Eleanor Burkett, University of Minnesota Extension Educator
Whether you live in the city or reside on a lake or river shore, managing stormwater runoff is worth considering for your landscape. Rooftops, roads, driveways and sidewalks create hard impervious surfaces which rainwater and melting snow cannot penetrate through to soak into the soil. Additional runoff created by impervious surfaces often is channeled into depressions on your property, often eroding soil along the way. The additional runoff also increases the amount of nutrients and sediment that are carried into lakes, rivers, and wetlands.
Simply put, a rain garden is a shallow depression filled with plants designed to collect rainwater runoff and allow it to filter into the soil, removing nutrients, sediments and other pollutants before reaching the groundwater. Small shrubs, flowering plants and ornamental grasses within a rain garden absorb nutrients, and the sediments settle to the bottom. Rain gardens add beauty to the landscape and may attract butterflies and birds.
Photo 1: Raingardens help to limit the amount of nutrients and sediment that are carried into lakes, rivers, and wetlands. David Zlesak
Rain garden designs can be simple or elaborate, depending on your gardening interest and experience. When designing a rain garden consider placement, soil type, size, desired shape, and plant species. To insure satisfaction, sketch a design before you start.
Rain Garden Placement
Rain gardens should be placed 10’ or more away from buildings to prevent water from the rain garden from entering basements and damaging foundations. They should also be 35’ or more from septic system drain fields, 50’ or more from drinking water wells and well away from underground utility lines. Call Digger’s Hotline (800) 242-8511to locate electrical, gas or telephone lines before designing your rain garden.
Photo 2: Rain garden at the edge of a forest. Brian Peterson
Test Your Soil
Of utmost importance is the soil in which a rain garden is constructed. The purpose of rain gardens is to infiltrate runoff water, so the soils need to be porous enough to quickly soak up water, ideally emptying within 48 hours. Forty-eight hours is the standard because most plants can survive inundation for this amount of time and it prevents a rain garden from becoming a mosquito breeding area. A simple test of a soil’s ability to infiltrate water is to dig a wide hole 10” deep and fill it with water; if the water disappears within 48 hours, the site is suitable for a rain garden. For clay soils mix in organic matter before planting.
Rain arden Size and Shape
Often several rain gardens are designed into the home landscape. For roof top runoff each down spout usually drains to one rain garden. Each rain garden should be about one-third the size of the roof area being drained. Rain gardens typically range from 100 to 300 square feet in size. Applying these same concepts, rain gardens may be built to capture runoff from other impervious surfaces such as driveways and sidewalks.
Rain gardens can be designed in any shape - crescent or kidney shapes are attractive. A long and narrow rain garden may be better suited to fitting between closely spaced buildings, sidewalks and roads.
Choose plants that are suitable for your soil type and will tolerate standing water for up to 48 hours. Many native plant species are well suited for rain gardens. If locating a rain garden near a lakeshore or riverbank it is recommended to use native plants, it may even be a requirement, so be sure to check with your local Soil and Water Conservation District or County Environmental Department.
Photo 3: There are many plants to select from that can tolerate periods of standing water such as Rosa palustris, the swamp rose. David Zlesak
Install the Rain Garden
Once the size, shape and location of the rain garden have been decided, construction can begin. Lay out a rope or garden hose in the shape desired as a guide for digging. On a slope the soil from digging may be used to create a berm on the downhill side of the rain garden. Otherwise, excess soil should be removed from the site. The depth of the depression may vary from 4” to 8”. For best infiltration the bottom of the rain garden should be flat and level. Finally, use plastic pipes, installed above or below ground, or a rock lined spillway to connect a downspout or other source of runoff to your rain garden.
For more information:
Downloadable “how to” manual by the University of Wisconsin Extension: http://learningstore.uwex.edu/pdf/GWQ037.pdf
Blue Thumb: http://www.bluethumb.org