Rain gardens are depressed or “bowl-shaped” landscaped areas that function as miniature wetlands. Typically planted with wildflowers and other native vegetation, a rain garden provides a place for stormwater to infiltrate, allowing approximately 30% more water to soak into the ground.
A simple rain garden can be planted in most landscapes with little or no modification to existing conditions. It is easiest to plant the garden based upon natural drainage flows (look for low spots where water ponds following a heavy rain).
Because the depth of a rain garden can be as little as six inches, heavy machinery is not necessarily required. Even sites with heavy clay or compacted soils can be conducive to rain gardens. For clay or hardpan soils, some excavation and replacement of soil may be necessary. When replacing soil, a mix of 50-60% sand, 20-30% topsoil, and 20-30% compost is recommended. Compacted, hardpan soils should be loosened, for drainage and root growth, to two feet deep. Ideally, the rain garden should be planted with native or locally adapted plant species. This provides for ease of care and maintenance as well as habitat for local, often beneficial, wildlife.
Sources: DC greenworks
View and detail drawings of example raingarden