Building a rain garden
Photo courtesy of Carolina Clear
The record-breaking rains drenching South Carolina this year are producing an abundance of storm-water runoff. Carried in this runoff are pollutants—pesticides, excess fertilizers, petroleum residues—that flow, if not intercepted, into storm drains, ditches, and the nearest stream, pond or lake, creating a multitude of environmental problems.
Fortunately, rain gardens provide an attractive and environmentally friendly way to manage runoff. They capture runoff in your landscape and allow it to infiltrate the soil before it reaches a stream. Soil and plant roots in the garden filter out pollutants, and the water is used by the plants or recharges groundwater.
“Rain gardens can treat up to 98 percent of the pollutants found in your typical residential storm-water runoff,” says Katie Giacalone, coordinator of Clemson Extension’s Carolina Clear water-quality education program.
A common misconception is that a rain garden is a pond or bog. Actually, rain gardens are depressions planted with landscape plants and are designed to capture, store and absorb water from a one-inch rain within 24 to 48 hours. A proper rain garden will be dry most of the time.
“Rain gardens are like any other garden in your yard, just inverted, so these plant beds are an amenity on your landscape,” Giacalone says.
To install a rain garden, begin with an appropriate location—somewhere that rainwater typically flows during a storm, such as downhill from a gutter downspout. Keep it at least 10 feet from your home’s foundation so it won’t flood your crawlspace or cause other problems.
Rain gardens need soil that allows water infiltration. A simple soil perk test will help you determine the infiltration rate. Dig a hole approximately 1 foot deep. If the soil is dry, fill the hole with water and allow it to drain completely. Refill the hole with water and measure the rate of drop. If it drops more than one inch in an hour, the soil is considered well draining and suitable for a rain garden.
As to size, your rain garden should measure 5 to 10 percent of the drainage area directed to it. For example, if the site handles water from a downspout off a 1,000-square-foot roof section, then the garden should be 50 to 100 square feet.
An effective rain garden needs a ponding depth of about 6 inches. Dig out the selected area to create this depression. You may include a berm on the downslope side of the garden and add a swale or pipe in the berm to allow excessive rainwater to escape without eroding or damaging the garden.
Loamy sand is the most recommended soil type for rain gardens. Chances are you will need to amend your soil before planting. Add organic matter (compost) and take a sample to your Clemson Extension office for testing to determine if lime or fertilizers are necessary.
With your garden prepared, you can begin the fun part: planting. Sunny and shady rain gardens will require different plants, but each needs plants that thrive in both wet and dry conditions, Giacalone says. Options include American beauty berry, obedient plant, Southern blue flag iris, coneflowers, asters, daylilies and ferns.
After planting, apply about 3 inches of hardwood mulch to reduce erosion and weeds.
Once established, your rain garden shouldn’t need much watering after the first year, except during prolonged droughts. Fertilizer is practically unnecessary, because the garden collects fertilizers from adjacent lawns and gardens. Periodic weeding and tending to plants will be your biggest chores.
For more information about rain gardens, including a free “Rain Gardens” manual to download, visitCarolina Clear’s Web page.
S. Cory Tanner is an area horticulture agent and Master Gardener coordinator for Clemson Extension based in Greenville County. Contact him at email@example.com.