GREEN AND GROWING
Rain is a free but fickle gift to the garden. It falls when it will and in unforeseeable quantities. Too much rain creates damaging runoff; too little rainfall may destroy a garden. The rain garden addresses these problems by drawing in and retaining rain water, rather than allowing it to drain across the property and escape as runoff. A rain garden captures excess water from surface flows, downspouts, and impervious structures so that it never becomes runoff, while storing it for later use by plants.
A rain garden is a shallow depression that has been modified to receive and collect water. The depression becomes a garden by planting plants that thrive on occasional flooding. By design, the rain garden soil stays wet for several days after a rain, allowing the collected water to slowly soak into the prepared substrate, while being absorbed and evaporated by the garden plants above it. Although the garden can be fed entirely by existing runoff patterns, the construction of swales, dry streambeds, and other devices to direct runoff will enhance its effectiveness.
Rain gardens are fairly easy and inexpensive to create. They have three basic elements. First, a depression is located where it can collect water. This does not need to be deep, but it does need to be sufficiently level to hold water. Because even a rain garden can flood, you need to prepare an overflow area or path. Further, you do not want to locate it close to your home’s foundations or septic field. A rain garden can be located in sun or part shade, but generally sun is better because water will evaporate faster. Second, the soil in the garden must be amended or replaced so that water can infiltrate easily and drain. Typically, rain garden soil has a substrate of gravel or expanded rock and a top layer of soil that has been heavily amended with organic matter to aid in the absorption of water. Properly prepared soil will not pond, or at least not for more than a day or two, even after the heaviest rain.
Third, the rain garden must be planted with native plants that can tolerate both short periods of flooding and extended periods of drought. It is important to use a variety of plants in the garden to maintain a biologically diverse ecosystem that will stand up to the extremes of water and drought that the garden is intended to handle. Plants with deep, fibrous roots are especially adept at holding soil and filtering pollutants. Bog plants do not work because they cannot take extended periods of drought. There are many resources in books and on the web recommending plants that meet these criteria in our area, both in shade and in sun. A good start can be found here.
A final, but optional element, is the use of swales, berms, and dry streambeds, with or without rocks, to slow and direct water into the garden. The rocks naturally must be heavy enough not to wash away in a storm.
When you have finished, you will have solved a difficult and recurring landscape problem. You, also, will have created an attractive, lush garden in which you will take pleasure and pride.
Tom Watson is a Volunteer Extension Master Gardener in Mecklenburg County. He has also received a Certificate in Native Plants from UNC-C and a Certificate in Horticulture Technology--Residential Landscape Design from CPCC. He and his wife, Sue Bartlett, own The Cedars Davidson Bed & Breakfast.