Union of Concerned Scientists
Greentips: April 2009
When rain falls faster than the ground can absorb it, it runs off into storm drains along with any contaminants in its path, such as oil and grease, de-icing salts, heavy metals, pesticides, and bacteria from trash and animal waste. The Environmental Protection Agency reports that urban runoff—in which 77 of 127 key pollutants have been detected—is one of the largest sources of water contamination nationwide.
We can all help minimize the problem of storm water runoff by planting rain gardens—6- to 12-inch-deep depressions filled with native plants. Rain gardens can capture hundreds of gallons of rainwater, filtering out up to 90 percent of pollutants while allowing the water to drain deep enough into the soil to help recharge groundwater supplies.
Whether you undertake this project on your own or with a landscaper, here are some factors to consider when planning a rain garden:
- Location. Site your rain garden where rain and snowmelt collect or run off—near downspouts or gutters, below a slope, or along sidewalks and driveways. However, avoid planting a rain garden within 10 feet of your home’s foundation, within a septic system’s drainage field, or above buried utility lines.
- Size. The square footage of your rain garden should generally be about 20 percent that of the area draining into it. For example, if your roof covers 800 square feet, a rain garden designed to collect all of the roof’s runoff should cover 160 square feet. To capture runoff most efficiently, a rain garden should be longer than it is wide, and aligned perpendicular to the slope.
- Materials. Rain gardens use layers of different materials to help maximize drainage. The bottom layer typically features an “underdrain” (e.g., a piece of perforated PVC pipe) pointed toward an existing storm drain and covered with gravel. The next layer is the planting medium, which should be a mix of about 20 percent compost, 50 percent sand, and 30 percent topsoil. A final layer of mulch helps prevent weeds and removes metals from runoff.
- Plant choice. Native plants are best because they establish deeper roots (which help the soil hold water), can withstand the local climate, need minimal care, and attract local butterfly and bird populations. See the Related Resources for a list of plants native to your region. And, if your rain garden is near a street treated with salt in the winter, ask your local nursery for salt-tolerant plants.