Answer: Mandevilla is a tender plant that is best grown indoors in cold winter areas such as yours. (Roots are hardy only to about 5F degrees, which is much warmer than your winter weather!) I would keep the plant in the pot and simply bury the pot in the ground until it's time to take it indoors in the fall. Take it inside when the nighttime temperatures start hovering around 55F degrees. Growth will probably slow down while the plant is indoors and you can cut or pinch back the stems to keep the plant tidy. It will begin growing again in the early spring.
Best wishes with your new plant!
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