Hydrangeas that bloom on old wood from last year’s growth are the earliest to flower – before July. Those that bloom later in the season – after July – are flowering from blossom buds on new wood that is growing during the current year.
If you feel uncertain about which variety you have, a safe rule for all types of hydrangeas is that no pruning is better than the wrong type of pruning. A simplified approach, suitable for all types of hydrangeas, is to limit pruning to:
Winter-killed wood — Remove all dead branches in the spring before or as the buds are opening. Test the stem for life by scraping the bark with a knife. If it’s not green underneath, it’s dead and should be removed.
Rejuvenation — Old wood dies back on even the healthiest hydrangeas. In the early spring remove dead or very old stems by cutting them at the base of the plant. This will stimulate new growth and produce a more vigorous bloom set later in the summer.
Dead flowers —Removal of old dried flowers, known as dead-heading, is the safest pruning and can’t be done incorrectly (however you don’t even HAVE to do this!).
Source: University of Minnesota Garden Newsletter