Although it comes as a surprise to many homeowners, the suburban neighborhood is a leading source of water pollution. Residential streets and driveways are inundated with oils and metals from cars and trucks, while lawns and gardens release fertilizers, herbicides and pesticides and pets deposit waste along curbsides. According to the Environmental Protection Agency, stormwater runoff from urban areas is the leading pollutant of rivers and lakes.
Homeowners can prevent pollutants from harming surface and groundwater by installing a rain garden, a pond-like recess shaped like a saucer or shallow bowl. Rainwater from the driveway, walkway and lawn is directed to flow into this depression, allowing the natural biochemical activity in mulch and soil to transform toxins into harmless compounds.
Following a storm, up to six inches of water settles in the garden basin, with the highest concentrations of pollutants in the first half-inch or first flush. Excess stormwater then flows into traditional stormwater outfalls or ditches. Water recedes in a couple of days through evapotransporation and infiltration. The trees, shrubs and perennials that landscape the rain garden absorb water and allow it to percolate through the soil, replenishing the groundwater table.
A rain garden contains three planting zones. The lowest will have periods of standing water and extended soil saturation. Plants for the lowest zone are selected for their tolerance to wet conditions. The middle zone will have periodic soil saturation, and the upper edge will be dry. Plants in the lower and middle zones must also tolerate fluctuating moisture. Plants in all zones will be subject to drought spells as well. A diversity of trees and shrubs native to local wetlands and streambanks are most suitable for the lower and middle zones. The upper rim of the garden can be planted in perennials.
Ten years ago, Larry Coffman, director of Maryland's Prince George's County Department of Environmental Resources, set out to address the detrimental impacts to streams that occur from converting forested land to impervious roadways, parking lots and rooftops. A rain garden was established along with other low-impact development techniques to reduce stormwater runoff. "Rain gardens combine environmentally sensitive site design with pollution prevention to form a comprehensive approach to water quality problems," says Coffman.
TABCO, the developer of Somerset, a residential subdivision in Prince George's County, was a pioneer in using the rain garden for stormwater management. Each residential lot features a carefully landscaped 300- to 400-square-foot rain garden at a low point. "TABCO was committed to …