Green roofs are subject to extreme environmental conditions, including prolonged periods of drought, high temperatures, and intense wind (Dunnett and Kingsbury, 2004; Getter and Rowe, 2006). In addition, the shallow (<15 cm) and lightweight growing substrates typically used on extensive green roofs do little-to-moderate temperature fluctuations, retain moisture, or allow plants to tap deep moisture reserves (Dunnett and Kingsbury, 2004). Consequently, a rather narrow range of drought-tolerant succulents [particularly stonecrop (Sedum) species] have been the dominant plant selections for extensive green roof applications (Getter and Rowe, 2006).
However, expanding the plant palette available for green roofs is desirable. Green roof vegetation composed of diverse growth forms can have higher stormwater and cooling performance relative to less diverse roofs (Lundholm et al., 2010). Also, the use of regionally native plants could help restore wildlife habitat and native biodiversity in the urban environment (Dunnett and Kingsbury, 2004; Snodgrass and Snodgrass, 2006; Tallamy, 2007). This use of regionally native plants is often an explicit goal of many green roofs, and native species have been successfully used on green roofs where designs incorporate natural soils to enhance their similarity to native habitats (Brenneisen, 2006). Yet, identifying suitable native species is not straightforward because of the unique environmental conditions on roofs and the constraints on substrate composition. Plants that display suitable traits in their native contexts may not do so in the context of an extensive green roof. For instance, many prairie species achieve drought tolerance through their extensive root systems associated with specific microbial symbionts (Snodgrass and Snodgrass, 2006). Alpine plants can tolerate extreme temperature fluctuations; yet, they often cannot tolerate the absolute highs encountered on an urban rooftop.
In addition, regional differences in climate can impose unique constraints on plant functional performance and the associated management requirements. The design of the living component of green roof systems should be tailored to these specific regional differences and to the specific functional goals (Simmons et al., 2008). Although there has been much work exploring these design relationships for roofs in northern Europe and the eastern United States, we know considerably less about the optimal design strategies for other regions and climate regimes (Dvorak and Volder, 2010). For instance, in temperate regions, current guidelines suggest that irrigation is needed only for plant establishment and that, with proper design (e.g., plant selection, substrate type, and depth) there is no need for permanent irrigation (Dunnett and Kingsbury, 2004; Getter and Rowe, 2006; Miller, 2003; Snodgrass and Snodgrass, 2006). However, it is far from clear whether this prescription holds true for green roofs in semiarid and arid climates. These climatic zones cover more landmass than any other climate grouping; yet, we have very little information about proper water management for green roofs under these conditions.
Despite its rainy reputation, the Pacific northwestern United States is dry through most of the summer. For example, Portland, OR, has an annual average rainfall of 37.5 inches, but only an average of 3 inches of this falls during the months of July, August, and September (Hale, 2009). Given typical extensive green roof substrate types and depth, irrigation may be necessary to maintain plants other than the most drought-tolerant sedums. In an effort to provide some guidance and to limit the potential use of irrigation on green roofs, Portland's Bureau of Environmental Services restricts irrigation on all new green roofs seeking to qualify for its floor area ratio (FAR) bonus. The FAR bonus loosens building height restrictions in the Central City District for buildings with green roofs. Building owners–awarded FAR bonuses are required to apply no more than 0.5 inch of irrigation to their extensive green roofs every 10 d during plant establishment and 0.25 inch every 10 d following establishment (T. Liptan, personal communication). These guidelines were established from observations on a single extensive roof setup, and there is a need for more comprehensive data on plant performance across a range of irrigation levels. No studies have investigated the efficacy of current water management practices in a summer dry environment such as the Pacific northwestern United States or evaluated the performance of green roof plant selections within the context of a specific water management regime.
This study tested the performance of a selection of Pacific northwestern U.S. native and non-native plant species under three different summer irrigation regimes. We also evaluated expected water use and associated management costs under each irrigation regime.
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