Answer: Rather than planting directly in the garden bed, keep your mandevilla in a container and sink the container into the ground. Mandevillas will not survive the winter in your garden. In fact, the minimum temperature they will tolerate is about 50F. At that temperature, growth slow down. You'll need to winter the plant over indoors, or store it in a dormant stage in a place such as a garage or basement.
Mandevilla as a houseplant can be trimmed back to make it manageable and needs a sunny window to flourish. Water once a week. You will get some leaf drop due to lower light and humidy indoors.
Another way to winterize mandevilla is in a dormant stage in a garage or cool basement with temperatures around 45-55F. Cut it back to about 12 inches above the soil line. Treat for insects if any. Keep it dry but not completely dry. Check periodically for soil moisture. It will drop its leaves. The important thing to keep in mind is that you want to keep it alive it through the winter. In the spring some of the old growth may be pruned back and when new shoots develop move the plant to a sunny window. Pinch the new growth to get a bushier plant. Repot in fresh soil and set outside when danger of frost has passed.
Best wishes with your new mandevilla.
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