Cultivating a Solution to Water Pollution

Tips for building a rain garden to protect freshwater resources

02-01-2002 // Melanie Radzicki McManus

NOT SO LONG AGO, Roger Bannerman noticed that whenever heavy rains pelted his front lawn, most of the water washed off his property and into the street. From there, it flowed into Madison, Wisconsin, storm drains and eventually wound up in nearby Lake Wingra. A watershed scientist with the Wisconsin Department of Natural Resources, Bannerman calculated that a relatively small amount of water--just two inches per hour--was absorbed by his lawn during those storms. And since the remaining runoff water was probably picking up oil, chemicals, pesticides and sediments built up on lawns, driveways and streets as it flowed along, he knew he was contributing to local water-pollution problems every time it rained.

Bannerman decided to solve the dilemma by creating a so-called rain garden in his front yard. Two years ago, he dug out a 180-square-foot section of his lawn, mixed compost with the soil and put native plants into the sunken area. Soon, his rain garden began catching a large amount of rainwater--more than doubling the previous water-absorption rate at about five inches per hour--while also filtering out pollutants. Impressed with the results, he built several additional rain gardens in his backyard last year. Apparently, he is not the only Madison resident turning to this pollution solution.

"In this area, we're seeing tremendous interest in the concept," says Bannerman. One new subdivision in Madison, he notes, now requires that every homeowner include a rain garden in his or her landscaping. There's good reason why.

"Storm water runoff is one of the biggest sources of surface water pollution in this country," says Craig Tufts, chief naturalist for the National Wildlife Federation. And with spring rains just around the corner, he notes, "winter is a good time for homeowners to begin planning their own rain gardens as a way to help protect freshwater resources."

Needless to say, the first step in creating such a garden is deciding where it should go. If you're building a new home, ask the landscaper to grade your lot so the water flows to two or three areas at its edges, where you can locate several rain gardens. How you select a site at an existing home depends on how and where the water flows in your yard when it rains, and how you use your property. Some of the best sites may be located along the sides of your house near downspouts, where you can snag roof runoff. (Side rain gardens can also create a beautiful living fence between your property and the neighbor's.) Just make sure that you locate downspout rain gardens at least ten feet away from the foundation of your house, so you don't inadvertently direct water into your basement.

Next, determine the garden's size. A rain garden should be at least 150 square feet, says Bannerman, and it should match your soil type. The more clay in your soil, the less porous it will be, thus requiring a larger garden. Bannerman estimates downspout rain gardens should be 15 to 20 percent of the size of the corresponding roof area if you have sandy soil, 30 percent for soil containing some clay, and as much as 60 percent for true clay soil. If it will be capturing roof runoff, the garden's size should also be linked to the square footage of the roof section draining into that downspout.

"But don't stress too much about these numbers," Bannerman says. If the calculated size is too big for your liking or takes up needed space, simply downsize. "Smaller rain gardens will accomplish less, but if a smaller size makes it possible for you to build one, do it," he adds.

Whatever its size, a rain garden should be created in a shallow depression roughly two to four inches deep. To enhance water infiltration, clay soils should be rototilled, then mixed with three or four inches of compost and rototilled again. Sandy soils can simply be mixed with compost. The base must then be carefully leveled, says Jane Kuzma, a registered landscape architect and owner of Bur Oak Designs in Wisconsin. "The trick," she notes, "is to make sure the bottom is level so you don't have water ponding in one end of the rain garden and not in another."

It is important to build up a berm around the rain garden's perimeter--use the dirt you shoveled out for the garden's base--then compact it firmly, leaving one section open where the water can flow into the site. Plant grass on the berm. This buffer serves as the first filter for the storm water; it also slows down the water's flow out of the garden.

If your lot has a steep slope or you live in an area with frequent mudslides, this traditional setup won't work. But there is an alternative, according to California resident Cynthia Brian, a professional gardener and coauthor of Chicken Soup for the Gardener's Soul. She lives on a hillside with a creek at the bottom. The creek frequently overflows, and during the rainy season, rainwater often flows heavily down her hillside. Brian tackled the problem by creating a dry creek bed out of rocks that winds down her backyard slope, ending in a rain garden near the creek. The dry creek bed channels rainwater into the rain garden, preserving the rest of her yard while giving the storm water a chance to be filtered.

Which types of plants grow best in a rain garden? Native species of trees, shrubs, grasses and even flowers that can survive in both dry and wet conditions do best, says Larry Coffman, associate director of the Prince George's County Department of Environmental Resources in Maryland, which developed some of the country's first rain gardens a decade ago. Species native to floodplains in your region are especially effective, but overall your choices are vast.

A rain garden guide published by Prince George's County lists nearly 50 suitable species, including red maple and sycamore trees; red-osier dogwood, nannyberry and sweet pepperbush shrubs; Virginia bluebell and goatsbeard perennials and annuals; grasses and sedges such as redtop and switch grass. Plants that can tolerate standing water and fluctuating water levels are typically planted in the center of the rain garden, while those at the outer edges grow in slightly drier conditions.

Bannerman warns that certain native prairie plants can grow as tall as ten feet. So if you're concerned about aesthetics, you may want to select species that grow to a maximum height of three to four feet. You may also want to choose plants that flower at different times of the year.

"One of the nicest things about these gardens is they attract wildlife," says Brian. "I've got quail picking around because the water washed down different seeds, for example, and there are frogs and crickets and pollywogs." Bannerman often sees birds and beneficial insects in the grasses in his rain garden. But, he adds, "I am particularly pleased that my yard is no longer contributing to local water problems. I figure that in two years my rain garden has filtered about 16,000 gallons of water."

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