Testing the Percolation of Your Soil
The whole idea of a rain garden is to capture runoff and allow it to percolate into the soil. Obviously, then, a rain garden has to be sited in soils that drain.
Dig a cylindrical hole x deep and x wide with a post hole digger.
Scrape the sides with a nail so they are not smooth.
Fill the hole with water up to a spot you've marked.
Measure how fast the water drains down.
Prince Georges County's reference on bioretention says that, ideally, soils should drain 1" or more an hour. If your soil percolates at that rate, you should not have to do the elaborate excavation that we did. If it percolates less than 1/4" per hour, it may not be a good place for a rain garden. Our soil drained 3/4" an hour, and our rain garden drains fine.