HORTSCIENCE 52(2):320–325. 2017. doi: 10.21273/HORTSCI11202-16 ponding. In the early 19th century, green roofs in Berlin did not use engineered media; rather, construction rubble was Green Roof Substrate Composition spread over tar paper roofs and the living systems developed overtime (Kohler and Affects Phedimus kamtschaticus Poll, 2010). Modern GRS composition is largely based on recommendations in the Growth and Substrate Water Content Forschungsgesellschaft Landschaftsent- wicklung Landschaftsbau (FLL), the German landscape industry’s guidelines for the de- under Controlled Environmental sign, planting, and maintenance of green roof systems. The FLL makes recommen- Conditions dations for particle size distribution and 1 organic content as well as specific physical Whitney N. Griffin , Steven M. Cohan, John D. Lea-Cox, properties such as water holding capacity, and Andrew G. Ristvey bulk density, and total porosity (FLL, Department of Plant Sciences and Landscape Architecture, University of 2008). Maryland, Plant Sciences Building, College Park, MD 20740 Beyond the basic FLL recommendations, GRS composition varies internationally and Additional index words. Phedimus, eco roof, living roof, evapotranspiration, plant available regionally, usually due to raw material water, Sedum kamtschaticum availability; however, the FLL recommen- dations have been adopted by municipalities Abstract. Phedimus kamtschaticus (Fischer) were grown in three experimental crushed and public entities around the world and brick-based green roof substrates (GRSs) with increasing organic matter (OM) content Ò applied to green roof components not (10%, 20%, and 40% by volume) and a commercially available blend, Rooflite ,in considered in or by the FLL. North Amer- single-pot replicates in a growth chamber for 6 months. Three unplanted replicates of ican GRS are largely composed of manu- each substrate were included in the design and received identical irrigation volumes as factured lightweight aggregates—usually planted replicates. Three destructive harvests indicated that increased substrate OM Ò slate, shale, or clay that has been kiln fired increased plant root and shoot biomass; however, plants grown in Rooflite demon- to create expanded mineral particles strated greater succulence in the second and third destructive harvests despite similar (Ampim et al., 2010). Particles of varying substrate OM content. By the end of the growth study, there was no difference in dry Ò diameter are mixed together to achieve ap- weight accumulation between the Rooflite and 40% OM treatment despite the propriate particle size distribution and phys- difference in succulence between the two treatments. Substrate volumetric water content ical properties such as water holding (VWC) ranged from 22.5% to below 5% during three consecutive periods of imposed capacity, total porosity, and bulk density water stress with no differences in evapotranspiration (E ), indicating plants were T (Handreck and Black, 2007). Although the accessing substrate water previously assumed to be unavailable. Cumulative water loss North American green roof industry largely (normalized for plant dry weight) indicated a likely shift into crassulacean acid uses manufactured aggregate for GRS, re- metabolism (CAM) around 60-hour postirrigation. Planted treatments (n = 6) lost more search from other countries indicates efforts water cumulatively (P < 0.05) compared with the unplanted controls (n = 3), although to use lower carbon recycled or natural there were no differences in total water loss between substrate treatments. materials for the inorganic component of GRS. For example, New Zealand GRSs are As researchers continue to investigate the from the next rain event. Although water does largely composed of naturally occurring zeo- effects of various green roof components and leave the substrate through evaporative losses, lite and volcanic rock (Fassman-Beck et al., system performance (Berndtsson et al., 2006; Starry et al. (2014) demonstrated that with the 2013). A study based in northern Italy used a blend of locally available naturally occurring Getter et al., 2007; Getter and Rowe, 2008; exception of large (>62.5 mm) rain events, mineral materials as the extensive GRS Mentens et al., 2006; Molineaux et al., 2009; green roof platforms planted in P. kamtscha- (Nardini et al., 2012). Molineaux et al. (2009) Rowe et al., 2006; Teemusk and Mander, ticus in the mid-Atlantic region were 30% reported in the United Kingdom, broken 2007; VanWoert et al., 2005a), the total green more efficient at removing storm water brick is a commonly used mineral portion roof area in North America increases (Erlichman through E compared with evaporation alone T of extensive GRSs. In Sweden, extensive and Peck, 