Answer: Most hardy plants can withstand very cold weather just fine when planted in the ground, but when their roots are not insulated by the ground it may be a different story so you are right to be concerned. Container grown plants are subject to additional stresses as well. You may find that some plants come through just fine, especially if they are in very large tubs or barrels with ample soil around their roots, but this will also depend on the low temperatures and the amount of sun and wind exposure they receive. The best thing to do is to sink the plants into the ground for the winter. The next best is to move them into a sheltered location out of the wind, be sure they are well watered before the soil freezes, and try to insulate the roots by heaping mulch around the containers or some similar method. Another possibility is to move them into a cold frame. Finally, some people have luck wintering them in an unheated garage or very cool basement. It may take some experimentation to see what works best for you. In any case I would not usually expect a container shrub or tree to live as long as it would if planted in the landscape without truly exceptional care including frequent repotting and optimal maintenance.
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