Rain Gardens

From Antivist

In a natural landscape�a forest, say�there's generally very little runoff. The soil and its dense cover of leaf litter and vegetation act as a sponge, absorbing most precipitation. But because we've replaced so much natural groundcover with impervious surfaces, rainfall no longer soaks into the soil as readily as it once did. As a result, huge volumes of runoff flow from countless roofs and compacted lawns down driveways and roads to storm sewers, carrying pesticides, motor oil, and other pollutants to nearby streams and rivers, fouling surface waters and destroying aquatic life through sheer physical force.

Rain gardens are shallow depressions designed to collect rain � typically from impervious surfaces such as roofs � and let plants, bacteria, and soils clean the water as it seeps its way into the ground. By keeping stormwater close to where it falls, rain gardens do help reduce flooding and settle out sediments. Yet they also prevent stormwater from becoming so contaminated with oils and other chemicals in the first place, and they actually remove pollutants from the water as it percolates through the soil on its way to becoming groundwater.

Benefits

  • Increase the amount of water that filters into the ground, which recharges local and regional aquifers;
  • Help protect communities from flooding and drainage problems;
  • Help protect streams and lakes from pollutants carried by urban stormwater Â� lawn fertilizers and pesticides, oil and other fluids that leak from cars, and numerous harmful substances that wash off roofs and paved areas;
  • Enhance the beauty of yards and neighborhoods; and
  • Provide valuable habitat for birds, butterflies and many beneficial insects.

Build your own

Begin by observing your landscape when it rains to determine how storm water runs off your property. Typically, the largest sources of runoff are the roof, paved surfaces, and slopes. You can locate your rain garden at the bottom of slopes or next to hard surfaces such as your driveway and sidewalks�along the front walkway or between the sidewalk and the street, for example, to keep runoff from flowing into the road and down the nearest storm sewer. You can also direct runoff from your roof to the side or back yard. Not only will a rain garden along the side of the house catch runoff from your roof, it will also create a handsome living fence between you and your neighbors. Wherever they are located, rain gardens can replace high-maintenance lawn that provides little in the way of visual interest or wildlife habitat. Just keep the plantings at least ten feet from your house�or your neighbor's�to prevent moisture problems in the basement.

There's no regulation size for a rain garden. Planting of any dimension will help reduce the runoff problem.

Unless you've got a naturally low area in your landscape, you'll need to create a depression by excavating soil. Typically, the deepest portion of the rain garden will be about three to six inches below the level of the surrounding land.

Some of the topsoil removed from the site will be used to create a porous planting mixture. If your rain garden is located on a slope, you can pack some of the excavated soil along the downhill side to increase the depth of the planting area with less digging. Or you can use the leftover topsoil elsewhere in your yard. It's a good idea to loosen the remaining subsoil in the depression with a shovel or garden fork. Next, return some of the topsoil, amended with compost, and sand if necessary. A blend of 20 percent organic matter such as compost, 50 percent sandy soil, and 30 percent topsoil will promote good drainage and help break down pollutants. Clay content, which inhibits drainage, should not exceed 10 percent of the soil mixture. The proper soil mixture will ensure that there is rarely standing water in your rain garden for more than a few hours, or a few days at most�too short a time to breed mosquitoes, which require about a week of standing water to reproduce.

To direct the storm water from the downspout or sump-pump outlet, attach a length of plastic pipe and bury it in a shallow trench that slopes down to the rain garden. Alternatively, you can simply lay the pipe on the ground or create a grassy swale leading from the downspout to your rain-garden depression.

Remember, rain gardens are meant to handle average storms, not major downpours. If you don�t want to drown your plants when a big rain event comes along, you also need to provide the water with a way to drain out. Try positioning your garden "so that when it overflows, the water goes into the lot�s existing drainage pattern." You may want to add an outlet furrow to your garden to ensure that excess water heads in the direction you�d like.

By all means, mulch: "A layer of mulch not only keeps weeds down... it also acts as a sponge to capture heavy metals, oils, and grease. As the mulch decays, bacteria and plant roots have a chance to break down the pollutants."

Plants

A variety of wildflowers, ferns, grasses, shrubs, and trees thrive in moist soil. Your rain garden can be divided into three wetness zones:

  • In the lowest zone, plant species that can tolerate short periods of standing water as well as fluctuating water levels, because a rain garden will dry out during droughts or at times of the year when precipitation is sparse.
  • Species that can tolerate extremes of wet soils and dry periods are also appropriate for the middle zone, which is slightly drier.
  • You can put plants that prefer drier conditions at the highest zone or outer edge of your rain garden.

Plant as many species as you can to enhance your rain garden's value as wildlife habitat. Here is a list of some 200 plant species that filter pollutants from water.

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