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Backyard rain gardens manage stormwater runoff

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By Judy Koehly
Master Gardener
Stormwater runoff is one of the leading sources of pollution in waterways, but rain gardens can be a great way to manage storm water. Rain gardens are shallow depressions planted with perennials and woody plants, which collect water from roofs, driveways, other impervious surfaces as well as turf grass (which, like a driveway, is terrible at absorbing water).
Rain gardens slow down and reduce runoff and thus help prevent flooding and erosion. In addition, the garden’s soil and plants filter pollutants in rainwater. When it rains, water initially pools in the garden’s plant zone, percolating quickly from there into the permeable layer underneath. The permeable zone then stores water until it seeps into subsoil. Rain gardens may improve water quality, as well, capturing common contaminants such as excess nitrogen and phosphorus.
To select a location for the rain garden, begin by observing your yard during rainfall. Notice where water is flowing from, and where it is going. Rain gardens should ideally be located between the source of runoff (roofs and driveways) and the runoff destination (drains, streams, low spots, etc.). The garden should not be within 10 feet of the house foundation and located at least 25 feet from a septic system drain field or a wellhead. Make sure to avoid underground utility lines.
The best location for the garden will be in partial to full sun; rain gardens should be constructed where the water table is at least two feet below the surface of the soil. If you hit the water table when constructing your rain garden, consider turning it into a wetland garden.
Rain gardens work best when constructed in well drained or sandy soils that are very prevalent in southeastern North Carolina. Rain gardens can be large or small; the size depends primarily on the site drainage area. The volume of water to be collected will be roughly equivalent to the amount of rain falling on impervious areas draining to the garden location, such as driveways and rooftops. In North Carolina, we typically try to capture runoff from an inch of rainfall.
The garden should be dug four to six inches deep with a slight depression in the center. The dug out soil will be used to create a berm along one side of the rain garden, which will allow water to be retained during a storm. If the garden is located on a slight slope, the berm should be located on the downhill sloping side of the rain garden. To prevent erosion, the berm should be covered with mulch or grass.
Once the rain garden has been dug, planting can begin. It is important to note that plants in a rain garden will have to tolerate fluctuating levels of soil wetness. To help plants survive extended wet periods, it may help to plant the plants “high” on the edge of the rain garden or on mounds within the rain garden to elevate the roots above the ponded water level. Associated plant lists are available for guidance in plant selection (www.bae.ncsu.edu/topic/raingarden/plants.htm).
Finally, the area should be mulched with two to three inches of hardwood mulch. Lighter mulches will tend to float, so avoid pine bark and pine straw mulches. Mulch is important in pollution removal, maintaining soil moisture, and in preventing erosion. Once established, your rain garden will be a low maintenance asset to your property and nearby waterways.
To see a photo gallery of rain gardens in coastal North Carolina, go to www.bae.ncsu.edu/topic/raingarden/coastalgardens.htm.
Send your gardening questions or comments to Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.