Answer: Mandevilla and Hibiscus rosa-sinensis are tropical plants and will not survive your winters outside -- not even with extra protection. But they can be overwintered indoors. You will need to bring them indoors soon, before temperatures drop below 45 degrees. Indoors, the hibiscus will do better if kept in a very sunny window, protected from drafts, and with as much humidity as possible. It may lose some leaves but should stablize. Stop fertilizing for the winter unless you have an extremely bright location for it. The mandevilla can also be kept in a sunny window, but it will tolerate being stored in a cool dark place such as a basement. Keep the soil just barely moist, not bone dry and the temperature about 45 to 50 degrees. Cut it back when you bring it inside as it will defoliate anyway. If you do not have a sunny window for the hibiscus, it too can be stored this way. Prune it back, also, either when you bring it in and/or first thing in the early spring. Next spring, gradually acclimate them to being outdoors, setting them out on sunny warm days and protecting them at night -- do not let them get colder than 50 degrees when they are in active growth. Good luck!
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