As days shorten and nights begin to cool, your rapidly growing mandevilla will begin to slow down in preparation for a restful winter. The beautiful, exotic mandevilla was never meant to live indoors, and it knows it. But it won't survive temperatures below 45 degrees Fahrenheit, so it will require a little special management for successful wintering indoors. This is much easier to do than you might think, and you won't need a greenhouse or any fancy equipment. A little common sense will ensure that your beautiful tropical vine makes it through the winter to perform beautifully next spring.
Dig up your mandevilla and pot it three to four weeks before the first predicted frost for your area, usually in September. Leave it next to its customary spot to begin hardening off for the winter. Give it one last seasonal feeding of water-soluble liquid fertilizer to toughen it up. Water a little only when the soil completely dries out for the rest of the winter.
Bring the mandevilla indoors when overnight temperatures dip to 45 F. This is roughly about the time that most people begin closing their windows and turning on the heat. Place the plant in a cool, sunny location. Don't be alarmed when it begins to drop its leaves because of lowered humidity, and don't try to increase the humidity. This is normal, and you can expect the mandevilla to begin to look ratty.
Treat the plant for insects with an application of insecticidal soap because you're bringing it inside. Water it only when the soil dries out completely.
Prune the mandevilla back hard the first of February. Cut stems to 12 inches above the soil line. Begin monthly maintenance feedings of half-strength water-soluble fertilizer.
Saturate the root ball with water one time in early spring when the mandevilla begins to develop new shoots. Thereafter, water only enough to barely moisten the soil surface. Always allow the surface of the soil to dry out before watering again.
Feed a full strength solution of water-soluble fertilizer in May or June, about three weeks before the last predicted frost in your area. Begin regular feedings per the packaging instructions and continue throughout the growing season. Begin pinching off new shoots to encourage bushier growth.
Move the mandevilla vine to a sunny spot outside for an hour each morning for several days in May, once daytime temperatures no longer drop below about 50 F. This will begin the plant's acclimation to living outside again. Bring it in before the sun gets hot for the first few days. Gradually increase the outside time over the course of a couple of weeks, working up to eight hours of full sun daily.
Replant the mandevilla in its prior outdoor location after all danger of frost has passed.