Optimal Growing Substrate pH for Five Sedum Species

To determine the optimal growing substrate pH values for Sedum plants, Sedum album, Sedum reflexum ‘Blue Spruce’, Sedum spurium ‘Dragon’s Blood’, Sedum hybridum ‘Immergrunchen’, and Sedum sexangulare were grown in containers using peatmoss and perlite-based substrates at five target pH levels (i.e., 4.5, 5.5, 6.5, 7.5, and 8.5). Optimal pH levels, calculated from dry weight regression models, were 6.32, 6.43, 5.71, 6.25, and 5.91 for S. album, S. reflexum, S. spurium, S. hybridum, and S. sexangulare, respectively, and 5.95 overall. Sedum spurium dry weight varied the most among pH treatments (i.e., 9.5 times greater at pH 6.3 vs. 8.3), whereas S. reflexum varied the least (i.e., 1.3 times greater at pH 6.3 vs. 4.4), indicating species-specific growth responses to growing substrate pH. These findings identified a narrow range of optimal growing substrate pH levels within a wider pH range tolerated by five Sedum spp. Therefore, by adjusting substrate pH to optimal levels, Sedum growth can be maximized. Sedum plants have been used in rock gardens and other landscape settings as groundcovers for many years. However, the high demand for Sedum plants only started recently when green roof installations became increasingly popular in North America (Getter and Rowe, 2007; Monterusso et al., 2005). Green roof installations often require green roof plant producers to provide plant materials with only a short timeframe from ordering to delivery. Understanding the ideal growing conditions for Sedum will allow for efficient Sedum plant production to meet the increasing industry demand. Growing substrate pH influences plant growth and performance, and different plant species have optimal substrate pH ranges that are unique (Reed, 1996). Substrate pH levels below or above optimal ranges may reduce plant root and shoot growth, influence nutrient uptake, and may even cause plant failure (Marschner, 1986). Many Sedum species are able to grow in a wide range of soil conditions (Stephenson, 1994) and even under extreme environments such as on the edge of cement driveways. However, a wide tolerance of soil conditions, including different soil pH levels, does not necessarily indicate plants thrive in a wide substrate pH range. Given that Sedum plants are in high demand for green roof installations, there is an urgent need for recommendations regarding optimal growing conditions, including growing substrate pH, for Sedum species. The objective of this study was to evaluate the hypothesis that Sedum species have optimal growing substrate pH levels or ranges. Specifically, this study aimed to determine the optimal growing substrate pH levels or ranges for the growth of five Sedum species commonly used in green roof installations. Materials and Methods Plant material and treatments. The research was conducted under greenhouse conditions with air temperatures of 27 C day/ 22 C night ± 2 C, and 55% relative humidity, with roof shade curtains programmed to close and open when global radiation was greater than 500 W·m and less than 150 W·m, respectively. The following five common green roof Sedum species were evaluated for plant growth response to substrate pH: Sedum album, Sedum reflexum ‘Blue Spruce’, Sedum spurium ‘Dragon’s Blood’, Sedum hybridum ‘Immergrunchen’, and Sedum sexangulare. Sedum plants were propagated directly into pH-adjusted substrate by planting 4-cm cuttings. Three cuttings of one species were planted per container (400-mL round, green plastic pots, 10 cm top and 6.8 cm bottom diameters) and thinned to one representative cutting per container once rooted. The growing substrate was a mixture of 80% Canadian sphagnum peat and 20% perlite (Therm-ORock East, New Eagle, PA). A 60-L batch of substrate was mixed and 250 g (i.e., 0.67 g nitrogen/L) of Polyon Homogenous NPK plus Minors 16N–2.6P–10.8K, 3–4 month controlled release fertilizer (Agrium Advanced Technologies, Brantford, Ontario, Canada) was incorporated evenly. To adjust the substrate pH to five target treatment levels (i.e., 4.5, 5.5, 6.5, 7.5, and 8.5), hydrated lime (61.1% CaOH2, 37.1% MgOH2, 0.4% silica and insoluble components, 0.4% iron and aluminum oxides, 0.3% sulfur trioxide, 140 calcium carbonate equivalency; Graymont Dolime Spray Lime, Genoa, OH) was added at rates pre-determined by regression analysis following laboratory titrations (data not shown). To reduce experimental error in this study, rates of hydrated lime were separately incorporated into pre-measured volumes of substrate for each container, and containers were filled and uniformly compacted. Substrate-filled containers were placed in the greenhouse and covered with translucent white shadecloth for 7 d before planting to facilitate the pH-adjustment reaction. Plants were watered as needed for the duration of the experiment using deionized water (pH 7.0 ± 0.5). Containers were spaced 3 cm apart in a completely randomized design on the greenhouse bench surrounded by a border of identical, planted, non-treatment containers to reduce edge effects. There were five replicates (i.e., containers) and five Sedum species for each substrate pH level. Treatment containers were re-randomized biweekly to reduce location effects. Data collection. To ensure treatment pH and electrical conductivity (EC) conditions were within the target range for the duration of the experiment, a pour-through extraction, adapted from Wright (1986), was conducted at 1, 3, 5, and 6 weeks after planting. Leachate obtained from the pour-through extraction was analyzed using a portable pH and EC meter (Oakton PC 300; Oakton Instruments, Vernon Hills, IL). At 3 and 6 weeks after planting, plant height, number of shoot branches, and plant width in two perpendicular directions (for calculating elliptical plant canopy area) were measured. Six weeks after planting, photographs were taken before plants were destructively harvested and fresh weight of the aboveground tissue was measured. Aboveground tissue was placed in paper bags in a drying oven at 60 C and dry weight was measured after 7 d once a constant weight was achieved. Dried aboveground tissue was ground to fit through a 1-mm screen, dry-ashed in a 450 C oven, digested using a HCl solution, and analyzed using the SNL-055 method to evaluate nitrogen, phosphorus, potassium, magnesium, calcium, copper, zinc, manganese, boron, and iron in plant tissue using an inductively coupled plasma optical emission spectrometer (University of Guelph Laboratory Services, Guelph, Ontario, Canada). Statistical analysis. All data sets were analyzed using GraphPad Prism Version 5.03 (GraphPad Software Inc., La Jolla, CA). The Shapiro-Wilk test was used to test data for normality. A one-way analysis of variance with a Tukey’s multiple comparison test was conducted for pH, EC, plant growth, and tissue nutrient data. Regression analyses Received for publication 26 Dec. 2012. Accepted for publication 28 Feb. 2013. This work was financially supported by the Ontario Ministry of Agriculture, Food and Rural Affairs/ University of Guelph Research Program, Landscape Ontario, LiveRoof Ontario, Sedum Master, and Carrot Common. We thank Linping Wang, Katherine Vinson, and Siobhan Dunets for their technical assistance. To whom reprint requests should be addressed; e-mail yzheng@uoguelph.ca. 448 HORTSCIENCE VOL. 48(4) APRIL 2013 were used to relate plant growth and tissue nutrient data to substrate pH and to estimate parameters for the best-fit regression model (linear or quadratic). Regression models for dry weight were used to determine optimal pH for maximum plant growth per Sedum spp. and estimate the pH range at which 90% of the maximum plant growth would occur. Pearson correlation coefficients were calculated to compare tissue dry weight with nutrient content. Treatment effects were evaluated using a significance level of P < 0.05.