But you can also start as early as June-July, because success rates are also very high.
Hydrangea cutting step-by-step
Chose one of the most vigorous stems from your hydrangea: new growth that has not born flowers.
Cut the stem about 8 inches (20 cm) from its tip.
You may cut several at once, this will increase your success rate.
Keep only the middle part of the stem – about 6 inches (15 cm) – removing the head and cutting just above a pair of leaves.
Delicately remove all the leaves except for the two topmost leaves.
Dip this portion in water for a few minutes, and then dip it in a mix called rooting agent that can be purchased in a horticulture store.
Cuttings may still succeed without this rooting agent, but using it will increase your success rate.
Plant the cutting(s) in special cutting soil mix, or a mix of soil mix and sand.
Space each cutting around 2 to 4 inches (5 to 10 cm) apart.
Stake each cutting to keep it vertical.
Place your cuttings outdoors in the shade.
Water regularly but not too much, or the young roots will drown.
Keep your cuttings in a cool, dry, ventilated place which does not freeze over the winter.