Fall is the best time to do this; cooler weather allows roots to become well established through winter. Follow these steps to get ornamentals off to a good start.
Step 1: Dig a hole no deeper than the rootball and about twice as wide. Set the container in the hole and lay a shovel handle at the top of the rootball, bridging the hole. 1- to 5-gallon plants should sit about 1 inch above soil level.
Step 2: Remove the plant from its container, loosen matted roots, and set plant in the hole. Fill hole halfway with soil, then soak well. Recheck the rootball's elevation and adjust if necessary. Finish filling the hole with soil and water thoroughly.