Answer: While the lilacs will be hardy to your gardening region, since they are in pots, they'll need some protection you wouldn't otherwise need to provide. The biggest problem will be the susceptibility of the roots to freeze solid. Plants in the ground are insulated; plants in pots can freeze solid because of the cold air that will surround them. You can either sink the potted lilacs into the ground for the winter or, since they are in decorative pots now, you can stuff insulation into the decorative pots, surrounding the nursery pots and thereby protecting them from freezing temperatures. So, for the utmost protection, wrap the nursery pots with insulation and set them in your garden house for the winter months. In the spring you can take them back outdoors and pop them back into their decorative pots.
I have a friend who does this with tree fuchsias every year and they overwinter perfectly.
Best wishes with your lilacs!
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