Depends on variety. They bloom on old wood , so only prune lightly.
ALL of my hydrangeas are cut back by the arborist who planted them in the first place. He put in several tree hydrangeas as well --and each year they get bigger and bigger, with the flower heads this year the size of dinner plates. Gorgeous plants which do need proper pruning. With the tree hydrangeas, they're cut back drastically, yet still rebound in Spring (North East). My heart sank the first time I saw him cutting them back so much--now I give him free rein.
PS: the plants averaged 7-8 feet tall this year and about 5-6 feet in diameter this Summer. No complaints at all. :)
Well it depends. Do your hydrangeas bloom on "new" wood or "old" wood. If they bloom on old wood and you "cut back" aka "prune," you won't have any blooms come next spring. Learn about your hydrangeas before you make a big mistake. These will help you.