Wild Ones   Rain Gardens on Clay Soil Sites  

By Beth Storey
Photos by Roger Bannerman, Wisconsin DNR

A well-planned and done rain garden need not look particularly different from the usual flower bed. A discriminating eye will of course recognize the native plants.


If all the world's water were to fit into a gallon jug, the amount of fresh water would be just one tablespoon full. Yet almost half of our nation's 3.6 million miles of rivers and streams are threatened or impaired. Search out practical tips on things you can do to protect and improve the quality of the rivers close to you. – The Nature Conservancy.


With changing federal laws governing water quality, many municipalities are enacting storm water management plans that require homeowners to disconnect their downspouts from the municipal sewage system. The most self-evident ways to deal with this water are to recycle it somehow, evaporate it, or infiltrate it into the soil. The procedures developed to handle storm water vary dramatically from community to community – catch basins, infiltration systems, and open ditches. Whatever method is chosen, it is more easily accomplished on a small scale than large. Many homeowners are opting for infiltration in the form of “rain gardens.” Anyone who has ever put shovel to soil knows that infiltration is most easily accomplished on sandy soil and almost impossible on clay soil.

The concept behind a rain garden is quite straightforward: it is designed with a central depression to retain rainwater runoff and give it time to seep into the soil. This infiltration helps recharge groundwater and protects local water quality by reducing polluted runoff. Rain gardens can feature hardy native plant species that thrive without fertilizers and pesticides. These native plants also have elaborate root systems that create channels into the soil causing it to more readily absorb water.

The traditional turf-style lawn creates an impervious surface, and standard turf-care practices often have a higher environmental impact than the limited care required for native plants. Your rain garden will also attract new friends. Not only will neighbors admire the vibrant health of a well-designed garden, but also birds, butterflies, and beneficial insects will find your yard more interesting than any mowed grass lawn.

This informal kidney shape is comfortable to lay out and work with. It's also easy to enlarge this shape if the need arises.

What About My Soil?

Whether over the soil or through it – water moves. And when it moves across hard surfaces, it moves very differently than when it moves across and into soil. Infiltration of water is key to a successful rain garden, and soils vary greatly in drainage capability. Your rain garden – or technically speaking, bioretention pond – must be able to absorb the water coming off your sidewalk or from your downspout within two days. According to most professionals, a true rain garden has wildlife and water quality benefits. So without this rate of drainage you’ll miss out on the groundwater recharge and storm water management advantages of soil infiltration.

Choosing a Site for Your Rain Garden.
Locate your garden at least 10 feet away from structures to avoid water seeping into foundations. Low, wet areas in your lawn are not good choices since these are areas where infiltration is slow. Look for sunny areas that are less than a 10% grade. Avoid areas near a septic system. For more info on preservation and restoration of native plant communities, see www.for-wild.org/native.html.

Sandy soil, which drains well, feels gritty and coarse. Clay soil is sticky, clumpy, and will require different preparation. If the site you’ve selected (see sidebar) seems to have clay soil, test the drainage with a percolation test by digging a hole 8 inches wide and 8 inches deep. Pour a bucket of water into the hole and allow it all to infiltrate. Pour in another bucket of water and monitor how long it takes to sink in. If infiltration is slower than 1 inch per hour you’ll have to do extra work to improve the soil’s absorption ability.

Note the roto-tiller in the background. To increase filtration, it's easier to break up the soil and work in amendments with a tiller.

Soil Replacement

If you have slow-draining clay soil, you’ll have to excavate the clay. Remove the top 10 to 12 inches of soil and replace with a composition of roughly 40% sand, 20% topsoil, 20% compost, and 20% clay. This soil mix will support plant growth and improve infiltration. Leaving a small percentage of clay in the mix will prevent a “layering effect.” In other words, if no clay is mixed into the replacement mix, plant roots may behave as if they were in a clay bottomed pot and stay in the top layer of soil, and water will not move through the soil as is your intention.

In a similar approach, 3 to 6 inches of clay soil can be removed, replaced with a sand/topsoil/compost mixture, and tilled 4 to 6 inches. Gardens with these types of replacement soils are often planted with wet prairie species, and the very center of the garden is made a bit shallower (less than 6 inches) to increase absorption rates.

