We recommend that you prune panicle hydrangeas back by about one-third their height each spring. This ensures that the growth for the season comes from the bigger, thicker buds lower down on the stems, and helps build a strong base while also encouraging new growth for abundant flowers.  We recommend spring pruning because the dried flowers provide some nice winter interest in the landscape, but if you wish, you may prune in the same manner in late fall, once the plant has gone completely dormant.