One inch of gentle rain on a flat plain evenly deposits more than 27,000 gallons of water per acre. My one-acre Panther Branch garden on flat, sandy, former farm land is the best case scenario for the even distribution of water from the sky.

Even the rains of tropical storms that, in a few instances, have deposited as much as eight inches in a day, have left little more than a few puddles in low spots that quickly dissipated after the sun came out. Flooding has never been a problem, but holding onto a little extra rainfall would be a lovely bonus.

In most of Wake County the story is very different. While a gentle one-inch rain falling on the impermeable surfaces of parking lots, driveways, roadways, hard clay slopes and rooftops also deposits more than 27,000 gallons of water per acre, the distribution of all that rain is dramatically altered, sometimes resulting in flooding and serious erosion.

According to Wake County Extension agents Mitch Woodward and Carl Matyac, "A week doesn't go by that we don't hear from homeowners dealing with drainage issues and wet spots in their landscapes."

An easy solution with a romantic-sounding name could be just the cure for homeowners and gardeners dealing with soggy bottoms, flooded driveways and clogged drainage arteries. The term "rain garden" may conjure images of lush tropical forests or lofty, misty waterfalls trickling down mountain valleys, but, in fact, rain gardens start with a hole in the ground. And getting that hole into the ground during the fall holds the best success for your rain garden.

How to do it

A rain garden is a shallow depression that you create to capture runoff from a driveway or roof that will allow the excess water to soak into the ground slowly rather than having it run across lawns where it collects pollutants that can foul streams and waterways. Once the rain garden is established with a healthy stand of well-chosen plants, it becomes a natural biosystem where the plants and soil work together to absorb and filter pollutants to percolate cleaner water slowly through the soil into nearby streams. Rain gardens also reduce flooding and provide habitat for beneficial insects and wildlife. Because water is typically only in the rain garden for a day or two, it doesn't become a breeding ground for mosquitoes, which require five days to develop.

Location. Rain gardens should be located between the source of runoff and the runoff destination (drains, streams, low spots, and so forth). Put the rain garden at least 10 feet away from the foundation of the house, being sure that it's also at least 25 feet from septic systems or drain fields. Ideally, for the plants, the garden should be in partial to full sun. The water table should be at least two feet below the surface of the soil, so if you hit the water table (you find water or the hole fills with water once it's dug) consider turning the site into a wetland garden -- the difference being in the types of plants chosen. In a rain garden, plant material ranges from ones that prefer wet feet in the moist areas to native grasses, shrubs, perennials and trees on the outer (drier) edges. A well-designed rain garden will appear to be just another beautifully landscaped area of your property. In wetland gardens, some standing water will usually be present, and you need to be more aware of mosquitoes.

Size. Rain gardens are being used successfully in all types of landscapes, from the smallest home plots to large commercial developments and public park lands. The size of the rain garden is gauged to accommodate the size of the property.

Let's say, for example, that you have a house 60 by 60 feet, with an estimated 25 percent of the roof area draining into a downspout, and a driveway of about 500 square feet draining into the location of the rain garden. You calculate the square footage of the roof area (3,600 square feet). Multiply that by the 25 percent of the runoff from the roof (900 feet) and add that number to the 500 feet of driveway for a total of 1,400 square feet. Divide that number by 20 to find the area the rain garden should be.

In this example a rain garden of at least 70 square feet will handle runoff from both the downspouts and the driveway. A 70-square-foot area could be configured in a garden about 5 by 14 feet or 7 by 10 feet. Both are relatively small areas that will provide big benefits in the long run.

Design and plants. Lay out a garden hose or spray paint the outline of your rain garden in whatever shape you prefer. Squares and rectangles are OK, but oblongs, circles and free form shapes are more natural-looking. Dig out the soil inside the outline to a depth of four to six inches, with the deepest part near the center. Use the excavated soil to create a low berm along one side of the rain garden to help retain captured runoff (on a slope, the berm should be on the downhill side). To prevent erosion, cover the berm with grass or mulch. For well-drained soils, add compost to the top layer of the garden to help plants establish and to help the garden retain more water. In compacted soils, add gravel or mulch to improve filtration. Choose plants that tolerate fluctuating moisture levels (native plants work very well) and remember that long dry spells may call for extra irrigation until the plants become established.

Planting your rain garden this fall gives root systems a head start. (Roots keep growing here all winter.) Choosing the right combination of plants will ensure that they'll be ready when the April showers arrive in the spring.