Water-retention additives increase plant available water in green roof substrates

Green roofs in hot and dry climates are frequently exposed to drought due to shallow depth and low water holding capacity (WHC) of substrates (growing media). Water-retention additives have the potential to increase substrate water availability leading to greater plant growth and survival, expanding the range of plant species suitable for green roofs. We determined whether two different water-retention additives (silicate granules and hydrogel) increased substrate WHC, plant available water (PAW), days until permanent wilting and growth of winter wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) grown in green roof substrates. Two substrates were compared; one based on scoria, the other based on crushed terracotta roof-tiles. Without additives both substrates had similar WHC (40–43%) but PAW was lower in scoria than in roof-tile due to greater air-filled porosity (AFP) and lower bulk density in scoria. Silicates increased WHC in both scoria and roof-tile substrates, but hydrogel only improved scoria WHC. The effects of additives on PAW differed between plant species, with additives increasing PAW in both substrates for wheat but only in roof-tile for lupins. PAW was not significantly different between hydrogel and silicates. Although hydrogels increased substrate WHC and PAW there was no increase in time until wilting in either substrate. Silicates on the other hand, increased time until wilting and total and root biomass for wheat growing in scoria and for lupins in both substrates. While there was a species effect on the efficiency of water-retention additives which requires further study, the addition of silicates is more likely to enhance species capacity to tolerate periods of low water availability on green roofs.

► Both additives increased plant available water but only silicates increased time until wilting. ► Silicates also increased plant growth. ► Silicates were effective in both substrates, despite differences in air-filled porosity. ► Silicates could be used to improve plant establishment on green roofs where water is limiting.