Road salt reduces plant cover in bioretention systems within road rights-of-way

Bioretention systems for managing urban runoff rely on healthy plants to reduce runoff and nutrient pollution via evapotranspiration and uptake. However, loss of plant cover is common and, in cold climates, potentially caused by the application of road salt. To investigate the impact of road salt on bioretention systems, we measured salt concentrations in the media and plant tissues and assessed plant cover at 19 sites in Toronto, Canada, in the field. Winter road salt was identified as the primary driver of plant cover loss: low-tolerance species accumulated excessive sodium and chloride, resulting in chlorotic and necrotic damage even under moderate salinity (median electrical conductivity (EC), 0.31-0.35 mS/cm, as measured in soil-water suspensions). Continuous EC monitoring showed no net salt buildup in any season, although salinity peaked in winter and was lower in summer. Low-tolerance species exhibited high salt ion uptake and substantial damage from legacy salt retained in the media. Although species-specific ion accumulation patterns were observed, they did not always align with species salt tolerance as described in the literature. Among the 14 species studied, Hemerocallis 'Happy Returns' (low tolerance) and Panicum virgatum (medium tolerance) significantly accumulated sodium, up to 2126 and 586 mg/kg, respectively, whereas Salvia officinalis (medium tolerance) significantly accumulated chloride (up to 20 mg/g); yet only Panicum virgatum displayed minimal damage (<5 %), while Hemerocallis 'Happy Returns' and Salvia officinalis displayed >50 % damage. These findings underscore the importance of selecting salt-tolerant species to ensure long-term bioretention performance.