A rain garden is a shallow depression or low spot in a yard or parking lot that is planted with native plants designed to capture stormwater runoff. The garden allows storm water to be filtered through the ground rather than running off into streets and stormsewers where it would capture more sediment and nutrients, degrading water quality. These “stormwater filters” typically range in size from 100 - 400 square feet in area and may hold hundreds of gallons of water at a time (Kassulke, 2003). While one individual rain garden may not have a large impact on water quality, over a whole watershed multiple rain gardens can significantly benefit water quality by capturing sediment and nutrients that contribute to algae blooms and degrade aquatic habitat. Rain gardens also increase groundwater recharge, which is vital for both the ecological health of streams as well as a source of municipal water supply for many communities (ENS, 2002).

Benefits of rain gardens for water quality: Rain gardens benefit water quality in numerous ways including: 1) Reduction in nutrient loads in stormwater runoff 2) Capture of solids (sediment, particulate matter, etc.) 3) Increased awareness of homeowners concerning the impact of stormwater runoff on stream and lake water quality.

The research literature shows that rain gardens are particularly effective at reducing solids and nutrients in stormwater runoff from residential yards and parking lots. Studies show that rain gardens can reduce total solids, nitrogen, phosphorous, and metals in stormwater runoff. A study at the H.B. Fuller lot in St. Paul, Minnesota found that a wetland area built into a parking lot to capture runoff reduced stormwater runoff volume by 73% and particulate matter export by 94% (Langer, 2001). Research done by the Center for Watershed Protection found that “BioRetention Facilities” installed in parking lots reduced total phosphorous measured in runoff by 65%, total nitrogen by 49%, and metals by 95-97% (Quigley and Lawrence 2001.

Rain gardens can play an important role an overall watershed management plan. However, they do not detain large volumes of stormwater runoff, due to the small volume of water they hold. Therefore homeowners should not be given unrealistic expectations as to the ability of rain gardens to solve certain problems, such as flooding.

Sizing of Rain gardens Rain gardens can reduce the peak rate of runoff and volume of runoff from storm events, if the gardens are sited correctly and large enough to treat the impervious area draining to the rain garden. Rainfall must often be directed from gutter downspouts to the rain garden. Rain gardens should be between 10-30% of the impervious area draining to the garden (roof, driveway or sidewalk area) (Kassulke, 2003). Roger Bannerman, a Wisconsin DNR non-point pollution researcher suggests sizing the garden at 15-30% of the impervious area, depending on soil types in the yard. If the major goal of the garden is only groundwater recharge, the garden should be around a tenth of the impervious surface area according to Ken Potter, a Civil and Environmental Engineering Professor at the University of Wisconsin-Madison (ENS, 2002). The garden can be smaller because the water trapped by the larger garden areas is generally lost to evaporation, (water lost to the air), or is used by plants (transpiration), neither of which contribute to groundwater recharge, though still benefiting water quality.

The role of rain gardens in watershed Rain gardens can play an important role in watershed-wide goals to improve water quality and augment groundwater recharge. In watersheds that have a high percentage of impervious surface area, rain gardens can be particularly important as there are limited opportunities for enhancing groundwater recharge in areas that have been completely developed. In new developments, it is much easier to build rain gardens and other water quality practices such as vegetated swales into the overall site plan.

In addition to their water quality benefits, over longer time-scales rain gardens will provide economic savings to cities and counties by reducing the size needed for stormwater ponds, and maintenance costs. Rain gardens will also help reduce costs spent on cleaning up and restoring lakes and rivers that are so important to the quality of life for most Minnesotans.

Enhanced stewardship of natural resources One of the main benefits of rain gardens is their value as environmental education tools. The construction of rain gardens make people more aware of stormwater and water quality issues which leads to improved stewardship of our lakes and rivers.

Aesthetics Rain gardens are much more attractive than traditional stormwater management practices such as stormwater ponds. A well-designed planting of native wetland and prairie species can be very attractive in its own regard, enhancing the aesthetics of a home or parking lot.

Rain Gardens Created by the Lower Phalen Creek Project We partner with the Capitol Region Watershed District, Ramsey-Washington Metro Watershed District and local property owners to install gardens that will capture and filter stormwater runoff in the Phalen Creek watershed. To date we have helped install twelve rain gardens that include more than 2,625 plants. The Maria-Bates Rain Garden on East Seventh Street next to the Swede Hollow Cafe is one of our largest projects and has been showcased as a model in numerous local and national publications. Other rain gardens installed through our efforts include seven residential rain gardens and a two-part rain garden at the Bruce Vento Regional Trailhead at Payne Avenue and East Seventh Street. Many more gardens are in the planning stages for 2008 and we frequently host workshops for local landowners.

Most of our rain garden installations have included work with local young people. In partnership with the Community Design Center of Minnesota, we have worked to ensure that the gardens provide hands-on education for youth who plant the gardens, and we have worked to weave rain gardens into the science curriculum of local schools.