Yes it can be done, but with qualifications. There are over 70 different species of Hydrangea with various hardiness requirements. In the North (where the over wintering question is most relevant) most success would likely be obtained from H. arborescens and H. paniculata and their cultivars. Both of these species flower on new wood so there is little danger of the flower buds being killed or damaged with a late spring frost. I would suggest a minimum of a 25 gallon container and if the pot must be over wintered outside on a balcony with little protection, at the very least sheltering against a wall, out of the wind, is advised. If the container can be moved into shelter of an unheated garage, better performance can be expected. Containers should not be moved until all of the plants have entered dormancy, and then the winter storage area should ideally remain between 24F and 40F ( -5C and +4C). All that being said, I would think that zone 5 is probably the limit of successful container overwintering of Hydrangea. But I’d love to learn if I am wrong – anyone out there having success in colder zones?