Answer: As a rule hydrangeas are pruned in the spring, if pruning is needed. The big leafed hydrangea (H. Macrophylla) is pruned in late spring but only as needed to remove any winter damage. Cut only the truly dead stems. The reason for this is it blooms on old wood or stems that grew the year before, so spring pruning reduces or prevents blooming.
If the plant is overgrown, you might remove a few of the longest stems by cutting them off low, close to the ground, but wait until right after it blooms to do that.
Hydrangea paniculata is a woody shrub that blooms on new growth of the season, so it can be pruned in late winter or early spring if you need to control its size or want to try to direct its shape a bit. It does not require pruning except to remove any dead or damaged branches.
Hydrangea arborescens also blooms on new wood. It is routinely pruned off short in very early spring, you can cut it off close to the ground or mow it with a lawn mower.
I hope this helps.
Q&A Library Searching Tips