Extension News

Rain Gardens: Filtering and Recycling Rain Water

Note to media editors: This is the Garden Column for the week of Aug. 18.


By Tigon Woline

Horticulture Intern

Iowa State University Extension


As our cities and towns grow, we continue to build houses, shopping centers and parking lots. All this new development compacts the soil and creates impermeable surfaces. Storm water run off from parking lots and buildings runs into the sewer systems taking with it soil and chemical residues. This material ends up in the ground water system that we depend on. To help control run off and return rain water to the ground water system without all the impurities, consider installing a rain garden.


A rain garden is a beautiful, landscaped garden that is planted in a slightly depressed area to collect rain water running off the roof and lawn. The water collects in the garden and slowly percolates through the soil. By filtering the rain water through the soil and plant roots, many of the lawn and roof chemical residues and soil particles are removed. Although developed in the 1990s, rain gardens have been slow to catch on until recently. Now, many residents, municipalities, and corporations are installing rain gardens to do their part in improving the storm water drainage situation.


There are a few things to consider before beginning construction of a rain garden. First, decide the best place for the garden. It is easiest to take advantage of the natural or existing drainage pattern. However, don’t put a rain garden where water already pools. This indicates the soil does not drain well and a rain garden may not work properly. The site should be at least 10 feet from the house to prevent water from seeping into the basement.


The soil type also is important. A sandy soil will drain quickly while a clay soil will drain slowly and may cause flooding during moderate rains. Clay soils and heavily compacted soil can be improved by excavating the site a little deeper than the final depth of the garden and replacing the soil with a sand-top soil-compost mixture. Once an appropriate site has been selected, design the ponding area in a shape that is pleasing and compliments the existing landscaping.


Next, select the plant material. Rain gardens in sunny areas are commonly planted with native plants. These types of plants are selected because they grow well, have deep, fibrous root systems that filter impurities from the rain water, and are attractive to many wildlife species. Some native plants suited for Iowa rain gardens include: Sweet flag (Acorus calamus), Swamp milkweed (Asclepias incarnata), New England aster (Aster novae-angliae), Tussock sedge (Carex stricta), Turtlehead (Chelone glabra), and many more. For a more complete listing of plants suited to Iowa rain gardens visit the URBAN Soil and Water Conservation webpage: http://www.urbanwaterquality.org.


The last step is installing the rain garden. Always remember to contact Iowa One Call (1-800-292-8989) to have the utilities marked before digging in the landscape. The entire ponding area needs to be excavated, even if drainage is not a problem. The finished grade should be 4 to 6 inches below the surrounding area. The depression collects and holds the rain water while it percolates through the soil. To prevent heavy rains from washing out the opposite site of the garden, construct a small berm along the backside. The soil that was excavated can be used to build the berm.


After planting, water well and place a layer of mulch over the garden area. Mulching the garden will help control weed growth while the plants are small. During the first few years, the garden will need weeding and minimal care to help the plants establish. Watering should only be done when there has been no rain for an extended period of time. Heavy rain may wash away the mulch where water enters the garden; a few well placed rocks can break up the flow enough to prevent the mulch from washing away.


Rain gardens are great ways to reduce storm water run off in your local area and they are relatively easy to maintain. Light weeding and mulching are required to keep the rain garden in working condition.


For more information on constructing rain gardens visit the following Web sites:








Contacts :

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu

There are no photos for this week's column.