Rain gardens offer a natural solution to runoff pollution
07-27-2015 // Mark Wexler
FOR DECADES, PEOPLE LIVING ALONG 8TH AVE NW in Puyallup, Washington, watched helplessly as their yards and street flooded during heavy rainstorms. With each flood, rivers of runoff tainted with lawn chemicals, car oil, animal waste and other pollutants poured into sewers and area waterways. But a few years ago, residents found a cost-effective solution to their problem: gardens designed to manage the rain.
Working with local authorities and landscape designers, residents installed bioretention cells, or rain gardens, in their yards and along the two-block-long avenue. “We haven’t had flooding problems since,” says homeowner Steve Hollis, “and the rain garden in our front yard has improved both the appearance and value of our property.”
Using Nature as a Sponge
The 8th Avenue project is part of a campaign—initiated by Washington State University Extension and the nonprofit Stewardship Partners in Seattle—to build 12,000 rain gardens on private property in the Puget Sound area by the end of 2016. Washington State scientists say at least 14 million pounds of toxic contaminants flow annually into the sound, much of it coming from urban communities. “The majority of our area’s water pollution is caused by rainwater runoff from our streets, driveways, lawns and rooftops,” says Stewardship Partners Program Manager Aaron Clark.
Across the country, hundreds of other municipalities are also supporting “green infrastructure” projects that utilize rain gardens and other natural processes. “Traditionally, cities have built huge, expensive concrete structures to control runoff,” says U.S. Geological Survey hydrologist Roger Bannerman, an expert on bioretention systems. “But the paradigm slowly is changing, and more and more communities are turning to natural solutions.”
Designed to capture and absorb rainfall where it lands, rain gardens are cultivated, bowl-shaped depressions, about 3 to 7 inches deep, strategically located near patios, roofs, lawns and other runoff sources. Their soils—sometimes augmented with compost or sand to improve permeability—work like sponges to absorb and filter the water, allowing it to percolate slowly into the ground. Researchers have found that such gardens collect about 30 percent more water than conventional lawns. When filled with native plants, they can also act as magnets for birds, butterflies and bees.
Ideal Form and Function
Most rain gardens will help reduce flooding, but ideally they should be big enough to hold all runoff from sources such as downspouts. Bannerman says a downspout rain garden should be 15 to 20 percent of the size of the roof area for homeowners who have sandy soil, 25 to 35 percent for those with soil containing some clay and 30 to 40 percent for homes with less-porous, all-clay soil. Such gardens should be about twice as long (perpendicular to the runoff source) as they are wide.
“Plant selection is very important,” says Jan Satterthwaite, a Seattle landscape architect and team leader of a local NWF Community Wildlife Habitat® project. She primarily uses plants native to the region that will thrive in wet and dry conditions and attract wildlife. Plants with deep fibrous roots provide the most cleaning and filtration benefits. Satterthwaite uses a mix of grasses and sedges, herbaceous flowering plants and native shrubs.
She has been designing projects for RainWise, a Seattle public utilities program that offers rebates averaging $4,000 to homeowners who install rain gardens and cisterns in areas where sewers overflow. Client Christina Burtner had sewer backups and flooding in her basement during storms. Satterthwaite put a 26-square-foot rain garden in Burtner’s yard using low-maintenance plants. “Not only has it solved our basement dampness and side-sewer woes,” says Burtner, “it also has added beauty to our property.”
Rain-Garden Tips
The following pointers can help you choose the best location and plants for your rain garden:
• For easier installation, look for flat areas and avoid steep slopes.
• Extend your downspouts or add rock-filled channels to direct roof runoff toward your garden.
• Construct the garden at least 10 feet away from your foundation to keep water from seeping into your basement.
• Don’t build directly over a septic system or utility lines.
• Avoid using an existing depression in your yard that constantly is damp or where water regularly pools, two indications that soil drainage at that spot is poor.
• Select a site that receives full or partial sun.
• For effective results, landscaping professionals suggest incorporating a mixture of deep-rooted native sedges, rushes and grasses with your flowering species (forbs).
• Select plants that bloom or produce berries at different times to provide continual habitat for wildlife. For names of appropriate native species, check with your local agricultural extension service or local native plant society.
For more natural gardening tips, visit www.nwf.org/nwfgarden.
Mark Wexler is editor-at-large of National Wildlife magazine.
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