Move them inside (garage, protected shed, etc) as soon as the temps get close to the point where the pots can crack. Water every two weeks or so. I would normally bring them inside once there was a hard frost that resulted in the leaves drying up but, since your pots may crack, keep them outside as long as you can.

I overwintered a few hydrangeas in pots in my unheated (but insulated) garage last year. They all did fine. However, I wondered if I did not wait until they were all really dormant before I moved them into the garage. I said this because they all had some unhealthy foliage growth throughout the winter. Then they did not bloom very well this past summer. Should I have waited longer before I brought them into the garage, so that they were truly dormant?

I have a few hydrangeas growing in pots. During last 2 winters I just brought them inside (my house is pretty cold during winter months). Plants were growing leaves, and one even had flowers in spring. All of them did OK inside on the window and looked well in spring.

Ostrich, I had similar results to yours when I first started to overwinter in pots. Since then I changed my pracice. I either wait untill leaves shrivel and fall off on their own (usualy after few consecutive nights in 20's) or if leaves are still present I just cut them off before bringing into cold frame. Also, I give them a good drink before put to sleep and do not water untill after the Christmas.
Everything is going fine since I start this practice.

I think that the Hydrangeas that I overwinter do best if they're allowed to get very cold and very dormant during the winter. The times I put them into a too warm place they don't do as well. And they always want to send out flowers long before I'm ready. It's a funny sight to go down to a cellar and see this plant with white, bleached out foliage and flower in the late winter. It still will do OK, but not the best. I've always thought it must be the equivalent of keeping me up all night. I look pretty OK, but I'm not really OK.

I'd keep them outside as long as I could and then try to find a place that was very very cold.

I end up keeping mine all outside under cover of some sort. That's always worked best for me.

If I have plants inside a cellar, say, I water very sparingly. Very.

Hay

George and Hay, thanks! I think that your experience just confirmed my suspicion. This year, I surely will just leave them out there until they are completely dormant then. Hopefully, they will grow much better when spring comes!

Winter is not here yet and I am already looking forward to spring 2009!!! :-)