Effects of Species Proportions and Fertility on Sedum Green Roof Modules

Abstract

Nursery producers are interested in supplying sedum (Sedum sp.)-vegetated modular units for the green roof industry. This research examined the influence of three rates of controlled-release fertilizer (CRF) (50, 78, and 108 g per unit of 15N–3.9P–10K, 8- to 9-month formulation) on production of modules containing one or two Sedum species. Six Sedum species were evaluated independently: ‘Angelina’ sedum (Sedum rupestre), gray sedum (S. pachyclados), ‘John Creech’ sedum (S. spurium), tasteless sedum (S. sexangulare), ‘Weihenstephaner's Gold’ sedum (S. kamtschaticum var. floriferum), and white sedum (S. album). The species were then paired as follows: white sedum/‘John Creech’ sedum, ‘Weihenstephaner's Gold’ sedum/‘Angelina’ sedum, and gray sedum/tasteless sedum. For each pair, three starting proportions of cuttings were studied. In 8 weeks, the medium and high fertility rates produced significantly more units with at least 95% vegetation coverage than the low rate. In general, increasing the fertility rate increased the fresh weight at time of harvest, except for ‘Weihenstephaner's Gold’ sedum and tasteless sedum at the high rate, which developed “melt out.” Melted-out tissues turned brown, desiccated and detracted from the visual appearance of units. ‘John Creech’ sedum and white sedum had the greatest fresh weight followed by ‘Weihenstephaner's Gold’ sedum, ‘Angelina’ sedum, and tasteless sedum. Gray sedum grew substantially slower than all other species and had the lowest fresh weight. For white sedum/‘John Creech’ sedum units, harvest fresh weight proportions were similar to starting cutting fresh weight proportions. For gray sedum/tasteless sedum units, tasteless sedum outgrew gray sedum and gray sedum was barely noticeable in finished units. Harvest fresh weight and digital image analysis (DIA) of ‘Weihenstephaner's Gold’ sedum/‘Angelina' sedum units indicated that ‘Weihenstephaner's Gold’ sedum colonized units faster than ‘Angelina’ sedum, and had greater visual impact in finished units. Units started with 25% ‘Weihenstephaner's Gold’ sedum cuttings, the smallest proportion tested, were roughly 73% covered with ‘Weihenstephaner's Gold’ sedum at harvest. We recommend growers use the medium fertility rate to produce the most units with 95% vegetation or more in the least amount of time and with reduced risk of melt out. To achieve desired final species proportions, growers may need to adjust the ratios of cuttings based on uneven species vigor.