Utilize Runoff to Create a Plant Oasis

Pictured above is the transformation of a horticulturally lost space. With the right soil, the right plants, and some hard work, the space has been completely renewed. Image courtesy UTIA. 

Warm temperatures mean different things to different people, but to University of Tennessee Extension specialist Andrea Ludwig, they mean the perfect time to build a rain garden. One of the top ten hot topics in landscape design for 2018, rain gardens are simply gardens designed to catch runoff water from rooftops, driveways, and parking areas.

According to Ludwig, a typical house roof will shed nearly 500 gallons of rainwater during a half-inch rain.  Since most of Tennessee receives over 50 inches of annual rainfall, this means nearly 50,000 gallons of rainwater becomes rooftop runoff each year. “This is water that can be nourishing healthy soils and lush landscaping around homes,” says Ludwig.

​For homeowners who are considering a rain garden, Ludwig says the best first step is to go outside when it rains and observe how water moves across the property. “Look for opportunities where the water slows down, in areas that catch runoff and aren’t being used otherwise.”

Rain gardens can come in all shapes, different sizes, and many colors. All of them bring countless benefits to the landscape that improve overall function, reduce mowing, attract beneficial wildlife, and protect community water resources. A rain garden can be as simple as planting water-loving, native Tennessee plants in a low lying area that already catches runoff to help soak it into the ground. A new depression can also be dug to create a rain garden at the end of a diverted downspout or set away from a driveway.

​If a new depression is the best choice, Ludwig cautions that a few considerations must be made in the process. First, make sure the sited location for the rain garden is in between the source of the runoff (e.g. impervious surfaces like rooftops or pavement) and where overflow will need to leave the property. The rain garden will be able to soak in runoff from most storms, but for anything over about an inch, the garden will likely be full and need to overflow the excess water safely to the storm drain system. Also, make sure there are no underground utilities, septic tanks or fields, or large tree roots. Place a Tennessee One Call at 8-1-1 before digging at all times.

Rain gardens should be located in the flattest area of the yard, at least ten feet away from structures, and absolutely must be built in soils that will infiltrate water within a couple days after a rain. Size is also important when building a rain garden, but will be partially determined by the soil type in the area. Soil samples can be sent to a soil testing center for analysis, and then homeowners can make the best decisions about the size and type of rain garden.

Once a rain garden is in place, wise plant selection, like native plants adapted to Tennessee soils and climate, will keep the soil healthy and able to absorb water. Choosing native plants found in wetlands or along stream banks will also supplement your rain garden. For more help in plant selection, consult the Tennessee Smart Yard native plant database at tynnativeplants.wordpress.com

For additional help or resources about rain gardening, contact your local county Extension office. For more information about creating a Tennessee Smart Yard, visit tnyards.utk.edu.  

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Dr. Andrea Ludwig, associate professor, Biosystems Engineering and Soil Science, 865-974-7238, aludwig@tennessee.edu