How To Create a Rain Garden
Corralling excess water in your yard is critical to prevent damage and reduce pollution downstream. Good news is it’s also easy.
By Oscar H. Will III
April showers can bring a lot more than May flowers, particularly if you have low spots in the yard that puddle, or slopes susceptible to damage from a real gully washer. Add in the potential to send fertilizers and soil into the local waterways, and spring showers can lead to significant environmental damage. You’ll know if you have a big problem on your place if your downspouts are currently discharging into a trench that you didn’t dig or that mud puddle in the middle of the backyard gets worse every year.
What can you do? One of the most sensible ways to handle runoff in your yard is with landscape features called rain gardens, and spring is a great time to get such a project going.
Rain gardens consist of a shallow depression in the soil or the equivalent created with berm on sloped ground. The structures collect runoff, slowing the flow so that water can percolate into the soil and prevent nonpoint source pollution downstream.
These carefully constructed earthworks are planted with perennials and shrubs that can tolerate high loads of fertilizer and can stand to have their roots either wet and dry for periods of time. Your County Extension office is a good place to discover which plants work best in your area.
Most rain gardens are self-contained—constructed so the excess moisture percolates into the surrounding soils. In some cases, however, particularly when the local water table is within several feet of the surface, rain garden design may call for under-draining. In this case, infrastructure is installed beneath the garden to carry runoff to a suitable and legal discharge point. Since permitting and construction complexities associated with under-drained rain gardens are more daunting, we’ll consider the self-contained rain garden here.
Study the water flow and pooling characteristics in your yard to site the rain garden. Be sure the location is not above the septic drain field and is a minimum of 10 feet away from the foundation of any building. Full sun for at least part of the day will allow for the largest selection of plants.
Call 811 to determine whether any utilities are buried in the location of your garden. Also, check your own maps and recollections of any gas, water, sewer, electric or Internet lines you might own. For instance, think carefully about lines that could be buried between the house and barn or shop.
Perform a percolation test by digging an 8- x 8- x 8-inch hole and filling it with water. Once drained, fill it again; if the water drains within several hours, the soil should be fine as it is. However, an inability to drain in a reasonable amount of time could mean your soil contains a preponderance of clay. If so, consider a larger excavation, and backfilling with layers of gravel and more suitable topsoil. Check with your county Extension office for some guidance.
Size your garden at 100 square feet to several hundred square feet, depending on the expected volume of runoff. Using a garden hose, outline the garden’s shape. A kidney bean shape with the concave side facing the runoff source is typical.
Calculate the volume of water the garden will need to contain from a 1.25-inch rain. If you are collecting from a roof, calculate the roof’s footprint in square feet and multiply that by 0.10 to get your garden’s cubic foot capacity. For example, a 1,000-square-foot roof footprint will deliver about 100 cubic feet of water, which would require a roughly 100-square-foot garden, with an average depth of about a foot. If, however, your soil continues to have more clay in it, increase the garden’s footprint to avoid digging it deeper.
Excavate the shape to the average depth you desire using hand tools, loader and backhoe. Pile the excavated soil along the downslope edge of the garden to create a berm that will help divert and contain the runoff; compact this soil firmly. Leave open the side where you expect the runoff to enter the garden. Plant grass or another suitable ground cover on the berm.
Loosen and amend the soil on the garden’s interior (a soil test is ideal), and work it so soil is level over most of the area, with gradual transitions to the berm. The more level the soil, the more area the runoff will have to move into the soil. Avoid creating dips where water will pool.
Plant perennials and shrubs, and water as needed the first year. Once established, your rain garden should be relatively maintenance-free, except for weeding and periodically replacing or adding plants.
In time, you might notice an accumulation of sediment or vegetative debris in the garden. Congratulations! That means your garden is working.
If the sediment buildup is significant, carefully remove it, while taking care not to create any low areas where subsequent rains will puddle.
Now sit back, relax and enjoy the beauty of your rain garden.