A crashing thunderstorm after a brutally hot day is one of Washington's summertime pleasures. The rain comes down in sheets, cooling the air and washing it clean.
But along with removing pollutants from the air, the rain gathers up any debris on your roof, lawn chemicals and fertilizer in the grass, antifreeze from the driveway, road salt, and dog waste. The stew rushes through storm drains into creeks and streams, then into the Potomac or the Anacostia River before emptying into the Chesapeake Bay.
Gentle soaking rains nourish lawns and plants. But in downpours--those inch-an-hour deluges--water can seep into basements, send topsoil down the road, and make a flood where the rose garden used to be. Downspouts, drainage pipes, and paving such as driveways all are designed for getting the water off a property and to the storm drain. After that, who cares?
An increasing number of soil scientists, hydrologists, and local officials care, because of the pollutants now found in rivers, streams, bays, and oceans, much of it contributed by private homes.
"Once water leaves your property, hits the curb, and goes into the storm drain," says Anne Guillette, a landscape designer in Pasadena, Maryland, who has much experience with storm-water issues, "there's no more opportunity to filter it of pollutants."
Many organizations and local jurisdictions are now so certain that saving the water begins at home that they're offering substantial subsidies or rebates--in DC and Montgomery County, up to $1,200--to homeowners willing to make some changes in how they handle the rainwater that falls on their property.
That's where rain gardens come in.
What Is a Rain Garden?
The purpose of a rain garden is to collect rainwater from all over your property--roof, downspouts, paved or impervious areas such as driveways and patios, and even hard turf--into a strategically located, bowl- or saucer-shaped place in your garden.
A rain garden is a spot in your yard that's been dug out to a depth of a few inches to a few feet, depending on the garden's size and the soil's consistency, then largely refilled with good soil and mulch and maybe stones or gravel, and finally planted with mostly native trees, flowering plants, grasses, and shrubs. Instead of rushing into storm drains, water collects in the rain garden and "percolates" slowly over a day or two through soil and roots and finally, largely cleansed of pollutants, downward to recharge ground water.
One of the miracles of science is that soil (even clay soil, according to a US Geological Survey study) in combination with native-plant roots is brilliant at absorbing and neutralizing pollutants.
You can get as scientific with your rain garden as you want to be. You can call it a "bioretention cell." You can follow to the letter the guidance offered in a detailed yet understandable 48-page booklet called "Rain Gardens Across Maryland." The booklet offers formulas for size, soil composition, construction, planting, and maintenance, among other things. It's downloadable for free at rainscaping.org or hgic.umd.edu.
What Can You Plant?
Rain gardens can be as beautiful as they are practical. You can arrange plants as you would in any other garden: Plant in sweeps of a single kind, and pay attention to different heights, textures, and silhouettes.
Tom Mannion, an Arlington landscape designer, has planned gorgeous rain gardens. One of the challenges of installing one, he says, is integrating it into the landscape. Adjacent to one of Mannion's rain gardens, in place of a lawn, he designed a small "meadow" as a transition between the rain garden and the rest of the yard.
"The meadow," he says, "is a low-maintenance sweep of native panicum grass and bronze fennel, which is a delicious herb for us and a food source for butterfly caterpillars."
Most environmental advocates suggest using native plants in a rain garden because they're more likely to be easy to grow and to attract native butterflies, birds, and other wildlife. However, plenty of nonnative plants can also work nicely--as long as, like the natives, they can stand to be in water for a day or two and can also persevere during times of drought, when even the rain garden dries out.
Among shrubs, you can't beat Virginia sweetspire (Itea virginica) in a rain garden--or any garden, for that matter. An azalea-size deciduous shrub that grows in sun or shade, sweetspire is a graceful plant with small but plentiful white flowers in late spring and gorgeous fall color. You'll find mostly the cultivar Henry's Garnet in local nurseries.
Slightly larger and best suited for rain-garden conditions is the spicebush, or lindera, also with sweet-smelling white flowers and fall color in an unusual soft orange that glows. Its color is best in full sun.
