Hydrangeas that do not bloom until August or later are blooming on this year’s new wood. Do not trim them back until late in the winter (March) because they will set their buds on the new wood next year---making them later, but more reliable bloomers after bad winters.
Oakleaf hydrangeas (our only native hydrangea) also blooms on old wood so cut them back as soon as blooming is finished, for most of us that is soon.
Finally, if you have “endless” blooming hydrangeas, deadhead throughout the summer, cutting the stem just a couple of inches below the spent flower For maximum flowers, do not prune the stems back to the ground until it has been growing in your garden for two or three years. At that time, the plant will benefit from new wood.