Green roof substrates: Effect of recycled crushed porcelain and foamed glass on plant growth and water retention

Abstract

A study was conducted in a controlled environment greenhouse to examine the potential of recycled crushed porcelain and foamed glass for use as a component of green roof substrates. Porcelain and foamed glass substrates were compared to heat-expanded shale which served as the control. Each finished substrate was analyzed per German FLL guidelines to determine granulometric distribution, bulk density, total porosity, water-holding capacity, saturated hydraulic conductivity, pH, soluble salts, organic matter content, and cation exchange capacity. Both substrates met FLL Guidelines except the porcelain substrate contained a greater percentage of larger particles and its maximum water holding capacity was lower than recommended. Two plant species were used in the study, Sedum album (stonecrop) and Ocinum x citriodolum (lemon basil). Data collected included substrate volumetric moisture content (VMC), plant growth, biomass accumulation, and plant stress as measured by chlorophyll fluorescence. Substrate VMC was generally greater in shale than in foamed glass or porcelain. At the end of the study plant growth index was greatest for stonecrop growing in shale, but there was no difference among the substrates for basil. However, plants of both basil and stonecrop accumulated the most total biomass when grown in shale. It is probable that water retention could be improved for both recycled crushed porcelain and foamed glass if more attention was paid to reducing particle size during processing. If so, then they may perform equal to heat expanded shale when incorporated into green roof substrates. In the case of porcelain, its use could divert some waste from landfills and greatly reduce the embodied energy required to construct a green roof.