Urban agriculture on extensive green roofs (EGRs), which are lightweight, shallow-profile systems with low maintenance, presents a viable opportunity to enhance food production and ecosystem services in cities. However, the harsh environmental conditions typical of EGRs pose challenges for crop cultivation, especially regarding water availability. This study investigates whether facilitative interactions with a common green roof cover species, Sedum, with bush beans (Phaseolus vulgaris) as our model crop, can improve crop performance under water stress. We tested 48 EGR modules with three planting designs (no Sedum, artificial Sedum, and live Sedum) under two watering regimes (high and low) at a green roof testing facility in Toronto, Canada, and measured above- and belowground functional traits of beans to assess productivity and trait plasticity. Results revealed that water availability had a greater influence on bean performance than planting design, where beans grown with artificial Sedum under high watering showed the highest shoot biomass and yield, while performance declined significantly under low water conditions, particularly with live Sedum. Leaf physiological traits were largely unaffected by treatment, likely due to the variable rooftop conditions, and root traits were significantly influenced by both water availability and planting design. Hypervolume analyses showed that beans exhibited greater root trait plasticity than Sedum, suggesting a stronger capacity to adapt to water stress. Contrary to the hypotheses, live Sedum did not consistently facilitate bean performance, and potential competitive interactions may have outweighed any stress-ameliorating effects. These findings show water management as the main management variable for crops on EGRs. To transition EGRs into viable agricultural spaces, future strategies should explore alternative facilitative species, nutrient amendments, and microbial inoculations to support crop resilience.

The newly described white-flowered annual Sedum ince closely resembles S. eriocarpum and the annual forms of S. hispemicum (S. longibracteatum), but differs from the former by its alternate instead of verticillate leaves, and from the latter by the 5-merous instead of 6 9-merous flowers and the ripe follicles which are divergent instead of patent and lack the distinct lips along the ventral suture. Molecular phylogenelic analyses indicate that S. ince is but distantly related to S. eriocarpum as well as to S. hispemicum. It belongs to a cytological distinct lineage of Sedum subgenus Gormunia sporting a basic chromosome number of x = 7. Within this lineage, however, S. ince holds a unique position and consequently it is classified in the monotypic S. series Elegans.