Written by Chris Campbell
Rain gardens are shallow planting depressions that catch and use runoff from impervious surfaces such as rooftops and driveways. They allow storm water to soak into the ground rather than runoff into sewers, streams and lakes where it has picked up pollutants such as fertilizer and pet waste along the way. Rain gardens have been shown to reduce pollution in streams by up to 30%. They are comprised of native and hardy introduced shrubs, ornamental grasses and perennials adapted to both wet and dry conditions.
For sunny locations, plants such as chokeberry, viburnum, Siberian iris, daylilies, beebalm, rudbeckia, switchgrass and reedgrass work well, while hydrangea, sweetspire, spiderwort, astilbe, hostas, ferns and anemone work well in shady locations.
The size of the rain garden is determined by the amount of runoff flowing into it. A good rule of thumb is a garden 25% in size of the total amount of runoff flowing into it. For example, if 400 square-feet of roof is draining into the garden, it should be at least 100 square-feet in size. Rain gardens are typically placed in front of downspouts or along impervious surfaces such as sidewalks or driveways. This allows the maximum amount of runoff to flow into the garden. They should remain at least six feet away from foundations to prevent water from backing up towards the house. Once the site is determined, the rain garden is dug to a depth of 4-6 inches. The bottom of the garden should be relatively level to allow equal distribution of water throughout the garden. In clay soils, compost can be worked in to improve the soil quality and drainage. On the low side of the rain garden (opposite to the inlet of flow), a small berm, usually 4-6 inches high, is constructed to catch and temporarily hold the water until it can infiltrate into the soil. A groundcover is suggested on the berm to prevent it from eroding over time.
Finally, shredded hardwood mulch is applied to the rain garden to suppress weeds and retain moisture during dry spells. Once established, a rain garden needs minimal care and irrigation.