The most important thing to know, in order to give good advice, is what cultivar or type of hydrangea are you asking about? Zone 5 supports H.arborescens, H.paniculata, some H.quercifolia and a few cultivars of H.macrophylla that bloom on new wood. The average garden variety macrophylla (pink/blue mophead) will not tolorate zone 5 winters and will not bloom in spring no matter when you put them in the ground. Extensive winter protection is their only chance for spring blooms.....yg