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Published April 2010

Every home needs a rain garden

By Linda Scherer

Rain gardens are a simple way for every property owner to do their part to protect water quality. By reducing and filtering rainwater runoff, rain gardens prevent pollutants from washing into sewers and water bodies. Besides helping the environment, the rain garden can be a beautiful addition to landscapes and provide a natural home for birds and butterflies.

Across the United states, rain garden projects have been encouraged by cities where neighborhoods and business districts have replaced forests and open fields. Because of the roofs, paved streets, and cemented sidewalks that come with development, there isn’t anywhere for the water to be absorbed and filtered, increasing the volume of untreated water runoff.

According to the Environmental Protection Agency, a typical city block generates nine times more runoff than a woodland area of the same size, because of impermeable surfaces.

The definition of a rain garden, according to University Extension Educator Eleanor Burkett, is a shallow depression filled with plants designed to collect rainwater runoff allowing it to filter into the soil, removing nutrients, sediments and other pollutants before reaching the groundwater.

Before designing a rain garden, take into account the placement, soil considerations, size, desired shape, and plant selection.

A rain garden should be placed near impervious surfaces so rainwater and snow melt will drain into the garden. Good locations for rain gardens would be close to gutter downspouts, alleys, and sidewalks, and near driveways and streets.

Other rules suggested by Burkett to follow when deciding where to place your rain garden would be:

• at least 10 feet or more away from buildings to prevent water from the garden from entering basements and damaging foundations;

• at least 35 feet from septic system drain fields;

• at least 50 feet from wells with drinking water; and

• away from underground utility lines.

More than one rain garden can be designed into the home landscape, according to Burkett. For rooftop runoff, each downspout usually will drain 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.

Once the size and location of the garden has been decided, the next step is to make sure the soil is porous enough to quickly soak up water, ideally emptying within 48 hours, according to Burkett. 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.

Rain gardens are designed using layers of soil. A blend of 20 percent organic matter, 50 percent sandy soil, and 30 percent topsoil is recommended, according to Hennepin County Master Gardener Lorrie Stromme.

A simple test of a soil’s ability to infiltrate water is to dig a wide hole 10 inches deep and fill it with water. If the water disappears within 48 hours, the site is suitable for a rain garden.

A shallow ponding depth of approximately six inches is preferred. If the soil did not pass the test for adequate drainage, excess soil from 2 to 4 feet deep should be removed from the site, to accommodate soils needed. For best infiltration, the bottom of the rain garden should be flat and level. Another option to consider besides a self-contained rain garden is to use plastic pipes, installed above or below ground, for draining; or a rock-lined spillway to connect a downspout or other source of runoff for the garden, according to Burkett.

Stromme also recommends a strip of turf or ground cover at the top edge of the rain garden to slow water as it flows into the garden and also filter sediments.

Plants that can tolerate the extremes of wet and dry periods are preferred for rain gardens, according to Stromme. There are some perennials that tolerate, or even thrive in moist soils and should be planted in the lowest part of the garden.

Native plants have several advantages, according to Stromme. They are best adapted to the local climate and once established, seldom need watering or fertilizing. Many are deep rooted, which enables them to tolerate drought. Native plants are attractive to diverse native butterflies and provide habitats for wildlife, especially birds. Native plants are low maintenance, but they still require care, occasional weeding, and control of debilitating diseases and insect pests.

Gardens on high-traffic streets should include plants that tolerate de-icing salts.

A mulch layer on the garden surface is suggested by Stromme to suppress weeds. Shredded hardwood mulch is recommended because it resists floatation, aids in the decomposition of organic matter, and helps to remove metals.

Winsted Lake Watershed Association member Petie Littfin suggests patience after installing a rain garden. She has multiple rain gardens and buffer gardens in her yard and has found it takes about three years to establish a rain garden. The first year, Littfin said the gardens are sparse. They begin to fill in the second year, and by the third year, they are looking good.

The plants in Littfin’s gardens that are four and five years old have multiplied and she is able to transplant some for other gardens or she offers them to friends.

Rain gardens and buffer gardens do not breed mosquitoes

A rain garden is not intended to retain water for long periods. Ideally, runoff will not be detained for longer than four days, to avoid concerns about mosquitoes breeding in standing water, according to Stromme.

Mosquitoes will not survive in wetlands that dry out in less than a week after a summer rain. The development of a mosquito from egg to adult takes 10 to 14 days. A mosquito larva must live in water for seven to 12 days before maturing to the adult stage.

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