Many viburnums are perfect for larger rain gardens. Most have prominent berries beloved by birds. You'll appreciate the sweet pepper bush (Clethra alnifolia) for its late show of very fragrant, creamy white or pinkish upright blooms and glossy leaves.
If you have room for large trees, start with the black gum (Nyssa sylvatica), a native beloved for its ability to grow in wet areas and for its fall color. Among small trees, the fringe tree (Chionanthus virginicus) is tops. It has feathery blooms and large, glossy leaves, grows to 25 feet or so, and usually has multiple trunks. Also multiple-trunked is the river birch (Betula nigra), with flaking cinnamon-and-white bark and a penchant for damp soil.
Among perennials, experts recommend black-eyed Susans, sweet-flag irises, bee balm (monarda), and the majestic joe-pye weed, which is not a weed but a five-foot beauty with purplish flowers late in the summer when everything else has melted. Other good choices include cardinal flower, an upright perennial with blazing red blossoms late in the season, and foamflower (tiarella), a low-growing flowering plant with maple-shaped leaves. Many ornamental grasses, with their mostly upright, sometimes arching habit, are also good bets.
These are some of the highlights; you can find a comprehensive list of other nice natives in "Rain Gardens Across Maryland."
Are Mosquitoes an Issue?
Here are some guidelines to keep in mind when it comes to rain gardens:
• Home rain gardens have ponding depths of five to eight inches.
• The garden's location will depend on where storm water flows naturally, where you can direct it with the use of downspouts, and how your land slopes.
• The garden's size will depend on how ambitious you are, the size of your lot, the amount of water you want to filter, and how much of your land is covered with impervious material such as driveway, roof, and patio. Most of the literature estimates 100 to 400 square feet or so for the average rain garden, but experts say not to worry too much about size; even a smaller garden can be beneficial.
• Rain gardens should be situated away from a house's foundation; experts recommend ten feet as a safe distance.
• Because the percolation of water through rain-garden soil and roots lasts just a day or two, mosquitoes have no chance to breed.
• Most gardeners already have some idea of the nature of their soil and how it drains. If you're not sure, there are ways to figure out how well your soil percolates. To determine whether it's mostly clay (slow to drain) or mostly loamy or sandy (quicker), you can perform a simple experiment: Dampen a handful of your garden soil and try to form it into a ball. Clay easily forms a firm ball. Sandy or loamy soils tend to crumble, and very sandy soil pours through your fingers.
• You can build a small rain garden yourself or with the help of a few strong friends. Some landscape contractors can do the job, but make sure they've done it before and know their stuff. Ask for references.
Design Help & and Rebates
Your best bet is to get in touch with one of the environmental organizations offering rain-garden plans, information, and, best of all, rebates. Although not all jurisdictions offer them, many do and the rest may soon follow.
In the District, for example, RiverSmart Homes--a program run by the DC government in conjunction with nonprofit environmental groups and local businesses--offers to provide up to $1,200 worth for rain-garden installation work on your property. Montgomery County offers a similar program. This includes site assessment, design and installation, and the purchase of plants. It can also include turf removal, planting of shade trees, and replacement of driveways and other impervious surfaces with pervious materials.
To do the work, RiverSmart Homes supplies local contractors trained in the DC Department of the Environment's requirements. You can also hire your own contractor.
Rebate programs are not yet offered in Northern Virginia.
Detailed advice is all around, especially online. These websites are particularly helpful: raingardens.org, arlingtonecho.org, raingardennetwork.com, and montgomerycountymd.gov/rainscapes.
For those still unsure about the design and placement of a rain garden, Maryland wetland scientist and environmental maven Spencer Rowe may have the best advice: "The rain garden does not have to be perfect to do its job, and it will change over time--that's one of the things that make it so rewarding. It's a living dynamic system. Dig a hole, relax, and let nature take its course. Observe and have fun."