Good question but the answer depends. The most common hydrangea with the big balls of blooms will flower on two year old wood so any branch that has old flowers from last year can be pruned back hard in February or March. Even later is you forget as these branches will not flower again until next year. This is not the case for the new \"Endless Summer\" reblooming hydrangeas. They flower on new and old wood and I prune my in fall and also in spring to shape them up. Spring would be the months of Feb - April. The Pee Gee hydrangeas (with cone shaped cream to pink blooms) need to be pruned in late winter or early spring - Feb or March as they flower on new wood. Just to add to the confusion, some growers claim that hydrangeas bloom best if never pruned at all - let them grow tall, wide and full of flowers the way nature intended. Keep growing, Marianne Binetti