Green roofs have many benefits, but in countries with semiarid climates the amount of water needed for irrigation is a limiting factor for their maintenance. The use of drought-tolerant plants such as Sedum species, reduces the water requirements in the dry season, but, even so, in semiarid environments these can reach up to 60 L m⁻² per day. Continuous substrate/soil water content monitoring would facilitate the efficient use of this critical resource. In this context, the use of plant microbial fuel cells (PMFCs) emerges as a suitable and more sustainable alternative for monitoring water content in green roofs in semiarid climates. In this study, bench and pilot-scale experiments using seven Sedum species showed a positive relationship between current generation and water content in the substrate. PMFC reactors with higher water content (around 27% vs. 17.5% v/v) showed larger power density (114.6 and 82.3 μW m⁻² vs. 32.5 μW m⁻²). Moreover, a correlation coefficient of 0.95 (±0.01) between current density and water content was observed. The results of this research represent the first effort of using PMFCs as low-cost water content biosensors for green roofs.