What is a rain garden?
A rain garden is a bowl-shaped garden that captures runoff and allows it to soak into the ground rather than run down the storm drain and into the creek. Its soil filters out pollutants, and its plants take up excess nutrients and prevent them from polluting the creek.
Before building a rain garden, it might be a good idea to talk to someone who's done it. Here is one woman's rain garden construction saga...
Besides helping water quality, a rain garden can drain wet areas of your yard, and it can be a beautiful garden that attracts birds and butterflies and doesn't require watering!
Size and Location
A rain garden should be placed at least 10' from your house (25' if you have a basement), 2' from any lot line. It should be fairly level and down grade from your house. Overflow should have a place to go, such as into a storm drain. Its shape does not matter--whatever pleases you. You can calculate how large it should be based on the size of the roof area you intend to drain. Or, consult the Northern Virginia Soil and Water Conservation District's Rain Garden Design and Construction manual.
Virginia clay soils are not ideal for rain gardens because they drain poorly. According to Prince Georges County, rain gardens should be sited in areas with an infiltration rate of 1" per hour. To see how well your soil drains, do a simple perc test. If you have clay soils, it may be a good idea to dig the area to a depth of 2'. Replace the clay with bioretention mix (50-60% sand, 20-30% leaf compost, and 20-30% topsoil). Fill the excavated area with bioretention mix, then 2-3 inches inches of shredded hardwood mulch (not bark-- it will float away.) The finished garden should be 2 to 3 inches below the level of the surrounding yard, and shaped like a saucer.
The plants in your rain garden should be able to tolerate wet feet, but also periods of drought. Native plants work best. See recommended natives for rain gardens in the Rain Garden Design and Construction manual.
Putting your rain garden to work
Finally, hook up one or more downspouts (or your sump pump) to your rain garden and watch it fill up with water!
After it rains, your rain garden should drain fairly quickly. If water ponds for 4 days or more, there is a risk that mosquitoes will breed in it.
Information about rain gardens
The Northern Virginia Soil and Water Conservation District Rain Garden Design and Construction: A Northern Virginia Homeowner's Guide offers practical instructions. It includes a size calculation worksheet and plant recommendations.