Book Description

Plants are arguably the most visible components of stormwater bioretention basins and play key roles in stabilizing soils, removing water through transpiration, and other processes. In cold climates, bioretention basins along roadways can receive considerable quantities of deicing salt in winter. This salt may variably migrate out of the systems prior to the onset of plant growth in spring or remain in the soil; the effects on plants presumably vary with time (due to annual weather patterns), space (because stormwater exposure is location-dependent), and biology (because plant taxa differ in their salt tolerance). The goal of this study was to investigate the magnitude of deicing salt's effects on bioretention plants, and how it varies with spatial, temporal, and biological factors. The study took place in a set of five bioretention basins in Philadelphia, USA that receive runoff from a major highway. We collected data on seasonal and depth-related changes in soil electrical conductivity (EC); plant size and mean leaf area, plant tissue sodium concentrations, and plant mortality. In a pair of years with below- and above-average temperatures, soil EC was significantly higher in basin soils compared to reference locations through all spring months. For the nine plant taxa evaluated, leaf areas and crown volumes were strongly reduced in proportion to stormwater exposure. These responses were largely consistent across taxa. In Hemerocallis 'Happy returns', leaf Na+ concentrations >500 mg kg-1 were typical for specimens in low-lying portions of the basins, confirming that deicing salts were responsible for inhibited growth. However, leaf Na+ was less strongly elevated in Iris sibirica 'Caesar's brother' and was not location-dependent. Finally, all Cornus sericea 'Cardinal' plants with stem Na+ concentrations >500 mg kg-1 died following a winter with below-average temperatures, despite having survived through two prior winters. Our observations demonstrate that deicing salts can have detrimental effects on plants in bioretention basins and highlight the value of installing salt-tolerant plants in basins likely to experience any frequency of deicing salt exposure.