Answer: This may be difficult. I would suggest trying to insulate both the branches and the roots as best you can. This hydrangea is special in that it is able to bloom on the old branches from the year before and also on the new growth of the season. For this reason you want to try to protect the old growth -- do not prune it now as this would remove the flowering wood for the early part of next summer. If it should die back despite your attempts to protect it, the plant should still be able to bloom later in the season and into the fall on the new growth.
Heap mulch generously over and around the container to insulate the roots and be sure to water the soil as needed to keep it slightly damp any time it is not frozen. In late fall after a few hard freezes have sent the plant into dormancy, n put a burlap wrap around the branches, gently stuff that with a nonpacking insulating material such as dry oak leaves or straw, then cover it with a sheet of plastic or a tarp to keep moisture out. Do not put plastic completely over the sides -- you do not want heat to build up and cause condensation, you just want to keep it fairly dry.
A shady northern spot is fine -- it will help the plant stay dormant. Put the protective material on in late fall, take it off gradually in mid spring. If a late frost threatens, cover it to protect the emerging spring growth. If there is any winter die back and you are absolutely certain a branch is dead, trim it off. Be sure to fertilize and water generously once it begins to grow again for the season. Good luck!
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