Substrate Depth Influences Sedum Plant Community on a Green Roof

Abstract

Because the waterproofing membrane beneath green roofs is estimated to last at least 45 years, long-term plant performance beyond initial establishment is critical. Plants that survive initially on a green roof may not exist in the long term because of variability in climate and other factors. This study evaluated the effect of green roof substrate depth on substrate moisture, plant stress as measured by chlorophyll fluorescence, and plant community development and survival of 12 Sedum species over 4 years in a midwestern U.S. climate during 4 years of growth. Plugs of 12 species of Sedum were planted on 8 June 2005 and evaluated biweekly for absolute cover (AC). Most species exhibited greater growth and coverage at a substrate depth of 7.0 cm and 10.0 cm relative to 4.0 cm. For the species evaluated, substrate depths of at least 7.0 cm are highly recommended. AC of Sedum was significantly greater at this substrate depth than at 4.0 cm. Mean volumetric moisture content of the three substrate depths followed the same pattern as AC. When averaged over time, the 4.0-cm substrate depth held less moisture than depths of 7.0 or 10.0 cm, whereas the 7.0- and 10.0-cm substrate depths were statistically the same. Species exhibiting the greatest AC at all substrate depths were S. floriferum, S. sexangulare, S. spurium ‘John Creech’, and S. stefco. In general, species that are less suitable at these substrate depths are S. ‘Angelina’, S. cauticola ‘Lidakense’, S. ewersii, S. ochroleucum, and S. reflexum ‘Blue Spruce’.