Abstract/Summary

Previous research showed that some herbaceous perennial species could offer summertime cooling better than succulent species on green roofs (GRs), if supplementary irrigation is available during times of drought. In light of increasing water shortages, use of greywater (GW) instead of mains tap water (TW) for this purpose comes into focus. A glasshouse experiment was conducted in summer 2015 at the University of Reading (UK) to assess the impact of GW irrigation on the health, growth and functioning (in terms of leaf stomatal conductance (gs) and associated water uptake) of four plant genotypes (Salvia, Stachys, Heuchera and Sedum), and their ability to deliver ecosystem services, particularly cooling. Twenty-two replicates of each genotype, plus controls of bare, unvegetated substrate, were irrigated with fixed volumes of TW or synthetic GW for 6 weeks. Plant growth and visual quality, and gs were measured. In Week 7, daily water loss from each container following saturation was determined. For the first 6 weeks, plant growth, visual quality and gs were similar for both TW and GW treatments for all genotypes, indicating no negative impact of short-term GW irrigation and no apparent impact on cooling when plants were well-watered. However, in Week 7 water uptake (and thus presumably gs) was significantly lower for plants irrigated with GW compared to TW for some genotypes (Salvia and Heuchera), especially as substrate became drier, suggesting a reduction in evapotranspiration and potentially reduced cooling service when the soil is dry.