Hydrangea pruning rules vary from one variety to the next, but none of them are too complicated. The easiest to remember is white-blooming hydrangeas – most white hydrangeas bloom on new wood and can be pruned as soon as the blooms fade. If you haven't pruned your white hydrangea this winter, you can go ahead and cut them back for full spring blooms.
For most macrophyllas, mopheads, and oakleaf hydrangeas, blooms form on at least one year-old stems. If you prune too severely you will not have any blooms. Other than cleaning out the dead and ugly parts, it's best to leave these alone unless your plant has become too large.
If you are pruning for cut-flowers, your goal is not to make the plant
look pretty, but to get the best blooms on long straight stems. First,
remove all of the dead or dying material that you would normally
remove. Second, you will cut the plant back to about a third of its
current size. Cutting the plant really low, will force it to grow long
stems, which is exactly what your want.