Starting a Rain Garden

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Locate the rain garden in a natural depression in the landscape near a downspout of the home.
Use a rope or garden hose to lay out the boundary of the rain garden in a curvy in shape with the longest length perpendicular to the slope of the land. The size of the rain garden will depend on the area of impervious surfaces and the infiltration rate of the soil.

Calculate the square feet of impervious surfaces (i.e. driveway, patio, roof, sidewalk) draining into the site.
Determine the type of soil. Soils with high clay content will infiltrate water very slowly while soils with high sand content will infiltrate more quickly.
The size of a rain garden for clay soils should be 60% of the drainage area. For example, if the area of your roof and driveway measures 1000 square feet and you would like to use all of the rain from these surfaces, your rain garden should be 600 square feet or 20'x30'.
The size of a rain garden for sandy soils should be 20-30% of the area draining to it.
If you plan to create the rain garden yourself and determine that your rain garden area needs to be greater than 300 square feet, then divide the drainage area between two or more rain gardens. Build each one as you can manage it.

The rain garden should be designed to hold about 6" of water above the ground surface.

Ideally, locate the rain garden in such a way that a low berm on the downhill side of the rain garden will hold back the appropriate amount of water. The top of the berm should not be higher than the uphill edge of the rain garden. A berm is a small earthen dam, no more than 12" high.
The bottom of the rain garden should be as level as possible, so some minor grading may be necessary.

A shallow swale or corrugated drain pipe (buried or above ground) will channel runoff from the roof downspout or paved surface to the rain garden.
The soil in the rain garden should be loose, sandy organic soil that allows water to quickly soak into the ground to nourish plant roots and recharge the groundwater. A general rule-of-thumb is to have soil that soaks in about one inch of water per hour. The following steps will help to achieve this:

Mix organic matter into the soil within the rain garden by spreading 2 to 4 inches of compost over the area and mixing the organic matter in with the existing soil.
If your soil is acidic (has a low pH), lime should be added to neutralize the pH of the soil. The Gwinnett County Extension Service will test a sample of your soil for a small fee. Visit their website or call 678.377.4010.
For soils with high clay content, it may be beneficial to remove about 1-2 feet of the soil and replace it with a more porous "rain garden soil." A soil mix suitable for rain gardens is a mix of 50-60 percent sand, 20-30 percent topsoil, and 20-30 percent compost. The clay content in the rain garden soil replacement mix should be no more than 10 percent.

Establish a grass or groundcover border along the upper edge of the rain garden to slow down the runoff water as it enters the rain garden. Do the same over the berm to stabilize it as a border of the rain garden.
Plant drought tolerant, wet tolerant, hardy plants. A mix of ornamental grasses, shrubs and self-seeding perennials are good choices. For more information on suitable plants for our area, please visit the Gwinnett County Extension Service website.
Once plants are in place, cover the rain garden with a 3" layer of mulch. Shredded hardwood is a good choice since it is less likely to float away.
Remove weeds on a regular basis and replenish mulch as needed.
IMPORTANT NOTE: Plan on providing an "overflow" path for water to take if the rain garden fills up before the rain has ended. This path should be stabilized with a hardy grass or groundcover.