Rain gardens, such as these in Knoxville, Tenn., can alleviate the negative impacts of stormwater runoff on local waterways. Photo by A. Ludwig, courtesy UTIA. Download photo.
As more of the Tennessee landscape changes from forest and meadows to urban and suburban settings, more landowners need to consider installing rain gardens.  University of Tennessee Extension biosystems engineer Andrea Ludwig explains why.
“The natural landscape in Tennessee is generally forest and meadows. When it rains on these landscapes, most of the rainfall is absorbed by the landscape,” Ludwig said. “When a house is built, that natural landscape is replaced with a rooftop, and often a lawn and driveway. When it rains on these surfaces, runoff occurs much faster and in greater volumes. This runoff is often referred to as stormwater. Rain gardens are one way that you can alleviate impacts of stormwater on local waterways.”
What sort of impact? As an example, Ludwig cited the average total annual rainfall for Knoxville, which is 48 inches. “This year has been a very wet year, and we have already seen 52 inches of rainfall by the first of September. This year an average Knoxville rain garden would have already diverted approximately 20,000 gallons of rooftop runoff from the storm drain and allowed the water to soak into the ground,” she said. “A single rain garden will not create a large improvement in water quality, but if every house and building used rain gardens to capture stormwater runoff, then the cumulative effect would go a long way towards sustainable, healthier communities.” 
Ludwig says rain gardens can be easily incorporated into landscaping in residential and small-scale commercial areas and provide the added benefits of increasing curb appeal, raising property value and decreasing the need for costly irrigation. She notes that research has shown that when impervious surface covers 10 percent or more of the land area in a watershed, streams and rivers begin to show signs of ecological degradation.   
“Rain gardens are designed to make the residential landscape mimic the natural landscape by soaking in the rainfall like the natural forest once did, restoring natural water movement and distribution in the landscape,” the expert said. “A rain garden is a planted depression in the landscape that captures runoff from rooftops or paved areas and infiltrates it into the ground. Rain gardens are installed in the pathway of runoff such that it is captured before it gets to the storm drain or creek. Rain gardens can be used in urban, suburban, and rural settings to decrease runoff and erosion.
Interested? If a rain garden seems like it would improve your property, Ludwig says before you take any other steps, you should go outside when it is raining and map out where the water falling on your property goes. “Do this by locating gutter downspouts and areas that collect and convey water away from your property,” she said. “The ideal spot for a rain garden is between the source of runoff, like a downspout or edge of a paved driveway, and where the water leaves your property, for example through a culvert or swale.”
Ludwig recommends against installing a rain garden where steep grades exist. “The slope should be less than 15 percent,” she said. “Keep rain gardens at least 10 feet away from building foundations, 20 feet away from septic systems and outside of tree drip lines and roots. Other things to consider are the location of buried utilities and how well your soil infiltrates (or soaks in) water.”
Ludwig says you can call 8-1-1 to request utilities be marked and perform a simple percolation test to ensure the soil where you want to put the rain garden can infiltrate the runoff. “If the percolation test hole drains within 36 hours, then the soils are suitable for an effective rain garden. Rain gardens will not work in soggy soils, because the water will not soak in,” she said. 
For more on how you can create ecologically friendly landscapes at your home, please visit our Tennessee Smart Yards website (https://ag.tennessee.edu/tnyards).
UT Extension provides a gateway to the University of Tennessee as the outreach unit of the Institute of Agriculture. With an office in every Tennessee county, UT Extension delivers educational programs and research-based information to citizens throughout the state. In cooperation with Tennessee State University, UT Extension works with farmers, families, youth and communities to improve lives by addressing problems and issue at the local, state and national levels.



Dr. Andrea Ludwig, 865 974 7238,