When do I prune my Hydrangeas?
Well, it depends on what kind of Hydrangeas you are growing in your garden. if they are herbaceous types, like the popular Hydrangea macrophylla that die down in the winter, the only pruning is for removal of cut flowers if desired during fall, or the cutting out of dead and damage stems (flowers grow on old stems each year, so no pruning in spring). Occasionally renovation is needed this is done by removing very old stems within the shrub while in full growth.
If they are Paniculata species, 7-8' tree size type Hydrangea with exposed trunk, are pruned in spring time removing old wood (Paniculata’s flowers form on new wood, so no problem pruning old wood heavily).
Courtesy of the NYBG Plant Information Service