From garden guru and friend, Ron Wilson, of Natorp’s
“How do I know when to prune my hydrangeas without messing with their flowers?” Pruning hydrangeas can get confusing as some flower on new growth, some on last year’s growth, and some on both. As a general rule to protect flowering, arborescens and paniculata selections flower on new growth, so prune in spring. Macrophylla, serrate, and quercifolia generally flower on old wood, so clean up deadwood after the winter and deadhead spent flowers as needed, leaving previous year’s growth. Some newer selections flower on old and new growth, but still try to maintain the old growth as best you can. Climbing hydrangeas flower on old wood, so prune after flowering as needed.