Answer: Mandevilla is hardy in USDA zones 9-11; you are in zone 6 so you'll need to dig it up and place it in a pot to keep it over the winter months. As the nights begin to cool off the plants will slow down. You can begin reducing the amount of water your plant receives and you can prune it down to about 12". When nighttime temperatures are about 50F, you'll want to move your plant indoors.
When you bring the plant inside place it in as much light as possible. You are looking for maintenance not growth. You'll probably get some quick new growth, just try to maintain it. Try to maintain the plant on the dry side throughout the winter.
When spring comes you can move the plant outside. Expect the growth produced during the winter to be burned off. New growth should follow and it will be hardy to the spring and summer weather.
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