Answer: This is a difficult question. You might be able to keep it in a sheltered but cool place for the winter, but to some extent it depends how stressed it has already become by being held in a container under home conditions. If you bring it inside, wait until it has gone fully dormant and bring it into a cool place -- above freezing but under 45 degrees. Keep the soil just barely moist, meaning not bone dry.
Whether or not to plant also depends when you expect the ground to freeze, a rule of thumb is to plant no later than 8 weeks ahead of that. Generally speaking it is better to plant than try to keep something in a container. You might also consult with your local county extension and/or the retailer where you purchased it.
If you opt to plant it now, do it right away and keep the soil slightly damp until the soil freezes up. Mulch generously over the root area in late fall after freezing weather has started. Also provide a windbreak for your plant this winter.
And, if it is a Hydrangea macrophylla, since your zip code places you in zone 5 you should probably plan on insulating it every year. To do that, wrap it in burlap and stuff around it with a dry non packing material such as oak leaves or straw, then put a tarp over the top to keep it dry. Allow gaps in the tarp on the sides so air can flow through and prevent condensation. Put this on in late fall when the plant is fully dormant and remove it in early spring so the plant can wake up naturally with the season.
Good luck with your hydrangea!
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