Shannon Kennedy, ANR Educator, UGA Extension Forsyth County
I have been astonished by the amount of rain we have received over the past few months. According to the National Weather Service, the precipitation total for the past five months is only 15 inches away from last year’s total. The National Weather Service predicts that the rain will continue through summer. This means our landscapes will have to deal with an incredible amount of rainwater runoff from impermeable surfaces.
One way to mitigate runoff is to construct a rain garden – a specially designed flower bed that captures rainwater runoff and allows it to soak into the ground. Rain gardens function to reduce flooding and drainage problems, recharge local aquifers, provide habitat for wildlife and pollinators, and add beauty to the landscape. They also decrease the amount of pollution from excess fertilizers, pesticides, and oils that wash into streams from paved surfaces. The first step for building a rain garden is finding where to place the garden. Observe your property during a rain event and ask yourself: where does water drain off your house or driveway? Where does it move?
You want your rain garden to be at least 10 feet away from the foundation of your house and perpendicular to water flow paths. To test soil permeability and drainage, dig a hole 6 inches deep and 6 inches wide in the area you want to put your garden. Fill the hole with water and watch the water level to see how quickly it drains. If the hole does not drain within 12 hours, your soil is not suitable for a rain garden. When it drains, fill it with water again and if it drains within 12 hours a second time it is suitable for a rain garden. Avoid areas where water accumulates because these areas have poor drainage and a rain garden is meant to facilitate drainage. If you wish to reduce ponding in an area, place the garden up-hill from where the ponding usually occurs.
The second step is to determine the size of your new garden. This is influenced by the amount of area the garden will drain, the type of soil, and the slope of the landscape.
If you are building a rain garden for a large area, the garden will have to be larger and deeper to accommodate more runoff. If your soil does not drain very well or if the slope is steep, increase the size and depth of the garden. After you have determined the size, shape, and location of the garden remove 4-6 inches of topsoil. Next, excavate the soil to the desired depth, (no more than 8 inches deep) being careful to keep the bottom of the bed level. Use the extra soil to create a berm on the downhill side of the garden. Lastly, amend the topsoil if needed and put it back into the newly constructed flower bed.
Finally, you can begin to add plants. Keep in mind that rain gardens are meant to drain water away, so the plants you choose must be tolerant of wet and dry conditions. Woody plants such as river birches, sweetbay magnolias, and oakleaf hydrangea are likely to work well in this type of environment. Herbaceous perennials to consider are aster, swamp milkweed, foam flower, and canna lilies.
A rain garden does not need any special maintenance. Simply fertilize it based on a soil report, use mulch to suppress weeds, and water plants during extended dry weather. If you have any questions about how to plan and construct a rain garden please contact the Forsyth County Extension Office, we would love to help!