Jennifer Schultz Nelson
Extension Educator, Horticulture
A relatively new method to deal with storm water that seems to be gaining steam as a new "garden trend" is rain gardening. In a nutshell, a residential rain garden is a depression in the landscape that is planted with native plants that tolerate wet soil, and is positioned to collect rainwater as it flows from downspouts and across lawns, patios and driveways.
Some people wrongly assume that a rain garden is a bog or pond that holds water for extended periods. On the contrary, water that collects in a rain garden is absorbed by plants and surrounding soil within 24 hours, so it is not really a true pond.
The main goal of having a rain garden is management of storm water runoff. As we build on and pave over more and more open ground, this minimizes the surface area available for storm water to be absorbed. Typically, runoff from rain and snow is diverted into municipal storm sewers.
But water is not the only thing flowing to the storm sewer. Along the way, the water picks up an assortment of chemicals, including motor oil from driveways and roads, pesticides, herbicides and fertilizer. Since many storm sewers directly feed into local bodies of water, this potentially creates pollution problems in local lakes and streams.
Nature had storm water figured out long before people disrupted her system with storm sewers. Before humans covered the land with buildings and roads, much if not most of the storm water was absorbed and filtered by soil and plants, and excess water eventually recharged underground water sources, or aquifers.
Plants and soil have an enormous capacity to filter and deactivate chemicals that could become pollutants if allowed to enter waterways unchecked. Even many man-made chemicals can be degraded by plants, soil microorganisms and chemical reactions in the soil. Rain gardens are really a way to rebuild and re-create a natural system to handle storm water runoff.
Rain gardens come in many shapes, sizes and situations. But there are some basic steps common to all rain garden applications.
The first step is choosing a location. The rain garden should be built more than ten feet from the existing home or other building to prevent damage to foundations. Rain gardens should not be placed where excavation will disrupt tree roots, and should never be placed in the drain-field of a septic system.
Generally the best location can be found by observing where runoff naturally flows on the land. Areas where water collects but dissipates within 24 hours are good candidates for rain gardens. Avoid areas where the soil is compacted, as they usually will not provide adequate drainage. Areas with steep inclines are poor choices since the bottom of the garden needs to be level.
Figuring out appropriate dimensions is the second step in building your own rain garden. Soil type is a factor in calculating size. Sandy and silty soils will naturally drain faster than heavy clay soils. This difference in drainage will affect how large the rain garden should be. Besides soil type, the other important factor in determining size is the square footage occupied by buildings or pavement. There are many different guides available to help with these calculations.
The third and fourth step in building a rain garden are designing the shape and layout of the garden, and then choosing plants appropriate for the design. Most resources recommend simple designs which will reduce maintenance and keep cost to a minimum. Native plants are popular choices for rain gardens since they naturally are adapted to our climate and soil types. Plant lists are available from many different sources, including the University of Illinois Extension office here in Macon County.
Before you start digging, it is important to contact your local utility company so that they can mark any underground lines that may be present. In Illinois, this service is called JULIE, and can be reached at 800-892-0123. You may need to modify your design to avoid digging up these lines. Use a garden hose or rope to outline the shape of the garden and modify as needed.
The initial digging is important in establishing the basic shape of the rain garden. To function properly, the sides of the rain garden should slope downward, much like a pie plate. The area should be excavated to a depth of five to seven inches, keeping the bottom of the garden level. Making the garden any deeper than this is not recommended, since it creates a potential trip hazard.
About two inches of compost should be added to the bottom of the garden and mixed with the existing soil. The compost provides additional nutrients for plants, and improves drainage of the surface soil. In cases where the soil is high in clay, it may be advisable to excavate further and amend the soil more extensively.
Plan on spacing most plants about one foot apart, and lay out your plants in their nursery pots according to your plan. Plant as you would in any garden, paying particular attention to minimizing how much you walk in the garden to minimize soil compaction which will interfere with drainage. Add shredded bark mulch at a depth of three to four inches to discourage weeds and minimize erosion.
It may not seem like a rain garden would make much difference in how much water enters the local storm sewer. Consider that in a typical downpour, each of the standard four downspouts on an average house will move approximately twelve gallons of water per minute into the storm sewer. One inch of rain doesn't sound like much, but on a roof only 20 x 25 feet, that equals more than 300 gallons of water!
The city of Rock Island, Illinois values rain gardens so much that it pays its residents to install them on their property, and gives a discount on their sewer bill as long as the garden is maintained properly. I had an opportunity to tour several of these gardens recently, and was impressed with how well they fit into the existing landscape. If someone hadn't told me they were rain gardens, I would have just thought they were a nicely landscaped area. The tour inspired me to start planning a rain garden or two in my yard to help with controlling runoff into the pond behind my house.
There are many resources for building rain gardens available free of charge on the internet. In Illinois, Lt. Governor Pat Quinn has an informative website detailing the Illinois Rain Garden Initiative at: http://www.standingupforillinois.org/cleanwater/