2013). As the layer that supports the from unplanted platforms. This contradicted biological function of any green roof system, GRSs were traditionally natural soil amended VanWoert et al.’s (2005a) conclusion that with naturally occurring lava or scoria, and GRSs retain water for plant growth, allow air brown or unplanted experimental roof plat- movement for root gas exchange, offer stability Emilsson (2008) reported the results of forms were as effective at evaporating storm a study using broken roof tiles as a component and structure for root anchoring, and provide water as planted experimental platforms. nutrients for plant uptake. Although sub- of extensive GRSs as an alternative to those Given plants’ substantial influence on ET strates retain a proportion of any rainfall mined minerals. water loss from a green roof system, the The organic content of GRS typically (buffering immediate storm water runoff), effects of GRS composition on plant growth plants provide the additional ecosystem varies depending on the design intent of and ET should be investigated to enhance the green roof system; however, most service of storm water removal via transpi- storm water retention predictions and inform rational water loss. In this way, water held ready-to-plant blends roughly follow the green roof system design. FLL guidelines of # 65 g/L (FLL, 2008). in the GRS is taken up through the roots and In general, any soilless substrate should cycled directly back into the atmosphere as This gravimetric recommendation is based be consistent in composition, free of patho- on verification via loss on ignition. How- water vapor, decreasing the water content gens and weed seed, and provide adequate of the GRS and allowing water retention ever, in practice, horticultural substrates are water, air, and nutrients for plant survival generally mixed volumetrically. The FLL and growth (Handreck and Black, 2007). In guideline is a weight per volume metric— addition to these properties, GRS must also a value that could therefore vary widely Received for publication 20 July 2016. Accepted have an adequate bulk density to resist wind depending on the bulk density of the blend for publication 17 Oct. 2016. uplift without surpassing roof structural if it is mixed volumetrically, as a typical 1Corresponding author. E-mail: whitneygriffin@ live load limits for the roof; they must also horticultural substrate. Griffin (2014) dem- tamu.edu. be engineered to rapidly drain to avoid onstrated that given the differences in 320 HORTSCIENCE VOL. 52(2) FEBRUARY 2017 MISCELLANEOUS densities of the mineral and organic portions that have evolved physiological responses idling’’ refers to stomatal closure for the of GRS, a substrate could have up to 40% to extreme heat and drought conditions. entire 24-h day, where no new carbon is OM (volumetrically) and still fall within the One such mechanism is a variation on metabolized but malic acid is still created FLL guidelines. Since OM provides cation the traditional C3 photosynthetic pathway nocturnally via the recapture of respiratory exchange and water holding capacity, vary- termed the CAM. CAM allows for a water CO2 (Borland et al., 2011). ing from the organic content of a GRS could use efficiency, or the weight of plant mate- Starry et al. (2014) evaluated P. kamtschaticus have significant impacts on substrate water rial per volume of water used, 6-fold great- for CAM metabolism and found it to be less holding capacity, plant growth, and ET. er than C3 plants (Nobel, 1996) because drought resistant with less evidence of CAM Green roofs present a unique engineered carbon uptake occurs nocturnally. CAM metabolism than Sedum album, but did report environment for plants—a thin substrate plants are adapted to keep their stomata some CAM activity for P. kamtschaticus. This layer requires a fibrous, nonaggressive root closed during the day to prevent water supported Butler’s (2011) findings that differ- system to avoid compromising the integrity loss—carbon dioxide (CO2) is sequestered ent succulents commonly found on green of the waterproof membrane of the roof; the at night when stomata open, and is stored as roofs can express variation in the extent to reduced rooting zone also limits the volume malic acid until sunrise. Even though sto- which they use CAM. Regardless of the of water that can be stored after rain events. mata are closed during the day (primarily photosynthetic pathway, the effects of GRS Green roof plants must tolerate extreme for water conservation), photosynthesis can water availability on plant growth and ET of diurnal temperature ranges, direct sun ex- continue during the day (albeit at a reduced green roof plants has not been studied in posure, and high wind exposure. All these rate) by converting