Run a garden hose at the base of the spiraea shrub for 20 minutes the night before transplanting it. Keep the hose on low volume so the water has a chance to soak in instead of just running off.
Measure out a 14-inch radius around the base of the spiraea shrub. Dig down 18 inches deep along the radial line using a trenching shovel or other long, narrow-bladed shovel. Sever all the roots connecting the shrub to the surrounding soil.
Stick the tip of the trenching shovel under the edge of the root ball. Pull the handle toward your chest to pry the roots loose. Repeat this at four equal points around the shrub.
Grasp the base of the spiraea trunk and lift it from the ground. Wrap the roots in wet burlap while transporting the shrub and preparing the new planting site. Remove the burlap before planting the shrub.
Dig a planting hole measuring 30 inches in diameter and 20 inches deep. Fill-in the bottom of the hole with a 1-inch-deep layer of loosened soil. Settle the spiraea into the hole. Fill in around the roots with loose soil.
Water the spiraea shrub deeply after transplanting it. Run water at the base of the shrub for 15 to 20 minutes once a week for the first month after transplanting it.