Dig around the central trunk of the hydrangea bush, making a trench spaced about 2 feet out from the trunk. Dig out the trench until the trench is deeper than the hydrangea's roots. Slide the shovel beneath the root ball and lift it from the soil.
Fill a potting container with 3 to 4 inches of a pine bark-based potting soil. Use a container that is at least 6 inches deeper than the height of the root ball and at least 3 inches wider than the root ball.
Set the hydrangea in the pot. Add more soil beneath the roots, if necessary, until the top of the root ball sits 2 inches beneath the pot rim.
Add soil to the pot to fill in the space between the roots and the pot sides. Add soil until the hydrangea is potted at the same depth it was growing at previously.
Water the soil until the excess moisture drips out the bottom of the container, as this ensures the soil is evenly moistened throughout its entire depth. Place the pot in a warm, sunny location and water as necessary to keep the soil from drying out completely.