A Lesson in 'Rainscaping': Building a Garden That Makes the Best Use of Water

The mid-Atlantic region typically has 100 storms a year. About half of them produce rainfall of four-tenths of an inch or less. That's only about 10 to 15 percent of annual runoff. The other half of the storms produce most of the rainfall, some several inches. "And in those storms," Schwartz said, "even green spaces on compacted fill produce runoff almost as heavily as paved areas."

So the first thing a homeowner planning a rain garden should do is check the soil. Schwartz offered 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 may 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 less permeable.

· To check for soil type, take some dirt 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 is an inch or longer before it breaks, that indicates high clay content. It's not desirable soil for a rain garden.

If the soil is permeable, the next step is to figure out how big a rain garden you need. You can put in a rain garden of any size, but be aware that a very small one will catch and filter only a small proportion of the water that falls on it. It can't hurt, but it won't be a lot of help. And rather than planting one huge rain garden, it's probably better to plant two or more smaller ones in different areas.

Schwartz suggests that you measure the footprint of your house and make a diagram showing all the gutters and how much of the roof each one drains. A rough idea would be that for every 1,000 square feet of roof, you might need a rain garden of 200 to 500 square feet. The garden should not be square; many guidelines suggest a kidney shape.

All rain gardens have benefits, Schwartz said: They're aesthetically pleasing, and they restore some normal hydrological function to the landscape. He and fellow researchers are studying ways to make larger areas function as rain gardens by using agricultural techniques. He foresees a day when an entire public park, for instance, might return to its natural role as a water-absorbing, water-cleaning landscape.

Here are some more tips to make the best of your rain-garden installation:

· If you have poor drainage or high clay content, soil might have to be removed to a depth of 24 to 30 inches and replaced with sand.

· Rain gardens should be installed at least four feet above your water table. They should be at least 100 feet from a well.

· 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 in a safe direction, into planting beds, away from structures or standing water.

Check the following Web sites for additional information: Chesapeake Ecology Center, http://www.chesapeakeecologycenter.org; the bio-retention manual of Prince George's County, http://www.raingardens.org/Rain_Garden_History.php (Scroll to "Maryland Rain Gardens," and click on the link for the manual); Wisconsin Department of Natural Resources, http://dnr.wi.gov/runoff/rg.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, http://www.gardenlerner.com.