By: Bob Howard, Mobile County Master Gardener
What is a Rain Garden? Spoiler Alert – it is not about the plants -- it’s about the rain!
A rain garden is an area in the landscape where rainwater from a roof, driveway, or street can be captured and allowed to soak into the ground rather than run off.
Planted with grasses and flowering perennials, rain gardens can be a cost effective and beautiful way to reduce runoff from your property. Rain gardens can also help filter out pollutants in runoff, and they provide food and shelter for butterflies, songbirds, and other wildlife. More complex rain gardens are sometimes referred to as “bioretention” zones.
Rain and tropical storms dump lots of water into the Mobile area. Our sandy soil drains quickly, but it can also wash away quickly. A rain garden is basically a planting basin to help control the rain – a garden bed that you plant with special deep-rooted native plants to help the water rapidly seep into the soil. The garden captures the water and, when properly designed, allows it to soak into the soil within a day. Mosquitoes are not an issue because the water should drain before they even have time to breed.
During our storms and heavy rain seasons, excess water may overflow the rain garden and run into the storm sewer system. Even then, however, the rain garden will have done its job – it will have channeled water away from your foundation and reduced the load on the sewer system. Additionally, a rain garden also reduces the amount of lawn chemicals and pet wastes that may otherwise run off into local lakes and rivers.
Where would you like to locate a rain garden? Stand out in the yard during a good rain – not a thunderstorm, but a heavy rain. Where is the source of the water flow – drainage from the roof, or from the driveway, or maybe from your neighbor’s yard? Do you need to build a berm across a slope to create a depression to capture the flow? Look around the yard -- Check the slope. You need a slope of at least 1 inch over every 4 feet for water to flow into your rain garden.
Your chosen spot should be at least 10 feet away from your home. Otherwise, water may saturate the soil close to the foundation or even back up against it. If you already have water pooling close to your home, channel it away with an underground PVC pipe to the rain garden area. This may mean tunneling under a walkway or other obstruction. Keep in mind a “big rain” may overflow your garden – so create an overflow zone, a slightly lower area on one side to channel water away once the garden fills. Locate it away from any residential structures, septic tank, or underground utility lines, and remember to call 811 (national number) to have your utilities marked before you dig.
How fast does your soil drain? Remove the grass and vegetation from the garden area and amend the soil for the garden. Dig a hole in your garden area about the size and depth of a large coffee can – approximately 8” x 8” x 8”. Fill it with water and then watch to see how long it takes for the water level to drop. If after one hour the water level has dropped by ½”, you can figure the soil drains 1” in two hours. At this rate, the garden soil will handle 12” of water in a 24-hour period, making the ideal depth of your garden 12”.
Next time: Building a Rain Garden – look for this in early 2024.
Go to aces.edu and type “Rain Gardens.” You will find lots more information.