Yellow cut-leaf coneflower and purple agastache bloom in this rain garden.
By Ann Atanasio Fauquier Master Gardener
A rain garden is strategically designed to direct rainwater away from your home and collect it in a cultivated depression planted with native vegetation.
Such gardens prevent untreated rainwater runoff from hard surfaces, such as roofs, driveways, sidewalks and compacted lawns, from flowing into storm drains and bodies of water. This runoff is a potential source of pollutants, including pesticides, herbicides, pet wastes, fertilizers and trace metals. Eventually, those pollutants can make their way into streams, rivers, lakes and beyond.
Incorporating a rain garden into your landscape will improve water quality by minimizing harmful runoff while diverting rainfall into a native habitat that naturally filters and cleans the tainted water that enters it. The rainwater will slowly percolate into the surrounding soil, thereby reducing/eliminating the need for irrigation. Unlike retention basins, the water in rain gardens should infiltrate the ground within a day or two and won’t become breeding grounds for mosquitoes.
The first step in establishing a rain garden is determining the location. The best choice is an area that collects runoff without holding water. Consider the slope of your yard, where rainwater pools and flows off. Although a low-lying area might seem the obvious choice, you don’t want to build a rain garden that holds standing water; those areas are water gardens, wetlands or ponds, not rain gardens. Your garden should be at least 10 feet from your house to prevent soil saturation near the foundation, and at least 50 feet from a septic system or well. To increase the effectiveness of the water flow, or if your property doesn’t slope, consider laying a river rock channel or running an underground PVC pipe from your downspout to your garden.
Next, you’ll need to decide upon the size and shape of your rain garden. The John Marshall Soil and Water Conservation District staff can instruct you on how to calculate the optimal size of your garden, based upon average rainfall, the size of your on-site watershed and the rough area of your roof. However, it’s also important to consider what size best fits into your yard. Any size rain garden, even a small one, will be extremely beneficial. Choose a shape for your garden that complements your overall landscape design. Experiment with shapes using a garden hose, and mark the final shape with spray paint. The conservation district office and the and the Virginia Department of Forestry have design ideas and technical information for planning your water garden.
Once you’ve decided upon a shape, but before you dig, call Miss Utility at 811. Before excavating, it’s recommended that you remove all sod up to 18 inches out from the edge of your design. By removing the grass, it won’t quickly re-sprout as it would if rototilled under. You can now dig your garden, sloping the sides gently and creating a flat bottom so that water will percolate down uniformly. The excavated topsoil can be amended and reused in your garden; the subsoil can be used to create a berm to capture the rainwater that enters the garden.
Fill your site with a mixture of 50 percent sand, 25 percent topsoil and 25 percent compost. Additional sand is fine, but it’s imperative that the clay content be minimal to ensure adequate filtration. Fill all but the top 6 to 12 inches of your garden with amended soil, creating a slight depression in the middle.
Now you can plant. When selecting plants, favor native species that are tolerant of local conditions, have deep roots, and provide natural food and habitat for local wildlife. The plant roots will work with the amended soil to filter the runoff water.
Typically, a rain garden contains three planting zones, with more moisture tolerant plants placed toward the center/depressed area of the garden, species that can occasionally tolerate standing water in the middle ring, and more drought tolerant plants along the outer edge. Consider the heights and textures of the plants, the direction the garden faces, and the amount of sun it receives.
Fauquier County Master Gardeners can help advise you on the appropriate plants for your garden (540-341-7950, ext. 1), and Virginia Tech offers additional information on Rain Garden Plants (Publication 426-043).
Finally, mulch your garden and weed regularly, especially during the first year. If too much water accumulates during a heavy rainfall, notch the berm on the low side of your newly planted garden to allow excess water to flow out. With a bit of care, you will have provided your yard — and your community — with a beautiful and eco-benevolent native habitat.