Size and Slope

The typical rain garden is 150 square feet (the size of a 10 x 15- foot room) and 4 to 6 inches deep. But for more detailed instructions on determining the appropriate size for your garden based on the area that will drain to it, see Rain Gardens: A How-To Manual for Homeowners from the Wisconsin Department of Natural Resources. (I also like raingardens.org but it suggests Equisetum which is invasive and impossible to eradicate). The garden will be deeper at the outer edge and slope upward to 3 to 6 inches at the center. The center of the garden should be as level as possible to create the greatest amount of surface area for water to infiltrate. Because creating a level garden base is so important, an area with a slope greater than 10% is not a desirable site for your garden. Follow these steps to find the slope of your lawn (see illustration below).

Determining your slope: Place a stake at the uphill end of your proposed garden site, and another at the downhill end. Tie a string between stakes and make the string horizontal using a level. Measure the width in inches between the stakes and the height in inches on the downhill stake between the string and the ground. Divide height by width; multiply by 100 to find the lawn's slope. It should be less than 10% slope, and 3 to 6 inches deep: Height/Width x 100 = % slope. Drawing by Bruce Webendorfer. University of Wisconsin Extension and the Wisconsin Department of Natural Resources. Raingardens: A How-To Manual for Homeowners, 2003. University of Wisconsin Extension, UWEX Publication GWQ037.


Plant Selection

The temporary pool created by an occasional summer thunderstorm will be attractive to thirsty birds and other wildlife, but this fluctuation in water level makes plant selection important. The soil will retain the greatest amount of moisture at the center of your garden and should be planted with species native that tolerate wet, clay soils and that are native to your area. Note that these plant lists are most appropriate for the Northeast and upper Midwest:

Sweet flag (Acorus calamus)
Swamp milkweed (Asclepias incarnata)
New England Aster (Aster novae-angliae)
Turtlehead (Chelone glabra)
Joe Pye Weed (Eupatorium maculatum)
Boneset (Eupatorium perfoliatum)
Prairie Blazing Star (Liatris pycnostachya)
Great Blue Lobelia (Lobelia siphilitica)
Mountain Mint (Pycnanthemum virginianum)
Blue Vervain (Verbena hastata)
Common Ironweed (Vernonia fasciculata)

Fringed brome (Bromus ciliatus)
Porcupine Sedge (Carex hystericina)
Fox Sedge (Carex vulpinoidea)
Cord Grass (Spartina pectinata)

Species suited to the drier soil on the upper edges of the rain garden include:

Nodding Pink Onion (Allium cernuum)
Lead Plant (Amorpha canescens)
White Wild Indigo (Baptisia leucantha)
Beardtongue (Penstemon digitalis)
Purple Prairie Clover (Petalostemum purpureum)
Black-eyed Susan (Rudbeckia hirta)
Stiff Goldenrod (Solidago rigida)

Sideoats Grama (Bouteloua curtipendula)
Prairie Brome (Bromus kalmii)
Copper-shouldered oval sedge (Carex bicknellii)
Narrow-leaved oval sedge (Carex squarrosa)

Native Plant Landscaping

Another option for implementing some storm water management in a clay-rich garden site is to replace turf grass with wet-tolerant native plants without creating a depression. Native plants will greatly improve the clay site by making the best use of water and soil resources, and also build up the soil. Landscaping in clay without a depression will not technically create a rain garden, but runoff will still be reduced since deep-rooted natives create channels for infiltration in even the tightest clay soils.

This garden will also attract desirable wildlife and beneficial insects. Some wet prairie species that grow in clay soils include blue flag iris (Iris versicolor), water plantain (Alisma subcordatum), porcupine sedge (Carex hystericina), and some bulrushes (Scirpus atrovirens, Scirpus acutus).

Mosquito Concerns

While planning their rain gardens, some homeowners worry about creating mosquito-breeding grounds. Mosquitoes, however, need at least five days to hatch eggs, and develop beyond the water-obligate pupa stage, longer than the two-day maximum that standing water should remain in a properly built rain garden.

A garden that dries out completely in the proper amount of time should not produce mosquitoes. This makes attention to good garden design even more important – garden size, slope, soil type, and drainage are crucial. And, once again, rain gardens attract beneficial insects and animals that aid in keeping mosquito populations in check.


Applied Ecological Services. Build Your Own Rain Garden. www.appliedeco.com.

Friends of Bassett Creek. 2000. Rain Gardens: Gardening with Water Quality in Mind. www.mninter.net/~stack/bassett/gardens.html.

Rain Gardens: A How-To Manual for Homeowners, Wisconsin Department of Natural Resources, DNR Publication PUB-WT-776-2003.

Rain Gardens of Western Michigan: 2003. Create a Rain Garden. www.raingardens.org.

Beth Storey lives six blocks from the Mississippi River, and spends her time ardently reducing the amount of turf grass in her Minneapolis yard.

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Updated: Oct 19, 2006.