Dig a hole where you want to replant the hydrangea. It should be three times the diameter of the root ball and the same depth of the root ball or the pot the hydrangea is in.
Prepare a soil mix to put back around the root ball once it is in the hole. Use the soil you took out of the ground, mixing in peat if it sandy soil or compost if the soil is more like clay.
Check the bottom of the root ball to see if the hydrangea is root-bound. If the roots circle the root ball following the contours of its pot, it is root-bound, and the roots should be snipped in a few places with sharp, clean pruning shears.
Set the hydrangea in the center of the hole so the top of the root ball is level with the ground.
Fill in around the root ball with your prepared fill soil, tapping as you go to be sure there are no air pockets around the roots.
Create a 2-inch-high lip around the plant that will hold water over the buried roots. This is called a berm.
Soak the area beneath the plant within the berm. Use a gentle flow of water and allow it to fully saturate the planting area.
Mulch with 1 to 2 inches of organic mulch to retain moisture.