I found Hydrangea's get "leggy" if not pruned back each year. Even though I'm only 4 miles from Florida, my hydrangeas go dormant during the winter. My best results have been pruning as soon as the leaves start to fall off in the fall.
Even if you don't get around to pruning until Spring and there are already buds on the stalks, pruning the "leggy" stalks will encourage the bush to fill out during the summer.