Be Part of the Solution: Create Your Own Rain Garden

by Gwendolyn Blanton, Environmental Specialist

These days, with so many roads, parking lots and rooftops, a lot of rain water never soaks into the ground — instead it flows immediately to the nearest storm drain and then goes crashing into a stream or river. Increased runoff causes more flash flooding and each rain event washes car leaks like oil, grease and antifreeze, excess fertilizers and other pollution into our rivers and streams.

Here’s a way that you can help reduce runoff and beautify your property at the same time — install a rain garden. A rain garden is a shallow pond area with soil, native plants and mulch that captures rainwater and lets it soak into the ground to help recharge vital groundwater and slow rainwater entering streams.

Choose Your Rain Garden Spot

Start by noticing where your rainwater flows and then try to put your rain garden in this area. If you have an area that collects standing water after heavy rains, this might be a good spot. You can create a small terrace or redirect rainwater to your preferred garden site by creating a swale or putting in underground piping. Keep these pointers in mind when deciding where to put your rain garden:

• Stay downhill and at least 10' away from building foundations and basements.
• Avoid septic system drain fields, underground utility lines and digging under large trees.
• Avoid black walnut trees as they put off chemicals that inhibit other plant species.
• Make sure any overflow from the rain garden will discharge to a suitable location.

Size Your Rain Garden

Ideally, the area of your rain garden should be about 20-30% of the surface area (roof or pavement) that drains into the garden, but remember any rain garden is better than none and you have to start somewhere!

Check Your Soil

Test the drainage in your garden site by digging a hole 8" deep and 8" wide. Pour in a bucket of water. If the water doesn’t go down at least 1" every hour, you will want to improve or replace your soil — or try a different spot. Even if your soil has good drainage, dig your garden a little deeper and then add some of the soil back, mixing with sand and compost.

Design Your Rain Garden

There are three zones in a rain garden: very wet, wet-to-dry, and dry. Plants that like very wet feet will be in the center of the garden (the deeper, flat part). Plants that like wet-to-dry will be toward the edges on the slopes leading up to the yard level, and plants that like dry will be at the top of the slopes and on the outer edges of the rain garden. Your garden will be scalloped like a bowl with a flat bottom, sloping down from the sides to about 4-6" deep in the middle.

To start, dig out your rain garden area 6" to 2' deeper than your finished level — make sure the bottom is flat. The more compacted or “clayey” your soil is, the deeper your initial dig should be. Mix some of the removed soil with sand and compost and put it back into the rain garden as a planting medium, or discard it and use a rain garden soil mixture of 50-60% sand, 20-30% topsoil and 20-30% compost as the bottom of your rain garden.

Before adding your soil back, make sure you don’t have perfectly smooth walls and bottom. You’ll want to scratch the scallop to help with infiltration before adding your mixture.

When you’re ready to plant, test how the rain garden will hold water by running a hose from the downspout. You may need to make adjustments like adding a berm on the lower side of the garden or revising the flow of water into the rain garden or the overflow away from it. This will also help settle the soil before planting. Let the soil dry some and you’re ready to plant!

Plant Native Tennessee Plants

Take into account how much sun your rain garden receives. You may want to plant some larger plants that will eventually give shade and plant the shade-loving things later. Also, your garden will actually be dry most of the time so choose plants that can tolerate both wet and dry.

Learn More

For a list of rain garden plants, or more details about building a rain garden, contact the HRWA office. You can also find more information at www.raingardens.org or check out

Gardening with the Native Plants of Tennessee by Margie Hunter for planting suggestions.

-From Voices for the Harpeth, 2004