Water-Saving Green Roofs for Pollinators

Abstract

Appropriately designed green roofs can function as water-saving irrigation systems and pollinator habitats. In this study, we evaluated a rooftop irrigation system using reclaimed water (simulated by diluted seawater up to 18 dS m⁻¹) to sustain vegetation (turf grass Cynodon dactylon and the Crassulacean acid metabolism [CAM] species Sedum kamtschaticum and S. oryzifolium) under drought and salinity stress. A stress factor, the time integral of the soil water potential that exceeds a threshold for normal growth, was used to quantify the accumulated plant stress. Moderate stress reduced total evapotranspiration but increased water use efficiency in CAM plants, indicating trade-offs between irrigation frequency and plant water use. Saline irrigation led to cation accumulation in leaves (Na⁺, Ca²⁺, Mg²⁺, and K⁺). Additionally, leaf extract electrical conductivity [ECleaf] was more reliable for assessing plant salt stress than drainage water electrical conductivity. However, ECleaf is not an absolute stress detector. Rather, it is a relative indicator that requires species-specific calibration. These findings support an integrated design framework that balances substrate properties, plant selection, climate conditions, and irrigation scheduling. Key recommendations include prioritizing drought- and salt-tolerant species (especially CAM plants) to reduce water demand, optimizing irrigation intervals to mitigate stress, and adopting renewable energy for automated irrigation. Integrating pollinator-friendly planting with efficient reclaimed water reuse can enhance the sustainability and resilience of green roof ecosystems. However, pollinator-habitat benefits remain theoretical. This demonstrates the need for direct field validation of flowering and visitation responses.