While pruning isn’t necessary, it’s useful if you want to control the size of the plant, shape it a bit, and perhaps get even bigger blooms. On hydrangeas that bloom on old wood (late spring/earlier summer blooming) such as many hydrangea macrophylla and oakleaf hydrangea, prune after the blooms have faded in late summer. Just lop off the old blooms and remove dead canes. End of season is also a good time to prune back extremely tall canes. On hydrangeas that bloom on new wood (later in the season and often until frost) such as hydrangea paniculatas, you can prune in late winter or early spring.