Answer: If you want to keep your mandevilla for next year then you must bring it indoors every fall. It will not survive the winter outside, it is a tropical plant and is damaged by temperatures below about 50 degrees. When you bring it in, you can try to keep it growing as a houseplant in a bright and sunny spot, or you can let it rest in a cool place until spring. If you need to repot it into a larger container, wait and do that in the early spring.
In order to grow well indoors these sunlovers need the maximum possible amount of sunlight -- these are tropical plants after all. In lesser amounts of light they will languish but survive; in lower light they will do better with lower temperatures (say in the 50's or 60's) which helps them to slow down naturally. Indoors, they may grow leggy due to low winter light levels. If your plant becomes leggy or loses its foliage, simply trim it back a bit. As growth slows, reduce watering and fertilizing proportionately.
Next spring, gradually reacclimate it to being outside to avoid "sunburning" it. Resume normal watering and fertilizing as it begins to grow again. Good luck!
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