Effect of Substrate Depth on Initial Growth, Coverage, and Survival of 25 Succulent Green Roof Plant Taxa

Abstract

<jats:p>Because of greater interest in green roofs in the United States, it is critical to increase the number and geographic range of proven plant resources for long-term survival on rooftops. Successful plant taxa for extensive green roofs must establish themselves quickly, provide high groundcover density, and tolerate extreme environmental conditions. Furthermore, dead load weight restrictions on many buildings may limit the substrate depth that can be applied. The objective of this study was to evaluate the effect of substrate depth on initial establishment and survival of 25 succulent plant taxa for green roof applications in the midwestern United States. Survival, initial growth, and rate of coverage were compared for plants grown in three substrate depths (2.5, 5.0, and 7.5 cm) on 24 roof platforms. Plant coverage was determined from image analysis of weekly digital photographs. Results indicate deeper substrates promote greater survival and growth; however, in the shallowest depth of 2.5 cm, several species continued to persist. Of the 25 species initially planted, only 47% survived in the deepest substrate of 7.5 cm. Recommended species at the depths tested for climates similar to southern Michigan include <jats:italic>Phedimus spurious</jats:italic> Raf. ‘Leningrad White’, <jats:italic>Sedum acre</jats:italic> L., <jats:italic>S. album</jats:italic> L. ‘Bella d'Inverno’, <jats:italic>S. middendorffianum</jats:italic> L., <jats:italic>S. reflexum</jats:italic> L., <jats:italic>S. sediforme</jats:italic> J., and <jats:italic>S. spurium</jats:italic> Bieb. <jats:italic>‘Summer Glory’</jats:italic>. Subsidiary species that are present at specific substrate depths but may not exhibit an ability to cover large areas include <jats:italic>S. dasyphyllum</jats:italic> L. ‘Burnatii’, <jats:italic>S. dasyphyllum</jats:italic> L. ‘Lilac Mound’, <jats:italic>S. diffusum</jats:italic> W., <jats:italic>S. hispanicum</jats:italic> L., and <jats:italic>S. kamtschaticum</jats:italic> Fisch. The primary deterrent for these subsidiary species was little to no survival at 2.5 cm. Deeper substrates promoted greater survival and growth for nearly all species tested.</jats:p>

Media depth influences Sedum green roof establishment

“…For example, at day 135 the 4.0 cm substrate depth exhibits an AC of 0.1589 while the 10.0 cm substrate depth is 0.5365. This confirms the results of Durhman et al (2007) who found that a 7.5 cm (3.0 in.) substrate depth had 96% plant coverage by the end of the second year, compared to 74% and 47% for 5.0 cm (2.0 in.)…”

“…In addition, some species are more suited for these temperature or water fluctuations than others. Durhman et al (2007) found that S. album was the only species of 25 that exceeded 1.5 cm 2 of growth per day at a substrate depth of 2.5 cm (1.0 in.). At a substrate depth of 5.0 cm (2.0 in.)…”

“…at three substrate depths. First year results showed S. acre, S. album 'Bella d'Inverno', and S. mexicanum exhibiting the greatest percent cover, although the latter did not survive the winter (Durhman et al 2007). …”

“…due to building structural weight limitations. Research at Michigan State University has shown that substrate depth influences rate of coverage and plant growth regardless of species (Rowe et al 2006b;Durhman et al 2007). Deeper substrates are beneficial for both increased water holding capacity and as a buffer for overwintering survival as shallow substrates are more subject to fluctuations in temperature.…”

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

“…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.…”