If your hydrangeas
are sited correctly, with enough room to grow, the only pruning required is to remove dead wood—be sure to take it off at the base of the plant if the whole branch is deadand spent flowers. In Dirr's experience, all hydrangeas benefit from regular dead-heading to encourage more blooms. Left unpruned, they will produce fewer flowers because of a growth-inhibiting chemical released by the terminal bud at the tip of the stem. Don't prune past August, though, because any new growth is susceptible to an early-fall freeze.
For more on caring for these flowers, see 5 Tips for Growing Gorgeous Hydrangeas