BEFORE DIGGING: Although you are not required to notify Call Before You Dig (CBYD) for shallow excavations done by hand, CBYD encourages you to call. Even hand tools can damage utility lines. If you are using any type of heavy equipment, you are required by law to call. Dial 1-800-922-4455, or 811. Visit the Call Before You Dig website for a homeowner’s guide to the CBYD service. One important note: this service will not locate small utility lines such as wires run from your house to outside fixtures. These things are your responsibility to locate.
Mark out the area of the garden, based on the size you calculated for the drainage area. If the area is flat, you can remove 8-9 inches of soil from the whole area. Shape a gentle slope from the bottom of the garden to the surrounding lawn area (see figure 1 below).
If the area is sloped, you can use some of the soil that you removed from the garden to build a berm at the lower end (photo, right). If you are removing turf to install your garden, you can use some of the turf pieces on the berm so vegetation can be established quickly, and erosion potential can be reduced. When you are constructing the berm, try to make it the same level across the berm. If there is one area that is lower than the rest, overflow from the garden during a large storm may concentrate there, and cause erosion as water flows out. Keeping the water spread out is the best way to avoid erosion (see figures 2 & 3 below).
Take a shovel full of soil from the bottom, in the area where you will be planting. If the soil at the bottom is looks the same as the top layer of topsoil that you removed, you can plant directly into it. If the subsoil is lighter in color, or appears compacted, you may need to loosen it and add an amendment like compost when planting (photo, right). A rototiller can be used to loosen the soil and integrate compost at the same time.
If your soils were slow to drain, it may mean that you have high clay content. If you have the means, you can make the garden a little bigger to account for the slower infiltration rate. Alternatively, you can add some compost to the bottom of the garden when you are rototilling. If these things are not possible, don’t worry. The garden will still infiltrate some water, and every little bit helps.
If you are using a pipe to get water to the garden, you may want to consider placing some stones in the area where the pipe or gutter enters the garden. During intense storms, water inflow from the gutters can wash out mulch and/or plants. The stones will help to dissipate this energy and prevent erosion.
Diagrams above adapted from the University of Wisconsin-Extension, Rain Gardens: A How-to Manual for Homeowners.
There are some general concepts that you can follow when planting to make your garden function well, and look pleasing to the eye.
Once your soil is ready, you can begin planting.