Cut a 5- to 6-inch-long branch from the hydrangea in late summer to fall. Use a branch of new growth with three to four leaflets and no flower buds. Make the cut just below a growing node.
Cut the bottom set of leaves off, but leave one to two leaves at the top of the cutting. Cut the tips off these remaining leaves.
Use a 3- to 4-gallon pot with a drainage hole for rooting. Larger pots offer the best possible balance and room during the rooting procedure. Fill the pot 3/4 full with a mixture of half organic compost and half quick-draining potting soil. Water the soil until it's moist.
Dip the bottom of the cutting in a rooting hormone and insert it 2 inches into the damp soil.
Put the pot in bright indirect or artificial light, but not in direct sun. Cover the pot and hydrangea cutting with a large plastic bag to maintain warmth and humidity.
Water the hydrangea cutting whenever the soil dries. Expect rooting in two to three weeks.