Use of organic wastes to create lightweight green roof substrates with increased plant-available water

Substrate design is important for stormwater retention and plant survival on green roofs. Green roof substrates are mostly inorganic, providing long-term stability, while organic components (< 20% v/v) reduce substrate weight and increase water retention, depending on rate and type. Selection of organic components depends on availability and organic waste materials can improve sustainability and reduce costs. We aimed to use organic waste materials to create lightweight substrates with good aeration and increased plant-available water (PAW) to support plant growth on green roofs. We evaluated the effects of five locally available organic waste components (coarse coir, fine coir, composted green waste, almond hull, and pistachio shell) on the physical and chemical properties of a scoria-based substrate when added at 0, 5, 10, 15, and 20% by volume. We also examined their phytotoxic effects on plant growth using lettuce (Lactuca sativa) as bioassay species. Overall, higher rates (15-20 % v/v) of coarse coir, fine coir and composted green waste all reduced substrate weight and improved PAW. Pistachio shells and almond hulls reduced saturated bulk density but adversely affected plant growth, likely due to phenolic compounds. For the other amendments, rates above 10-15 % addition also decreased root and shoot growth in lettuce. This indicates a potential trade-off between water availability and growth in these substrates. Therefore, if a green roof needs to be amended to reduce weight but have improved PAW, coarse coir, fine coir and composted green waste would be the best choices.