Vegetated roofs, or green roofs, are multilayered systems containing plant and substrate materials. Green roof substrates are not similar to field soils, but have characteristics in common with shallow-drained soils and/or greenhouse container substrates (Beattie and Berghage, 2004; Spomer, 1990). There is no single ideal substrate for green roofs in all locations throughout the United States due to regional climatic differences; however, a blend of characteristics provides optimal container capacity, nutrient-holding capacity, pH, aeration, and bulk density. Typically, green roof substrates are 80% to 100% mineral and 0% to 20% organic matter, which contribute to water- and nutrient-holding capacities (Beattie and Berghage, 2004). Organic matter also can act as an adhesive between soil particles, resulting in improved moisture-holding capabilities (Alexander, 1996). Compost is the preferred source of organic matter incorporated into green roof substrates primarily due to nutrient, microbial, and social benefits (Friedrich, 2005). A maximum of 15% (Rowe et al., 2006) or 0% to 25% (Friedrich, 2005) organic matter content (by volume) is recommended for green roof substrates, although there are few research reports of compost effects on green roof substrate performance. Compost components reported in green roof substrates include green waste compost (Dunnett and Nolan, 2004), composted yard waste, and composted turkey waste (Durhman et al., 2007; Rowe et al., 2006; VanWoert et al., 2005). Nonsucculent native plant growth is more dependent on organic matter content than succulent stonecrop species, although visual ratings of ‘Diffusum’ stonecrop (Sedum middendorffianum) and ‘Royal Pink’ stonecrop (Sedum spurium) increased 9.4% and 15.8%, respectively, with the substitution of 15% (by volume) organic matter consisting of peatmoss, aged poultry manure, and composted yard waste (Rowe et al., 2006).
Substrate depth determines the vegetation forms and species planted on green roofs [Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL), 2002]. A 1-inch-deep substrate often can be used without supplemental irrigation or major structural modifications, resulting in lower installation and maintenance costs (Friedrich, 2005). However, plants grown in 1-inch-deep substrates have reduced survival over those grown in 2-inch-deep substrates (Durhman et al., 2007). Winter plant damage is less in 4- or 6-inch-deep than in 2-inch-deep substrates, possibly due to lower minimum temperature and higher temperature variation (Boivin et al., 2001). Shallow-depth substrates (1.6–3.1 inches deep) support moss-sedum vegetation types (FLL, 2002). Green roofs with 1.6-inch-deep substrate hold less water and likely dry out more rapidly than 2.8- or 3.9-inch-deep substrates (Getter and Rowe, 2009). Lower survival rates of native herbaceous plants on a 4-inch-deep substrate was probably due to low container capacity (Rowe et al., 2006) and water availability was hypothesized to be the limiting factor for coverage of ‘Weihenstephaner Gold’ stonecrop (Getter and Rowe, 2009). ‘Summer Glory’ stonecrop is a good choice for shallow-depth green roof substrates and is capable of 75% survival within a 1-inch-deep substrate after 482 d or 100% survival on a 2-inch-deep substrate after 482 d (Durhman et al., 2007). ‘Weihenstephaner Gold’ stonecrop, ‘Tasteless’ stonecrop (Sedum sexangulare), stonecrop (Sedum stefco), and ‘John Creech’ stonecrop (S. spurium) are suitable for 1.6- or 2.8-inch-deep substrates (Getter and Rowe, 2009).
Water-absorbent crystals, or hydrogels, expand into pliable gels when hydrated and are added to horticultural substrates to reduce plant stress and act as a water reservoir supply during periods of drought (Johnson and Leah, 1990). Potassium propenate propenamide copolymer-based hydrogel amendments delayed wilting during drought conditions for marigold (Tagetes erecta) and zinnia (Zinnia elegans) following incorporation into a peat-lite mix at a rate of 4 to 16 kg·m−3 (Gehring and Lewis, 1980). Polyacrylamide-based hydrogels incorporated into a sandy soil (4 g·kg−1) increased survival of buttonwood (Conocarpus erectus) under drought stress (Al-Humaid and Moftah, 2007). In addition, coarse sand amended with hydrogel at 0.5 to 5 g·kg−1 increased mean shoot fresh weight and reduced evapotranspiration for lettuce (Lactuca sativa), radish (Raphanus sativus), and common wheat (Triticum aestivum) when subjected to temporary drought (Johnson and Leah, 1990). After incorporation with a polyacrylamide hydrogel (SuperSorb·C; Aquatrols Corp. of America, Paulsboro, NJ) at 1.8 kg·m−3, there was a 6.9% increase of container capacity (and a concomitant 22.8% decrease of aeration porosity) in pine bark substrate and a 6.0% increase of container capacity (and a concomitant 33.0% decrease of aeration porosity) in a mix containing (by volume) 80% pine bark and 20% sand (Fonteno and Bilderback, 1993). SuperSorb·C also increased water retention in a mix containing (by volume) 50% peatmoss and 50% pine bark (Wang and Gregg, 1990). It has been suggested that hydrogels also may be used during land restoration to address water limitations during seed germination and seedling establishment (Mangold and Sheley, 2007).
