Use ground layering to propagate hydrangea. It's easy and the success rate is very high, but it does take several months from start to finish. Begin the process in early spring, as soon as the hydrangea has leafed out. It is possible to simultaneously root several cuttings at the same time from the same mother plant.
Choose a thin branch near the outside of the bush. Its diameter should be approximately the same diameter as a pencil.
Bend the branch over until it touches the ground next to the hydrangea bush. About 8 inches from the tip of the branch, remove a pair of leaves. The spot where these two leaves emerge from the stem is the "leaf node." Lay the branch on the ground and mark the spot where this leaf node touches the ground.
Use your garden trowel to dig a trench about 2 inches deep at the spot you marked in Step 2.
Push the leaf node of the branch into the hole you dug in Step 3. Cover the branch with the soil you removed from the hole. Hold on to the branch with your other hand or it will pop up out of the ground. Do not cut the branch off of the mother plant at this point.
Put a heavy rock over the part of the hydrangea that you buried to keep it under the soil. The branch will continue to get nutrients from the mother plant, and roots will form at the leaf node in six to eight weeks.
Check to see if roots have formed by removing the rock after at least six weeks have past. If the branch pops up out of the ground, rebury it and check it again in a few weeks. If the branch stays in the ground, roots have begun to form. Replace the rock and allow the roots to form for another three to four weeks.
Cut the rooted cutting off the mother plant eight to ten weeks after beginning the rooting process. Make the cut at ground level, just past the part that is buried and that is closer to the mother plant than the roots. Do not dig up or remove the new cutting from the ground at this point or it may go into shock and die. Allow it to grow on its own for another two to three weeks.
Dig up and transplant the newly rooted hydrangea at least three weeks after severing it from the mother plant.