It could be a NIKKO variety which is a zone 6 plant, however I've grown a nikko in a sheltered spot in a zone 5b area. I guess you don't have much of a choice but to plunk them in the ground. However as these have been indoors for a while, do take steps to harden the plant and don't put them in direct sun because they will burn.

FWIW, I would second ianna's advice. I'd add that a sheltered spot close to your home would be a good idea. The heat from the house, and the protection may give your hydrangea a better chance during the winter. As well, if you're so inclined, take a cutting and then you'll have two!

I have one in my garden that was an origional Mothers day present. It has been there for close to 5 years and thriving. I will be moving it this year and dividing but this time I need to place it again in a spot that is somewhat sheltered and very important a spot that can keep moist. This plant is very thirsty and will wilt right away when dry.