There isn’t a common answer to the common question, “When should I prune my hydrangea?” because there are numerous species of hydrangea. Among the most popular types in our area are:
Hydrangea macrophylla (bigleaf hydrangea) is sometimes easy to recognize because its flower color changes with the soil pH: blue in acid soil; pink in alkaline. The leaves are coarsely serrated and glossy, dark green. H. macrophylla also include the lacecap hydrangeas, whose flowers look like a circle of unopened buds surrounded by open petals. Bigleaf type hydrangea set their flower buds during late summer to early fall (i.e., bloom on old wood.) Pruning bigleaf hydrangea in the spring or even late fall, after the buds have been set, will remove the flower buds and any chance of getting flowers that season. Bigleaf hydrangea should be pruned as soon as the flowers have faded in the summer. There are also newer remontant cultivars of H. macrophylla such as ‘Endless Summer’, which bloom on both old and new wood. This means that if the old growth is damaged by late spring frosts or by spring pruning, you will still get flowers on the new growth during the summer.
Hydrangea arborescens (smooth hydrangea or sevenbark) cultivars, including ‘Annabelle’ and Hydrangea paniculata (hydrangea or hortensia) cultivars,including 'Pee Gee',have (usually white) flowers which are set only on new growth. They can be cut back immediately after flowering and may rebloom or they can be cut back in late winter or very early spring and still be expected to flower the following season. How far you cut them back is based on personal preference. You can choose not to prune H. arborescens or H. paniculata at all, but pruning makes for a neater plant and, if you want long stems for fresh or dried flowers, pruning back hard encourages long stems on older plants.
Hydrangea quercifolia (oakleaf hydrangea) is a native plant characterized by its oakleaf-shaped foliage. Oakleaf hydrangea blooms on new wood and can be pruned in late winter or early spring, while dormant, to remove dead wood. If it has experienced winter dieback, prune back to below the point of injury.
Hydrangea anomala petiolaris (climbing hydrangea) is a vine, not a shrub, and requires little to no pruning. Once climbing hydrangeas become established, they can grow quite vigorously and may only need occasional summer pruning to stay in bounds.