Answer: This hydrangea should be root hardy into zone 4 which is where your zip code places you. It is unique in that it is able to bloom on both the old wood that grew the previous year and also on the new growth of the current season. In your zone you would expect it to freeze back to the ground during the winter, with winterkill ruining all of the old stems. However, to help protect the crown, I would wait to do any pruning until spring. Also, this dieback means it will bloom later in the summer and fall on the new growth so do not prune it except to remove the winter damaged/killed stems.
You should mulch it heavily in late fall after it is dormant. I hope it is planted in a sheltered location with protection from the winter wind. You may want to wrap it in burlap and heap a dry insulating material over the plant, oak leaves or straw for example. Then top that with plastic to try to keep it mostly dry. Do not completely enclose it with the plastic however as you do not want to cause condensation to build up on sunny days.
Good luck with your Endless Summer!
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