Want to help improve your landscape while also helping provide sustainability for our environment? Sure you do! One way you can do that is with a rain garden! Rain gardens increase water infiltration back into the earth (rather than run-off and erosion) while cleaning up the water before it goes into our lakes and streams! It’s a win/win thing!
If this is your first real introduction to rain gardening, you’ll need to know rain gardens impact watershed management as well as water quality. That’s a good thing! Complicated? Not really. A rain garden is basically a shallow depression in a landscape capturing water and holding it for a short time to reduce downstream flooding. Notice, I did say a "short" time, so get the swamp or pond scene right out of your mind! It is not there!
To build a rain garden, you’ll want to look for an area that holds water or where water runs from downspouts or driveways and sidewalks. You’ll want to avoid those "mushy" areas where water stands most of the year. Soggy areas like that can be made into an entirely different thing – crawfish ponds. Just kidding! Those can be made into wetland areas. Be sure to watch out for septic systems, field lines, gas lines, cables, etc., in your site selection.
Dr. Cathy Sabota, Urban Extension Horticulturist, provides some additional information on testing your site and sizing your rain garden.
The next step is to test the water-holding capacity of the soil. Dig a hole in the middle of your site about one-foot deep and one-foot wide. Fill the hole with water. If it takes less than 12 hours to drain, you can plant a regular rain garden. When the drain time exceeds 36 hours, your site is considered a wetland. But if it drains in less than 36 hours but more than 12 hours, you will have to zone your rain garden (add-in high spots or mounds).
Sizing the rain garden. For water quality, the rain garden will need to capture a depth of six inches. Water should get in easily and, in the case of larger storms dropping more than one inch, the water needs to overflow into the lawn area without causing damage or erosion. If you will be collecting water from the downspouts, calculate the square feet of the area of the roof served by those downspouts. You may be adding your driveway, as well. Measure or pace off the driveway and sidewalks draining into the rain garden. Add the square feet from the roof area, driveway and sidewalks, and divide by 20. That will give you the area requirements for a garden water-depth of six inches. Divide the area figure by ten instead of 20 for a three-inch-deep rain garden. If, for example, the first floor area of your home is 1,800 square ft and you plan to collect water from gutters serving a quarter of the roof area, your calculation would be: 1,800 sq ft X ¼ = 450 sq ft/20 = 22.5 sq ft or a four-and-a-half ft by five ft garden that will hold six in. of water would be sufficient.
Remember, a rain garden should receive its water at the flush end and the water should move toward a berm (or elevated section) at the other end of the garden. The garden should be dug four to six-inches deep with a slight depression in the center. Hint: Keep the soil you dig out to use later in building the berm! Cover the berm with mulch or grass to prevent erosion.
Planting a rain garden isn’t rocket science, but you do want to choose plants tolerant of fluctuations in soil moisture. A plant list is available at www.aces.edu/waterquality/mg.htm. Also, remember to plant "high" on the edge of the rain garden or on mounds for plants not as tolerant of the moisture.
Be sure to water your plants regularly (once per week if it doesn’t rain) and to fill your garden with an even coat of mulch, two inches deep.
If you’ll look at some photos of rain gardens, you’ll see they are not only functional and helpful to us and our descendants, but they are also very attractive. Start today to learn more about rain gardens. It’s the smart thing to do!
Jerry A. Chenault is an Urban Regional Extension Agent with The Alabama Cooperative Extension System, New & Nontraditional Programs division.