Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions

Abstract

<jats:p>Plants suitable for extensive green roofs must tolerate extreme rooftop conditions, and the substrates in which they grow must fulfill horticultural and structural requirements. Deeper substrates may retain more water for plants during dry periods, but will also weigh more, especially when near saturation. A study in central Pennsylvania was conducted to evaluate the influence of substrate type and depth on establishment of five green roof plants. Two stonecrops [white stonecrop (<jats:italic>Sedum album</jats:italic>) and tasteless stonecrop (<jats:italic>Sedum sexangulare</jats:italic>)], one ice plant (<jats:italic>Delosperma nubigenum</jats:italic>), and two herbaceous perennials [maiden pink (<jats:italic>Dianthus deltoides</jats:italic>) and saxifrage pink (<jats:italic>Petrorhagia saxifraga</jats:italic>)] were planted in three depths (30, 60, and 120 mm) of two commercially available green roof substrates (expanded shale and expanded clay). Study flats inside a plasticulture tunnel received three drought treatments (no drought, 2 weeks early drought, and 2 weeks late drought). The two stonecrops performed well under most conditions, although tasteless stonecrop was stunted by early drought. Ice plant only grew well when provided with water. When subjected to any drought, the herbaceous perennials had the fewest survivors in the expanded shale. Saxifrage pink flowered profusely wherever it survived. The study plants were most affected by substrate depth, except for maiden pink, which responded solely to drought. When subjected to early drought conditions, the herbaceous perennials did not survive in 30 mm of either substrate, or in 60 mm of expanded shale. Although the stonecrops performed well in 60 mm of substrate when subjected to drought, their performance was superior in the expanded clay compared with shale.</jats:p>

Effect of green roof media depth on Crassulacean plant succession over seven years

“…General conclusions regarding depth agree with many previous studies on succulents, as well as herbaceous perennials and grasses (Benvenuti & Bacci, 2010;Dunnett, Nagase, & Hallam, 2008;Durhman et al, 2007;Thuring et al, 2010). Deeper media depths allow for greater diversity in plant material and generally encourage healthier plants with greater biomass.…”

“…When 18 herbaceous perennials and grasses were evaluated in 10 cm of media without irrigation in Michigan, only four species (Allium cernuum, Coreopsis lanceolata, Opuntia humifusa, and Tradescantia ohiensis) still existed after three years (Monterusso, Rowe, & Rugh, 2005;Rowe, Monterusso, & Rugh, 2006). Other studies have shown similar results (Dunnett, Nagase, & Hallam, 2008;Durhman et al, 2006;Thuring, Berghage, & Beattie, 2010). In all of these studies, if irrigation had been available, then a greater number of species would have likely survived.…”

“…Regardless of species, media depth influences how well plants perform (Benvenuti & Bacci, 2010;Dunnett, Nagase, & Hallam, 2008;Dunnett & Nolan, 2004;Durhman, Rowe, & Rugh, 2007;Rowe et al, 2006;Thuring et al, 2010). Deeper media depths are beneficial for both increased water holding capacity (VanWoert et al, 2005a(VanWoert et al, , 2005b and as a buffer in fluctuating winter temperatures (Boivin, Lamy, Gosselin, & Dansereau, 2001).…”

Effect of substrate depth on initial growth and drought tolerance of Sedum lineare in extensive green roof system

“…Early studies (Durhman et al, 2007;Lassalle, 1998) have also demonstrated that deeper substrate can retain more water, which allows plants to maintain a better physiological state when supplementary watering is restricted on green roofs. Therefore, deeper substrate tends to promote growth rate, survival and coverage of Sedums (Durhman et al, 2007;Thuring et al, 2010), and improves floral assemblage and species richness on green roofs (Olly et al, 2011 andRowe et al, 2012). Secondly, a smaller plant is less sensitive and more tolerant to drought stress than larger ones, because it usually uses less water (Blum, 2011).…”

“…Substrate depth generally range from 2 to 15 cm on extensive green roofs, limited by the dead load weight of many buildings. Green roof plants grown in deeper substrate tend to have better growth and higher biomass accumulation and survival (Durhman et al, 2007;Thuring et al, 2010;Nektarios et al, 2011;). Rowe et al (2012) studied the impacts of media depths on the succession of 25 succulents over a course of seven years and found that the number of species was reduced at a faster pace with shallower depths.…”