Lynda Ellis, UM Extension Master Gardener in Anoka County.
A rain garden can capture rain that would otherwise flow down your lawn and driveway into streets and storm sewers, and use it to make your property more beautiful. It does this if it is located where rain can be directed into it, built so that it can temporarily hold the water (a rain garden is not a pond), and planted with appropriate plants to add beauty and attract bees, birds and butterflies. While a handy homeowner can create a nice rain garden, assistance of a landscape professional with experience in rain gardens can be very helpful.
As in real estate, the first three things are location, location, and location. The rain garden should be at least 10 feet from buildings. If one of your downspouts drains into your lawn, perhaps you already know where your rain garden should go. If it instead drains onto the driveway, could it be redirected toward a section of the yard?
Does the soil in that section drain readily? Dig a wide hole 6 inches deep and fill it with water. Wait 24 hours. If the water disappears within that time, the location is suitable for a rain garden.
Can you carve out a large enough space to handle the amount of rain you will get during downpours? Rain gardens range from 100 – 300 sq ft and can usually handle rain from hard surfaces (roof, driveway) three times their size. More than one rain garden may be needed to handle rain from larger areas. Use a hose or rope to outline a curved shape for the proposed garden and move it around until you are satisfied with its placement.
Unless it is located in a depression, you will have to excavate between 4 – 10 inches to form the level bowl of the rain garden. If the site is not level, use some of the excavated soil to make a berm on the downslope side to further prevent overflow. You may also wish to add a border.
Next comes the fun part – planting. Rain gardens can be located in sun or part shade. They have distinct planting areas: the bowl and the upper part, and a transition zone between them. The bowl area will need plants such as spiderwort and blue flag that can handle wet feet. Plants for the entire rain garden should also handle dry conditions. Native plants are often preferred for that reason. You can use perennials, shrubs, or even small trees in a rain garden. Don’t forget mulch. Wood chips made from hardwoods will not float away. River rock can be used for the inlet.
Mulch will help keep down weeds, but you will have to weed, especially the first season. You will have to water your rain garden in dry seasons. The final result is a beautiful garden that captures rain that would otherwise overburden storm sewers and water treatment plants. For more information, see: http://www.metroblooms.org/, http://www.bluethumb.org./, and http://learningstore.uwex.edu/assets/pdfs/GWQ037.pdf