Two reviews summarized known green roof substrate information (Beattie and Berghage, 2004; Friedrich, 2005). Nonetheless, there are few research reports that evaluate physical and chemical properties of shallow-depth substrates and their relationship to initial plant growth. Greenhouse and laboratory trials were conducted to determine the physical properties of substrates with increasing concentrations of compost and hydrogel, and to evaluate initial growth of two stonecrop species in 12 substrates by measuring shoot dry weight and coverage area.
Al-Humaid, A. & Moftah, A.E. 2007 Effects of hydrophilic polymer on the survival of buttonwood seedlings grown under drought stress J. Plant Nutr. 30 53 66
Beattie, D.J. & Berghage, R. 2004 Green roof media characteristics: The basics Proc. 2nd North Amer. Green Roof Conf.: Greening rooftops for sustainable communities Portland, OR 2–4 June 2004 Cardinal Group Toronto 411 416
Boivin, M.-A., Lamy, M.-P., Gosselin, A. & Dansereau, B. 2001 Effect of artificial substrate depth on freezing injury of six herbaceous perennials grown in a green roof system HortTechnology 11 409 412
Day, M. & Shaw, K. 2001 Biological, chemical and physical processes of composting 17 50 Stoffella P.J. & Kahn B.A. Compost utilization in horticultural cropping systems CRC Press Boca Raton, FL
Dunnett, N. & Nolan, A. 2004 The effect of substrate depth and supplementary watering on the growth of nine herbaceous perennials in a semi-extensive green roof Acta Hort. 643 305 309
Durhman, A.K., Rowe, D.B. & Rugh, C.L. 2007 Effect of substrate depth on initial growth, coverage, and survival of 25 succulent green roof taxa HortScience 42 588 595
Fonteno, W.C. & Bilderback, T.E. 1993 Impact of hydrogel on physical properties of coarse-structured horticultural substrates J. Amer. Soc. Hort. Sci. 118 217 222
Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau 2002 Guidelines for the planning, execution and upkeep of green-roof sites Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau Bonn, Germany
Friedrich, C.R. 2005 Principles for selecting the proper components for a green roof growing media Proc. 3rd North Amer. Green Roof Conf.: Greening rooftops for sustainable communities Washington, DC 4–6 May 2005 Cardinal Group Toronto 262 273
Gehring, J.M. & Lewis, A.J. 1980 Effect of hydrogel on wilting and moisture stress of bedding plants J. Amer. Soc. Hort. Sci. 105 511 513
Handreck, K.A. & Black, N.D. 1994 Growing media for ornamental plants and turf University of New South Wales Press Randwick, Australia
Johnson, M.S. & Leah, R.T. 1990 Effects of superabsorbent polyacrylamides on efficiency of water use by crop seedlings J. Sci. Food Agr. 52 431 434
Mangold, J. & Sheley, R.L. 2007 Effects of soil texture, watering frequency, and a hydrogel on the emergence and survival of coated and uncoated crested wheatgrass seeds Ecol. Res. 25 6 11
Nektarios, P.P.T. & Chronopoulos, I. 2004 Comparison of different roof garden substrates and their impact on plant growth Acta Hort. 643 311 313
Olszewski, M.W., Trego, T.A. & Kuper, R. 2009 Effects of peat moss substitution with arboretum and greenhouse waste compost for use in container media Compost Sci. Util. 17 151 157
Pill, W.G. & Jacono, C.C. 1984 Effects of hydrogel incorporation in peat-lite on tomato growth and water relations Commun. Soil Sci. Plant Anal. 15 799 810
Rowe, D.B., Monterusso, M.A. & Rugh, C.L. 2006 Assessment of heat-expanded slate and fertility requirements in green roof substrates HortTechnology 16 471 477
Spomer, L.A. 1990 Evaluating ‘drainage’ in container and other shallow-drained horticultural soils Commun. Soil Sci. Plant Anal. 21 221 235
U.S. Environmental Protection Agency. 2009. Green roofs for stormwater runoff control. EPA-600-R-09-026.
VanWoert, N.D., Rowe, D.B., Andresen, J.A., Rugh, C.L. & Xiao, L. 2005 Watering regime and green roof substrate design affect Sedum plant growth HortScience 40 659 664
Wang, Y.-T. & Gregg, L.L. 1990 Hydrophilic polymers-their response to soil amendments and effect on properties of a soilless potting mix J. Amer. Soc. Hort. Sci. 115 943 948