Answer: You didn't mention whether or not you piled mulch over the tops of the plants after you cut them back. It's an important step to keep the roots at a tolerable temperature. Mandevilla can be borderline in your growing region. The coldest temperatures they can tolerate is 45 to 50 ? F. Without adequate protection, the plants won't survive outdoors. Since yours are in containers, you may be able to cut the plants back in the fall and then bury the containers in the ground in a protected place near the foundation of a building, then pile some protection over them (pine boughs, shredded leaves, etc.) Burying the pots in the ground will help insulate the roots from temperature extremes. Failing that, you might want to consider taking the potted plants indoors for the winter.
Best wishes with your mandevillas!
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