To care for a mandevilla during winter, move the plant indoors when temperatures are below 60 degrees Fahrenheit or when outdoor temperatures are similar to indoor temperatures. Place the plant in a sunny location indoors and water it once weekly throughout the winter. If the plant becomes dormant, a stage where the leaves fall off, water the mandevilla sparingly until the next growing season, and store the plant in a room with temperatures of 55 to 60 degrees Fahrenheit.
If necessary, prune the mandevilla back by one-half before moving it indoors. To care for a mandevilla planted in the garden, transplant the mandevilla to a container slightly larger than the root ball, and cut the stem back to 12 inches above the soil. Water the plant sparingly throughout the winter.
If pests are present in the soil, flush the soil with water to remove the pests before bringing the plant indoors. Begin fertilizing the plant with a phosphorus-rich fertilizer three to four months before the next growing season.
After the danger of frost passes and temperatures are consistently at or above 60 degrees Fahrenheit, place the mandevilla outdoors and resume the regular watering schedule. Expect winter growth to fall off the plant. The winter growth is replaced with new growth as the season progresses.