Pruning:

• Hydrangea panicula:
  • Bloom on new wood.
  • Prune in late winter or early spring.
  • Take off up to 30%-50% of the old growth before they leaf out to create a roundish shape.
• Hydrangea quercifolia:
  • Bloom on old wood.
  • They require little pruning. Limit it to the removal of dead or broken branches as needed.
• Hydrangea macrophylla:
  • Bloom on new and old wood
  • These plants produce buds in late summer to early fall (August-September) that will form next year’s flowers. It is best to wait until new growth emerges in the early spring to be sure that you are only removing dead or weakened parts and not removing live stems with flower buds.
  • In very old and declining hydrangeas, hard renewal pruning may be needed. Cut back all of the branches to the ground. This will eliminate the blossoms for that year, but the next year should be quite productive.
• Hydrangea Arborescens:
  • Let these plants grow a season or two before doing any serious “hard” pruning.
  • Bloom on new wood
  • Prune in late winter or early spring.
  • Flower buds will grow on this season’s growth or new wood.
• Hydrangea Petolaris:
  • This rugged, gnarly vine needs little to no pruning except for removing any dead wood that develops.

Check out this guide from Monrovia for more information:
https://www.monrovia.com/be-inspired/when-to-prune-hydrangeas.html