By Jeanne Huber
A RAIN GARDEN is a way to turn a problem–excess storm water– into an amenity.
Instead of dumping your gutter water into a storm drain or letting it empty next to your foundation, where it can make your basement or crawl space damp, you funnel the water to a low spot on your lot. There, plants soak up some of the water and the rest mostly percolates slowly into the ground. If you do it right, you’ll get lovely, low-maintenance landscaping that attracts birds and butterflies.
Rain gardens are designed to replicate the way thick grasses and forests once absorbed rainfall. By holding back the rain, these gardens help reduce flooding and allow microbes in the soil to break down pollutants, such as spilled automobile oil, before they reach rivers and streams. Rain gardens are “a beautiful solution to pollution,” says Larry Coffman, a private consultant who originated the rain garden concept while he was associate director for programs and planning with the Prince George’s County (MD) Department of Environmental Resources.
Coffman guided planning for the first rain gardens, which were installed in 1998 in Somerset, MD. The developer skipped the usual storm drains and storm sewers and instead installed a 300- to 400-sq.-ft. rain garden on each 10,000-sq.-ft. lot. This approach saved $4,000 per lot and worked so well that communities across the country are now promoting the idea as a way of reducing the harmful effects of development. The District changed its plumbing and building codes last year to encourage the concept. Storm water can now be diverted into rain gardens (as well as green roofs, rain barrels and cisterns), while previously it had to go into a storm sewer system, which allowed untreated water to reach Chesapeake Bay.
So how do you build a rain garden?
Engineers need to calculate water runoff and specify a specific mixture of gravel, sand, soil and compost when they design large-scale rain gardens that handle runoff from commercial buildings and parking lots. But, if you want to build a rain garden at your house, you can “engineer in place” by starting small and watching what happens when you begin using your storm water instead of throwing it away.
You might begin by directing water from one downspout to a garden bed that’s at least 10 ft. away from your house (so water doesn’t seep into the basement or crawl space). Or you can also direct the water to a low spot in your yard and turn that area into a garden.
If your soil is relatively sandy, you can probably use whatever plants you like. If the soil contains a lot of clay, choose plants that like wet soil. Native plants are best because they are adapted to the climate and provide shelter and food to a wide variety of birds and other wildlife.
Good choices for the Mid-Atlantic include swamp milkweed, joe-pye weed, New England aster, wild bergamot, and black-eyed Susan, all native to the Chesapeake region. Lord Baltimore hibiscus–a hybrid of the native hibiscus–purple coneflower, coreopsis, bee balm and blueberry are other options. (Editor’s Note: Google “native plant society” for your state recommended species for where you live)
After you see what plants thrive with the amount of care you’re willing to provide, you can expand your rain garden or build more, so that each downspout is connected to one. To maximize water absorption, you may want to invest more energy and money in amending your soil. Test it first to determine how well it is draining. Dig a hole 8 inches deep and 8 inches wide and dump a bucket of water in it. If the water goes down at least 1 inch per hour, you’ll need to dig down just 6 to 12 inches and refill half way with a fluffy soil mixture that contains one part compost to 10 parts soil.
If the water drains more slowly, you can make your rain garden function better if you excavate 2 feet down and refill most of the hole with a mixture of 50 to 60 percent sand, 20 to 30 percent topsoil, and 20 to 30 percent compost. Where the soil is especially heavy with clay, you might want to excavate even deeper so you can add gravel before layering the fill soil on top. Spaces between the gravel act as a reservoir, allowing the rain garden to hold more water. Or you can add a trench drain to carry away the excess water.
Before you begin deep digging, call your local utility company to find out how to locate underground wires or pipes. Also spread a tarp so you can store the excavated soil without making a mess of nearby landscaping. Add the compost and perhaps sand as you refill the hole. Be sure not to incorporate clay in the replacement soil. Don’t mound the enriched soil into a raised bed. A rain garden needs the opposite shape — a shallow depression, usually about 3 inches, where water can pool during the rainstorm and then slowly sink into the ground. “That’s the point,” Coffman says. “We want to keep water where it falls and retain pollutants on the land instead of washing them off into our rivers.”
Check your work by watching what happens in heavy rainstorms. If the rain garden overflows, make it larger or add a trench so the runoff can go to another rain garden or even into the storm sewer or whatever solution you now use. Even a small, functioning rain garden is better than no rain garden — or the ultimate rain garden that exists only as a plan.–AP