Big leaf Hydrangeas bloom on old and new wood so you need to protect the next season's flower buds that form in the fall.
In July remove some stems, then in fall shear off the top third off your plant. Late November or early December, it is very important mulch heavily, or even better, tie the branches together and wrap with burlap. Fill in the burlap cage with mulch. In the Spring uncover when you do your perennials. Once the stems produce leaves, cut out all dead ones and prune it back to a finger length above the first set of leaves.