The 2012 ASHS Annual Conference

Green roofs are a popular technology used to address the environmental challenges of society, but their use in arid climates is claimed unsupportable due to the lack of precipitation characteristic of such regions. The current perception is that, without adequate precipitation, the thin green roof growing substrates will desiccate in the extreme heat of arid climates making plant growth untenable. As a result, the misconception that arid climate green roofs are theoretical concepts must be overcome without the luxury of thinner and lighter weight profiles of growth substrates. The objective of this study is to determine a minimum growth substrate depth that will support growth of traditional green roof taxa Sempervivum spp.(hens and chicks) and Delosperma nubigenum (hardy iceplant) in arid climates. Plants were grown in a greenhouse located at the Fabian Garcia Science Center in Las Cruces, New Mexico.  The plants were grown at depths of  7.5, 10, and 12.5 cm in a 6:3:1 heat-expanded clay: washed sand: worm castings (Hydroton, Willich, Germany; Quikrete Playsand, Atlanta, GA; Wiggleworm Soil Builder, Racine, WI) growing substrate by volume. The growing substrate had a bulk density of 0.8 g cm^-3, an air-filled porosity of 42%, and a pH of 7.3. The experimental design was a split-plot with four replications. The whole plot was growing substrate depth and the sub-plot was plant taxa. Four plants from each species were randomly selected and harvested destructively to assess initial plant growth characteristics. Plants were evaluated every 15 days for survival, coverage, and visual quality. Initial results revealed that Delosperma had a mean canopy area of 130 ± 7 cm², which was greater than that of Sempervivum which had a mean canopy area of 66 ± 1 cm². In addition, Sempervivum had a mean root weight of  0.8 ± 0.2 g and a shoot weight of 3 ± 0.5 g, which was less than Delosperma which had a mean root weight of 4.6 ± 0.9 g and a mean shoot weight of 4.1 ± 0.3 g. While the growing substrate is adequate to support growth, a lighter formulation is being developed for further tests. Data collection is ongoing, but at day 15 substrate depth had no effect on plant growth.