To find a correct site for a rain garden, look for a 2-12% slope on your property that water flows into or across naturally. If you have a natural depression and water does not pond for an extended period of time, this could be an excellent location.
In order to be sure the water flowing into your rain garden will drain property, it is necessary to conduct an infiltration test. Simply dig a hole 8 inches across and 8 inches deep. Fill the hole with water. Fill it once if you have just had a rain, fill it twice in consecutive order if you are in a dry spell. If the water in the hole drains within 8 hours, this is a good site for a rain garden.
Size and shape are important factors in a rain garden. A rain garden's size should be approximately 1/3 the size of the area draining into it. For example, if the rainwater entering your garden is from your roof's downspout and size area of your roof is 300 square feet, you rain garden should be 100 square feet.
Most people choose their rain garden shapes to be organic and flowing, such as kidney bean shape. Whatever shape you choose, it is important that the garden be longest parallel to the runoff entering the garden. This will insure your rain garden captures maximum rainwater runoff.
You will need a shovel, rake, and bucket to handle the soil. If available to you, a small tiller would be very useful.
To measure the size of the garden, you will need a tape measure. Additionally, a rope, garden hose, or marking paint could be used to draw the shape of your garden.
As the bed of the rain garden should be level, you will need a leveling device.
The bed of the garden should be between 4-6 inches deeper than the ground level. If your garden is on a slope, you should dig down this amount or slightly deeper.
Some of the excess soil you dig out, can be used to create the berm. The berm of the rain garden should be equal height to the ground level on the "up" side of the rain garden.
You should not plant a rain garden if your soil has more than 40% clay. If your soil has some clay, but not much, you can add soil amendments such as compost or manure to help with soil health and infiltration rates.
After all the excess soil is removed and soil amendments are added, till the soil to a depth of 6-8 inches. This will improve infiltration rates. After you are finished tilling, assure that the garden bed is level. This is important for a rain garden because the rainwater runoff entering the garden should disperse and infiltrate at equal rates.
Before planting, spot and space the plants where you want them in your rain garden.
When finished planting, cover your rain garden with a shredded, hardwood mulch. This type of mulch will resist floating when rainwater enters the garden.