What is a rain garden?

A rain garden is a garden of native shrubs, perennials, and flowers planted in a small depression. It is designed to temporarily hold and soak in rain water runoff that flows from roofs, driveways, patios, lawns, or other hard (impervious) surfaces, preventing it from entering the storm sewer system. Soil and plant roots use natural processes to improve water quality by filtering pollutants. Rain gardens are effective in removing up to 90 percent of nutrients and chemicals, and up to 80 percent of sediments from the storm water runoff. Compared to a conventional lawn, rain gardens allow for 30 percent more water to soak into the ground. The water is held by the garden and allowed to slowly infiltrate the soil, providing an important role in recharging ground water supplies and reducing storm water runoff volumes to local streams. A rain garden is not a pond or wetland, but is dry most of the time and typically holds water for not more than two days during and following a rainfall event.

What are the benefits of rain gardens?

Rain gardens are designed to improve local water quality and reduce the impacts of stormwater on area streams. Communities around the country have experienced dramatic reductions in storm water pollution, due to many homeowners installing rain gardens on their properties. According to the US Environmental Protection Agency (EPA), 70 percent of all water pollution comes from pollutants carried in stormwater runoff. A few examples of these pollutants include pet waste, fertilizers, oils and greases from automobiles, and trash (i.e. non-point pollution sources). Rain gardens are effective in removing up to 90 percent of nutrient and chemical pollutants and up to 80 percent of sediments from storm waters flowing into them. This polluted water would otherwise reach nearby streams, rivers, and lakes untreated. Not only are rain gardens helpful to water quality, they also create beautiful additions to any landscape and can help reduce localized flooding or standing water in nearby streets. Constructing, installing, and maintaining a rain garden will help reduce pollution and flooding, and help keep our local water supplies and recreational areas healthy.

• Improve water quality by filtering out pollutants
• Provide localized stormwater and flood control
• Easy to maintain after establishment
• Preserve and promote native vegetation
• Attract beneficial birds, butterflies, and insects
• Provide aesthetically pleasing landscaping
• Provide a stormwater management solution for homeowners who properly disconnect their downspouts from the sanitary or combined sewer system

Will my rain garden have standing water?

Rain gardens are designed to infiltrate water in about one day. If it rains several days in a row, it is possible that your rain garden may have standing water until the rain stops and the water has time to soak in. If designed and installed correctly, rain gardens typically do not have standing water for more than 48 hours. Be sure to test your soil type and infiltration rate, or percolation rate, before beginning your rain garden. Rain gardens may not be appropriate for all locations; high water tables, clay soils and bedrock locations may inhibit infiltration.  The below brochure provides additional information about testing your soil’s infiltration rate.

Build Your Own Rain Garden

Can a rain garden be too big or too small?

A rain garden should have an area about 20 percent the size of the roof, patio or pavement area draining into it. A typical residential rain garden is between 100 and 300 square feet. If a smaller rain garden than recommended for a lot is chosen by the landowner, the garden will still function. Any size rain garden can make a positive impact by infiltrating some storm water.  Rain gardens can vary in size and complexity depending on your site constraints and how you would like them to function. The “Build Your Own Rain Garden” brochure can give you more guidance on size calculations and other factors of rain garden design.  Or, simply seek help from a landscaper or other professional.

Will road salt damage the plants in my garden?

It is better to locate the rain garden away from direct salt discharge. There are some salt-tolerant plants that you might use if the only place you can locate the rain garden will be subject to salt spray and runoff from streets and sidewalks. For large parking lot and street applications, pre-treatment structures may be helpful, such as a sediment settling area. For larger rain gardens for commercial parking lots, an underdrain system would be best; the salty runoff is filtered through the rain garden, then carried away by the underdrain system. This prevents possible groundwater contamination with chlorides.