If you haven’t heard of rain gardens yet, it’s time to catch up with the latest way to give purpose to your plantings.

Briefly defined, a rain garden is a shallow basin filled with native plants that is designed to trap and filter rainwater. It captures runoff from roofs, driveways and pavement that otherwise would find its way to storm sewers, streams and, ultimately, our water supply — complete with a freight of pollutants.

A rain garden is not a pond, since a well-built one will drain completely in a day or so as water percolates through the soil and recharges underground aquifers. Think of it as a compact bit of man-made wetlands acting to neutralize contaminants that compromise water quality in our reservoirs, lakes, rivers and bays. (Runoff typically carries fertilizers, pesticides, motor oil and other chemicals flushed from lawns and roadways.)

Like natural wetlands, rain gardens can control flooding, too, especially in a built-up, paved-over state like ours. By keeping water out of overburdened drainage systems, rain gardens curb the flash floods and erosion that often result from heavy downpours.

“In rain gardens, personal gardening meets public water policy,” says Isaac Martin, vice president of the Native Plant Society of New Jersey and owner of Ladybug Landscaping LLC in Freehold. “This is a great way to solve any number of problems at once.”

Larry Coffman, associate director of the Maryland Department of Environmental Resources, is widely credited with developing the rain garden concept in Prince George’s County, Md. in 1990.

Coffman worked with developer Dick Brinker to design a 300- to 400-square-foot bioretention area on the property of each home in one new subdivision. This network of planted drainage basins cost about $100,000 — $300,000 less than the standard curbs, gutters and storm drains. The term “rain garden” came into use because it sounded more appealing than “bioretention area” or “planted drainage basin,” and an environment-friendly idea was launched.

Besides contributing to the health of public watersheds, rain gardens have other positive values. Planted with native species adapted to extremes of climate and fluctuating moisture levels, rain gardens are oases for wildlife. If you build one, the bees, butterflies, hummingbirds and dragonflies will come — but not the mosquitoes. By drying out quickly, rain gardens deny these pests the standing water their eggs need to mature and hatch, a process that takes at least a week.

So, what looks like a simple patch of wildflowers can be a hard-working flood-preventing, water-purifying, wildlife-attracting, mosquito-killing supergarden that works in just about any residential or commercial setting. The idea is so appealing and so effective that it is catching on in many states, including Wisconsin, Michigan, Pennsylvania and Missouri. Kansas City, Mo. has launched a campaign to see “10,000 Rain Gardens” built over the next decade.

The Native Plant Society, headquartered at Cook College in New Brunswick, is among the organizations promoting the rain garden concept in New Jersey. At the group’s Web site, www.npsnj.org, you can download a 35-page pamphlet that walks you through planning and construction, recommends sources for native plants and even offers eight rain garden designs suitable for the soil conditions found in the state.

The Rutgers Cooperative Extension Service is getting into the act, too. The extension’s Middlesex County office has built a demonstration rain garden at its EARTH Center in South Brunswick. Visitors are welcome to check it out any day during daylight hours.

RAIN GARDENS MUST BE located to intercept and temporarily hold drainage water from roofs, downspouts and paving. Beyond that, they are open to interpretation.

“You can get creative from there,” says Martin, who has installed a half-dozen rain gardens in Monmouth and Ocean County this year. “They are getting more popular as people realize that this is gardening with a purpose.”

Bill Young, a fellow society member who lives in Point Pleasant, installed a rain garden to process water pumped from his backyard pool, which relies on biofilters rather than chlorination for cleanliness. At the end of the season, Young can discharge pool water into an area planted with species that don’t mind getting their “feet” wet periodically.

Some homeowners build rain gardens near downspouts to slow the flow from gutters or near sump pump outfalls to process water lifted from basements. Others put them in their side yards, where the garden becomes a privacy screen obscuring the view of nearby houses. Another option is to locate these gardens in front yards, between water-shedding surfaces such as roofs and driveways and storm drains in the street.

What all rain gardens have in common is a shallow, level-bottomed depression excavated about 6 inches below the surrounding grade. Excavated soil is used to create a berm so water doesn’t flow in the uphill end of the basin and out the downhill side, creating gullies.

It helps to know the composition of your soil and its drainage characteristics — and here’s where a soil test available for about $10 from your county extension service office is a big help. The basin should be backfilled with a mix that water will pass through readily. A good formula is 50 percent sand, 30 percent excavated soil and 20 percent decayed leaves (leaf mold), often available free from municipal composting operations.

As for size, 150 to 300 square feet is generally recommended. Ideally, the size of the garden should relate to the size of the roof area or driveway being drained, but even smaller rain gardens will be an asset.

The location you choose should be at least 20 feet from the foundation of homes with basements to prevent water from getting into cellars. Shallow, grass-covered ditches or perforated plastic pipe can lead water to the rain garden. Any outfall from the planted basin designed to carry overflow from big storms should direct water away from the house, too.

After the rain garden is excavated, filled with suitable soil and planted, the final step is to cover the soil surface with an organic mulch. Shredded hardwood bark is recommended as a good trap for heavy metals and other contaminants; it won’t float and wash away as wood chips tend to do.

Costs will vary depending on the number and species of plants you need and whether you do the digging yourself or hire it out. Typically, you can build a 150-square-foot garden for $300 or so, including soil amendments and mulch. Hiring a designer will assure an eye-catching result, but will add to the price.

Choosing plants for the rain garden is “the fun part,” says Martin, and a process likely to introduce you to beautiful, undemanding species not commonly found in the aisles of most home centers. Native turtleheads, Joe Pye weed and meadow rue bear lovely flowers. Cardinal flower (Lobelia cardinalis) and jewelweed attract hummingbirds. Native grasses and sedges offer year-round foliage accents.

“People have to stop thinking in terms of the overly familiar evergreens and perennials you see in every yard,” Martin says. “They need to develop a new aesthetic.”

The beauty of a rain garden is a bonus, a reward above and beyond the satisfaction that comes from being part of the waste water solution instead of part of the problem. When your new garden is aflutter with birds and butterflies, when a great blue heron stops by for lunch or a frog appears for a drink and a dip, your yard will seem like a more hospitable place. You may even find yourself hoping for a rainy day to fill your garden — and your sense of satisfaction — to the brim.

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