Answer: Overwintering containerized plants can be a challenge. Bitter cold, fluctuating temperatures and wind are the main enemies. Using larger containers (such as half-whiskey-barrel size) helps, as does insulating the containers with mulch or other materials packed around them. Heavy mulching will also help protect the plants from freeze/thaw cycles which can heave the plants out of the soil. Make sure the soil is kept evenly moist (but not soggy) up until it freezes. Consider using an antidessicant spray according to the label instructions, too. Moving the containers to a sheltered location up against the building or erecting a windbreak may also help.
You are not too late to take these precautions. Since soil takes a while to cool down, it's unlikely that much damage from the cold has occurred this early in the winter.
Some plants may be hardy in your zone when planted in the ground, but not survive in your zone when planted in a container because the soil is more exposed. Note that ceramic and terra cotta pots can crack or shatter when frozen because the soil expands as it freezes, so make sure you are using winterproof containers.
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