Residential rain gardens are being installed at an ever-increasing rate across the globe to infiltrate and dampen urban stormwater flow. However, we know little about how plant-soil dynamics control rain garden hydrology because rain garden design varies widely. Our objective was to test the hydrologic response of rain gardens supporting three common vegetation treatments (turfgrass, wet-mesic prairie, and shrubs) relative to non-vegetated controls. We hypothesized the volume and timing of soil-water drainage (i.e., exfiltration) would differ by vegetative treatment because of plant-induced changes in soil water storage, transport, and uptake. To test this hypothesis, we used a controlled field experiment of closed-cell rain gardens in a complete, randomized block design where each vegetative treatment was replicated three times. Each rain garden was sized to residential scale, representing 17% of a contributing roof area that provided stormwater for this experiment. We continuously monitored stormwater input, soil exfiltration, and volumetric soil moisture to assess differences in the hydrologic response of each rain garden via vegetative treatment. We also quantified indirect measures of soil structure (saturated hydraulic conductivity, water retention), root biomass, and evapotranspiration as potential mechanisms controlling rain garden hydrology.

Results/Conclusions

During the 2009 growing season (June-October), the exfiltration response of the rain gardens differed by vegetative treatment in support of our hypotheses. Stormwater was delivered to the rain gardens on a total of 28 dates in 2009, following rain events of at least 1.3 mm (0.05”). On average across all stormwater events, the rain gardens planted with turfgrass, prairie, and shrubs all reduced the volume of exfiltration relative to the non-vegetated controls by approximately 25%, 30%, and 50%, respectively. Differences in exfiltration volume by vegetative treatment depended on the volume of stormwater applied on a given date (p < 0.0001). Similarly, depending on storm magnitude and antecedent soil moisture, all vegetation types significantly decreased the peak flow rate (p < 0.001) and duration (p < 0.0001) of the exfiltration response. Preliminary differences in soil structural development, root dynamics, and evapotranspiration by vegetation type (p < 0.05) support these vegetative-mediated responses in hydrology. Our data suggest that changing the vegetation type in urban rain gardens may yield marked differences in the hydrologic function of these plant-soil systems via shifts in ecohydrological processes.