Rain gardens are residential bioretention practices widely used to manage urban runoff, yet their design as plant-soil systems lacks understanding. We hypothesized that vegetative treatment (turfgrass, prairie, and shrubs, plus a non-vegetated control) would alter the volume and rate of drainage from 12 replicate mesocosms (i.e., rain gardens) through changes to the belowground system. Roof runoff was collected on-site and distributed equally among the mesocosms following natural rain events for two growing seasons. We monitored stormwater input, drainage output, and soil moisture to assess differences in hydrology by treatment, explained by indices of soil structural development (infiltration, saturated hydraulic conductivity, soil water retention). Drainage volume and response dynamics differed as predicted by vegetative treatment in support of our hypothesis. The greatest reductions in drainage volume were observed beneath shrubs and prairie following smaller stormwater inputs, and accelerated drainage responses were observed beneath turfgrass following larger stormwater inputs. Differences in infiltration, saturated hydraulic conductivity, and plant-induced changes in antecedent soil moisture among vegetative treatments help explain these plant-mediated drainage responses. This study shows that plants can alter the hydrologic dynamics of rain gardens and thus are a critical component of the design and intent of these plant-soil systems.

Prairie plants of the U.S. Midwest are used for stormwater management because of their purported belowground biomass, but species‐specific data are lacking by which hundreds of prairie species may be evaluated with respect to potential biomass. We hypothesize that aboveground traits could be used to define differences in biomass accumulation to leaves, stems, and roots among prairie plants. Sixteen species of wet‐mesic prairie vegetation common to urban plantings of the Midwest were grown in monoculture to quantify accumulated biomass after 2 yr. Species were grouped in accordance with aboveground morphology and flowering phenology to test for differences in biomass accrual and crown characteristics. Although the species of forbs were as different from one another in leaf, stem, and root biomass as were all forbs together vs. graminoids, grouping plants by aboveground form and flowering phenology helped discern functional differences. Differences among groupings based on aboveground traits are supported by conceptual frameworks through which plant biomass accrual is linked to crown height, leaf area, and growth strategy. These results suggest aboveground traits could inform the selection and/or substitution of prairie species for different uses within urban plantings.