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Ann English, a landscape architect who designs rain gardens, said gardeners in Seattle have created effective versions by excavating as little as 6 inches. In an online manual she has written for homeowners, she suggests that beds be excavated between 8 inches and 3 feet. The less deep the amended soil, the wetter the site, and the greater the need to select flood-tolerant flora.

English, a member of an environmental organization in Beltsville, Md., named the Low Impact Design Center, suggests a soil mix of 50 percent coarse sand, 30 percent low clay topsoil, 15 percent shredded hardwood mulch and 5 percent peat moss.

The Virginia Department of Forestry suggests 50 percent sand, 25 percent topsoil and 25 percent compost or leaf litter.

English has also designed plans of plant combinations that can be used in given situations: a 450-square-foot rain garden to attract and sustain birds, a 900-square-foot hedgerow for partial shade, a 150-square-foot corner bed in full sun, and so on. (You can find them at www.lowimpactdevelopment.org.)

At Brookside Gardens in Wheaton, Md., English worked with others to design the public garden's large-scale demonstration rain garden near the conservatories.

Formerly a series of cascading ponds, the hillside garden was routinely washed out in storms as water sheeted down a broad pathway and poured off a sloping field. It was so bad that staff had to drain the ponds before anticipated deluges, and even then gardeners would routinely have to retrieve and replant perennials and bulbs that had washed into the parking lot.

Completed with grants and donated labor and materials from a contractor, J&G Landscape Design, the $80,000 rain garden features an upper shade garden covering 175 square feet, where water seeps and drains into the lower garden of 550 square feet in full sun. The garden is framed by paths made from permeable pavers laid on an 18-inch bed of gravel. In its first really wet spring, it has performed admirably, said Lisa Tayerle, its gardener.

"In March, the garden had six to eight inches of rain, and two hours afterward, two-thirds was gone," she said. "It's working really well."

Permeable and beautiful

One of the key features of the upper basin, said Brookside's Phil Normandy, is the creation of a lip to form a bowl to hold the water. The depression is planted with wood asters, goatsbeard, lady ferns, two species of wood ferns and wild geranium.

The lower garden is a medley of native plants, including two species of amsonia, goldenrod, coneflowers, New England asters, baptisia and great blue lobelia.

Normandy said the concept of a rain garden evolved from the use of stormwater management ponds commonly found wrapped in chain-link fencing in shopping malls or town house developments, empty, littered and weedy.

"These are basically dry ponds full of water when it rains and an eyesore the other 11 months of the year," he said. "The idea was that you can make this permeable space beautiful, and this is a surprise to a lot of people."

"In my mind," English said, "the ideal rain garden is one where you don't walk by and say, 'Aha! A rain garden.' It's just a beautiful garden." Even if it's in your backyard.

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