Sometimes creating a rain garden is as simple as directing rain to an absorbant swale in your yard and perhaps putting in a few plants. If you have no convenient place to direct your roof runoff, or you want a formal garden design, then it's time to get down and dirty.
Soils on developed land have been compacted by heavy construction equipment. The soils will need to be dug up and loosened to a depth of two feet, or replaced with rain garden soil mix (50-60% sand, 20-30% topsoil, 20-30% compost), not only to prepare for planting the garden but so rain can soak in. If you have extra soil left over after this loosening process, use it in another part of your yard.
Preparing a rain garden is just as much work as creating any other perennial garden. Get out your shovel and dig to produce beautiful results.
Define the Borders
First, define the borders and shape of your garden at the location you have selected. When we prepare a rain garden, we outline the area to be dug by spraying with non-toxic soccer-field paint. Another method is to lay a hose along the shape of the garden, then dig along the hose. This gives a nice flowing border to the garden area. Or, you could simply choose a rectangle as the shape of your garden.
If the area is lawn, you will have to remove the turf. You can either use this either in another area of your yard, or it can be composted to help improve your soils. There is a special tool for removing turf that can make this task easier.
If you are not replacing your soil, double-digging to a depth of two feet is recommended. Remove the soil from the hole and pile it to the side; then dig and loosen the earth in a new hole, and pile it loosely into the first hole. This is the time to mix compost into the soil, if you are doing soil improvements. Continue until the whole garden is prepared. A great tool for this purpose is a digging fork. Try it, you'll like it!
To be certain that your rain garden will function properly, simply replace the soil with the recommended rain garden mix: 50-60% sand, 20-30% topsoil (no clay), and 20-30% compost. This will give your new plants a great start, and the soil mix is designed to soak up rain.
If your site soils are clay, soil replacement is probably in order. You may also want to add a reservoir of gravel at the bottom of the garden bed, or add tiles or an underdrain that leads to another area. This will avoid having your rain garden become waterlogged. The idea is to create a living sponge of soil, plants, roots and mulch, not a soggy bog.
How much soil is needed for replacement?
1 cubic yard=27 cubic feet.
A rain garden that is 2 feet deep X 5 feet wide X 10 feet long will need 100 cubic feet of replacement soil, or about 3.7 cubic yards.
Improve the Soil
In most of West Michigan, adding compost or other organic matter is the best way to enrich your soil. A compost of decomposed yard waste such as dead leaves and grass clippings is generally the best bet (this is another resource that shouldn’t be wasted!). If you do not have your own compost, compost can also be purchased at garden centers and many other retail locations.
Grading the pond area
Grade the surface of your prepared rain garden bed in such a way that the water entering it can spread out over a large flat area and soak into the soil. This may involve removing a lot of soil.
The depth of the dip should be about six inches. The depth can be graduated from the edges of the garden to the middle. This avoids creating a crater that people can trip and fall into.
When your pond area is ready and the soil is nice and loose, it's time to plant. You can prepare a rain garden bed and then cover it with mulch until later; then, plant through the mulch. Or, you can plant immediately, then mulch the plants. The choice is yours. The sooner the plants are in, the faster your rain garden will get established.
Next, select the right plants for your garden.