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Rain gardens hold moisture

Joel M. Lerner, APLD
Special to The Washington Post
A community of plants can slow draingage on a slope or serve as a rain garden. Illustrates GREENSCENE (category l), by Joel M. Lerner, special to The Washington Post. Moved Monday, June 29, 2009. (MUST CREDIT: Photo for The Washington Post by Sandra Leavitt Lerner.)
THE WASHINGTON POST | THE WASHINGTON POST

WASHINGTON – Precipitation this year has been fickle.

So, when it does rain, it’s important to keep the water on your property for as long as possible. When it rains too hard or for too long, a lot of excess water runs off into storm drains, streams, and into the watershed. 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, Md., 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, Md. 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.”

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.

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.

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.

• 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.

• Lerner is president of Environmental Design in Capitol View Park, Md., and author of “Anyone Can Landscape”(Ball 2001).