Use of water-retention additives to improve performance of green roof substrates

Abstract

Green roofs provide an opportunity to make our cities more liveable by reducing the impact of urban stormwater runoff through the combined effects of substrates (growing media) absorbing rainfall and plants using water. Stormwater retention is determined by substrate s ability to absorb and store water (water holding capacity; WHC) and supply it to plants (plant available water; PAW). As green roof substrates are largely mineral-based with minimal organic matter, water-retention additives might provide improved stormwater and plant performance. We evaluated the effects of adding biochar, silicates and hydrogels on WHC, PAW, permanent wilting point and bulk density of a scoria-based green roof substrate. Permanent wilting and PAW was assessed in two drought experiments with wheat growing in substrate with and without water retention additives. Hydrogel and silicates were added at 0.01 and 1% v/v, respectively, whereas biochar was added at 10, 20, 30 and 40% v/v. Although hydrogel and silicates increased WHC, only hydrogel increased PAW, but this did not delay permanent wilting. Biochar addition greatly increased WHC and PAW, and reduced bulk density, with greater rates of addition resulting in lighter substrates which also held more water. The onset of permanent wilting was delayed by up to 2 days (30 and 40% biochar). When modelled for Melbourne conditions, a green roof installed with scoria and 40% biochar substrate would retain 12% more annual rainfall from runoff and reduce potential water deficit conditions by 77% compared with un-amended scoria substrate. This has significant implications for green roof performance in terms of stormwater water management and plant survival. As green roof substrates are similar to other designed containerised substrates these findings also have wider applicability to urban horticulture.