|I have my plants in pots. Can they survive the winter or do they need to be planted?
|Unfortunately, I don't think this will do well in your area in the cold winters you typically have -- whether in the ground or in a container, unless you are able to provide it with some extra winter protection such as a coldframe. It is only rated winter hardy to USDA zone 7 or zero to 10 degrees.
You could try planting it in a sheltered spot out of the wind, then put a wire mesh frame around it, then place an insulating material such as dry fluffy oak leaves or loose straw inside the frame. Then cover the top only of the frame with a tarp to keep excess water out and prevent the insulating materials from getting soggy and possibly smothering the plant. You would do this in late fall after the weather has been quite cold, and unpack it gradually in the spring. This is a lot of work, though, unless you really like the plant.
Another thing to try would be bringing it into an unheated garage in late fall after it has gone dormant, keeping the soil just barely moist, then take it back out again in early spring to wake up with the season. Unfortunately, I can't promise you that either of these methods will be successful. Good luck with your hydrangea, it is a pretty one!