I picked up 10 potted hydrangea cheap at end of season sale. Most only had a few leaves, as I was planting them I noticed lots of buds on stems. I know its to close to freezing weather for them to not freeze, will this kill the bush?
It may. It would help to know what type of hydrangea do you have, not to mention what zone you are in. I just bought a batch of Quickfire on sale, which I was quite enthused about as well. Placing a layer of several inches of mulch around the entire rootball would help. Perhaps even surrounding the plant with a tomato cage or similar apparatus filled with leaves would help as well. The goal will be to protect the plant from freeze/thaw and wind damage. Still, much of the plant could be killed back during a harsh winter, but new shoots may emerge from the ground if well protected.
If you experience cold winters (Zone 6 pr colder), another option is to keep the plants potted in a garage, water them once every 1-2 weeks and plant them in the ground about 2 weeks after your area's average date of last freeze. Do not fertilize them now; the potting medium probably already has those round fertilizer pellets anyways.