Water need and water use efficiency of two plant species in soil-containing and soilless substrates under green roof conditions

Despite the significance of urban landscapes, there are limiting factors like spaces and water resources to expand them across the world. These limitations necessitate the development of water-conserving strategies in vertical infrastructures such as green roofs. One water-conserving strategy is precise irrigation regimes based on the plant species’ water needs. We investigated the water need of Carpobrotus edulis and Aptenia cordifolia under treatments with different soil-containing and soil-less water-absorbing substrate amenders and humic acid applications. The experiment was factorial based on a randomized complete block design with three replications and was conducted from May to September 2020. The first factor was the substrates with different green roof substrate compositions including soil-containing and soilless substrates with varying bentonite percentages. The second factor was humic acid levels (zero, 100, and 200 mg/l), which were applied as fertigation every 15 days during the experiment. Water needs were determined using the lysimetric method. The results showed that despite the soil-containing substrate with bentonite, the soilless substrate alone could not lead to optimal plant growth. The highest water use efficiency and the least evapotranspiration were obtained from the substrate containing 20%Soil +20% leca +20% perlite +20% mineral pumice +20% leaf litter plus 12% w bentonite, combined with A. cordifolia. This plant species showed a better performance compared with C. edulis. During the spring and summer months, the soil-containing substrate with bentonite and A. cordifolia can create a sustainable green roof system by creating better coverage, more water conservation, and a more aesthetic appearance. Based on the results, the application of the highest concentration level of humic acid (200 mg/l) increased the water use efficiency by about 40% after the establishment of the plants. Also, using this level of humic acid reduced the evapotranspiration rate in A. cordifolia up to 10 ml/day and in C. edulis up to 15 ml/day.