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Swales follow the contours around the base of a natural or created slope, redirecting storm water and filtering runoff as it sinks into the soil, instead of keeping it in one place, like a rain garden. Plants suck up the water along a swale’s gently sloping banks and sometimes down the center of the channel. Fast-draining soil maximizes drainage efficiency, and perforated pipe underneath the gravel can help with heavy water flow.

Small swales can direct gutter water to a dry well, while more substantial ones could run along the base of a hill to divert water around a low-lying house. Jan Johnsen, a landscape designer in Mount Kisco, New York, landscapes swales with a river rock lining and plants that thrive in moist conditions along the sides: evergreen ferns, sedges, winterberry, grasses, and Siberian and Louisiana irises. She also uses rugged prairie plants or other natives to fast-draining soil.

Photo by Saxon Holt

Creating a Swale

You can build a small swale yourself, but should hire a pro with earth-moving equipment for a long, wide one. Consult a landscape contractor or a civil engineer if you live near a bluff, have a septic tank nearby, or are on a slope that drops more than 1 foot over a horizontal distance of 20 feet.

A swale should carry water to a place where it can be released safely, such as a garden bed with good drainage or a buried dry well; allowing it to be absorbed on-site, rather than flow into a storm drain, is important for protecting natural waterways. The sides of the swale should flare so they ­extend out three to four times more than they are tall, and the first 8 inches of soil should drain well. If your soil drains quickly (at least ½ inch per hour), it can just be loosened. If water sinks in half that fast, amend it with 40 percent compost. Where drainage is ­slower still, replace soil with 60 percent screened sand and 40 percent compost. If the swale itself can't be made big enough to handle all the water, consider excavating another 6 to 8 inches, lining the trench with filter fabric, laying perforated pipe, then covering it with round ¾-inch gravel. Top the gravel with at least 8 inches of a compost-rich soil mix.

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