I have a "Ruby Slippers" that barely survived its first winter in the ground.
As a last chance I am thinking of putting it in a container. What would be the smallest container that I could use with the hope of overwintering it in an unheated attached garage.
In a container
I am going to quote Dan Hinkley: For those gardening in less benign climates and wishing to grow hydrangeas, they do respond well to pot culture. A gallon sized plant should be moved directly into a 15 - 25 gal container using a bark based soil less compost which can be purchased through gardening centers. Move the containers into an unheated porch, garage or cold greenhouse during the winter season, protecting only during the coldest temperatures. Fertilize and water as for those plants grown in the ground - replace the compost (planting medium) every three years or as needed.
Luis