Many factors determine survival of green roof plant species including substrate depth (Dunnett et al., 2008a; Durhman et al., 2007; Getter and Rowe, 2009), solar radiation levels (Getter et al., 2009a), and climate with the limiting factor usually being substrate moisture (ASTM, 2006; Durhman et al., 2006; Emilsson, 2008; Monterusso et al., 2005; Wolf and Lundholm, 2008). Also, fertilization often promotes biomass accumulation which in some cases makes the plants more vulnerable to drought (Rowe et al., 2006). There is evidencethatevapotranspirationratesinfluencestormwater retention and energy conservation (VanWoert et al., 2005b; US EPA, 2009; Voyde et al., 2010)