By Joel M. Lerner
Special to The Washington Post
Saturday, July 10, 2010; E01
Usually by this time of year in the Washington area, we have had about 20 inches of precipitation. As of this writing, we are within a couple of inches of the average at Dulles International and Baltimore-Washington International Marshall airports, but Reagan National is more than six inches below normal. We have seen worse than this, but precipitation this year has been fickle, in the form of winter rain and snow -- followed by significantly below-average rainfall in April, May and June.
So, when it does rain -- often in midsummer cloudbursts -- it's important to keep the water on your property for as long as possible. When it rains too hard or for too long, absorption into the soil is difficult, causing excessive runoff into storm drains, streams, and eventually into the Potomac and Anacostia rivers and the Chesapeake Bay. It's much more valuable to the landscape -- and much less wasteful -- if you can harness and clean this runoff water.
Zora Lathan, executive director of the Chesapeake Ecology Center in Annapolis, suggests doing so by modifying the land itself. "One solution to cleaning up the waterways is to put the contours back into the land -- to create rain gardens that imitate nature and allow rainwater to infiltrate the ground, to be filtered of pollutants and to recharge the groundwater."
A two-tiered, large-scale version of such a rain garden is in action at Brookside Gardens in Wheaton. It slows the flow of storm water through a shade garden, after which the water runs slowly into the primary rain garden below. The lower garden is installed along a paved area. The rain garden diverts the runoff before it reaches the paving, keeping the water on site and allowing it to percolate into the soil.
The Indianapolis Museum of Art has a rain garden that runs along a portion of the parking lot and, as Lathan suggested, "puts contours back into the landscape." The dip in the landscape edging the parking lot supports a large shrub border separating the museum's strolling gardens from the parking area. The depression is filled with moisture-loving shrubs and perennials that receive lots of water from the parking lot with every rain.
The science of rain gardens is still evolving, and there are many formulas for the required soil profiles. For example, the Low Impact Development Center in Beltsville suggests soil mixtures 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.
Rain gardens don't have to be deep gullies. They can be as natural as the permeable paving Brookside Gardens installed to allow visitors to view their rain garden, or a woodland filled with native plants, a wildflower meadow or any area where flora will do well with "wet feet." The depth of the rain garden can range from six inches to three feet, depending on drainage, soil type and runoff volume.
There are many national initiatives to promote the creation of rain gardens. Kansas City, Mo., launched a public-private effort in 2005 aimed at creating 10,000 rain gardens in the metropolitan area. Atlanta's Clean Water Campaign is encouraging their creation with a detailed how-to brochure. Closer to home, Pepco installed a rain garden at its Benning Road Service Center in the Anacostia River watershed in Washington. There have been efforts in Maryland by Pepco and environmental organizations to promote the installation of rain gardens by others.
Lathan launched a major public education campaign to encourage people in Anne Arundel County to practice bay-saving landscaping techniques such as green roofs, rain barrels, pervious paving and rain gardens. She calls it "rainscaping." Her efforts include posters, brochures and interactive sites where residents can see what these elements look like and how they work. Lathan always points out that rain gardens are simple to install in a shallow depression. "The garden in a saucer" is how Lathan describes it.
Some sites and locales are simply not right for rain gardens. It's important that homeowners do some research before beginning work. One of the most important factors is that the soil underneath the garden be permeable.
The first step is to test the soil. There are two simple tests.
-- Dig a hole six inches deep and fill it with water. If it takes more than 24 hours for the water to drain, that's a bad spot for a rain garden. You could have compacted fill or heavy clay. If you have a place in the yard where water typically pools after a storm, that's also not a good place for a rain garden, as the soil is obviously less permeable than needed.
-- To check for soil type, take some soil in your hand, add a couple of drops of water, and work it between your forefinger and thumb into a ribbon until it breaks. If it feels smooth, and the ribbon reaches an inch or longer before breaking, that indicates high clay content, which is not desirable for a rain garden.
If the soil is permeable, then determine how large a rain garden you want. You can install a rain garden of any size, but be aware that a very small one will catch and filter only a small amount of the water that falls on it. It can't hurt, but it won't be a lot of help in the larger scheme of things. And rather than planting one large rain garden, two or more smaller ones in different areas might be better.
Measure the footprint of your house and make a diagram showing all the gutters and how much of the roof each one drains. For every 1,000 square feet of roof, you should install a rain garden of 200 to 500 square feet. All rain gardens have benefits; they're aesthetically pleasing, and they all restore some amount of normal hydrological function to the landscape.
There are several rules to follow for rain garden installation:
-- If you have poor drainage or high clay content, soil might have to be removed to a depth of 24 to 36 inches and replaced with sand, compost and topsoil.
-- Rain gardens must be installed at least four feet above your water table and should be at least 100 feet from a well. They don't solve problems of underground standing water, but will keep underground water on site in well-drained areas.
-- The slope into a rain garden should be no steeper than a 20 percent grade.
-- Keep rain gardens two feet from property lines. Make sure that overflow will discharge into planting beds, away from structures or areas of standing water.
Visit these rain gardens in the Washington area for ideas on planting and to better visualize exactly what a rain garden is:
Chesapeake Ecology Center, http://www.chesapeakeecologycenter.org. Brookside Gardens, Wheaton, http://www.brooksidegardens.org. The Children's Rain Garden at Powhatan Springs Park in Arlington, 703-533-1839.
Historic London Town and Gardens, Edgewater, 410-222-1919, http://www.historiclondontown.org.
Joel M. Lerner is president of Environmental Design in Capitol View Park.