern green roofs were described as “extensive” (<20 cm medium depth) or “intensive” in 2007 (>20 cm medium depth; Oberndorfer et al., 2007). Since then (2017), green roof systems were further divided and described as extensive (4–12 cm), semi-intensive (12–30 cm), and intensive (>30 cm; Lata et al., 2017). Adding a green roof to an existing building often involves installing an extensive green roof system that usually does not require building structural modification for additional weight (Oberndorfer et al., 2007). In contrast, adding an intensive or semi-intensive green roof to an existing building can require additional building structural modification for the additional weight of the medium, plants, and retained water (Oberndorfer et al., 2007).
An unirrigated extensive green roof can be a challenging environment for plants to grow in as temperature, light, wind, and/or drought stresses are more severe and/or frequent on a rooftop than in-ground plantings (Brown and Lundholm, 2015; Dunnett and Kingsbury, 2004; Shafique et al., 2018). Because weather can differ with region, which species perform well on a rooftop in different regions can vary (Dvorak and Volder, 2010; Getter and Rowe, 2008; Shafique et al., 2018). Many green roof studies have been conducted in temperate regions of North America and Europe often moderated by water bodies (Getter and Rowe, 2006; Köhler, 2006). Recommended species varied tremendously among sites and included species as diverse as succulents, grasses, and mosses. However, many species recommended for extensive green roof systems from these studies die on rooftops in the north-central region of the United States (Minneapolis-St. Paul, MN; personal observation). Little information is available on which species survive and thrive on an extensive green roof system in a dry, northern climate such as Minneapolis, MN.
Aside from identifying which species survive and thrive on a rooftop in a cool, dry climate, there is increasing interest in identifying plant species that provide aesthetic and/or ecological benefits to a rooftop (Nagase and Dunnett, 2012; Speak et al., 2012). Different plant species can provide color and/or texture to a roof, can capture air particulates, can be edible, and/or can support pollinators or other native insects such as butterflies (Baraldi et al., 2018). A mix of species on a roof can deliver multiple aesthetic and ecological outcomes (Lundholm, 2015; Lundholm et al., 2010). We initiated a multiyear study to 1) characterize medium and weather conditions on rooftop in a cool-dry climate and to 2) identify plant species with horticultural and/or ecological attributes that survive and thrive on an unirrigated extensive green roof in a cool-dry climate.
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