Do you know if your hydrangea blooms on old wood or new wood? I would advise not to prune in the fall as you might be cutting off flower buds on old wood. If you really need to prune it, do it right after it blooms or wait until leaf break in spring and trim out dead wood. The last two winters have been hard on hydrangeas and even those that bloom on new wood have been killed above ground in certain regions but will usually resprout from the base of the shrub.
