Here is the advice that Sonia Uyterhoeven, gave in a post to NYBG's Plant Talk blog in 2011, when she was the Gardener for Public Education.

"Mandevilla (Mandevilla splendens) is native to Brazil. It has glossy leaves and striking trumpet shaped flowers. It is generally grown as a vine but can also be pruned to maintain a shrub-like upright shape. Mandevilla (Mandevilla x amabilis) is a hybrid that grows to be a large vine reaching 8 to 10 feet tall. They are generally grown on trellis.

Both the species and the cultivated variety love light and good drainage. Wait until it starts to dry out before you water. Fertilize your mandevilla every other week with a liquid fertilizer when it is actively growing. Unlike many tropical plants, mandevilla does not like to be pot bound so give it room to grow.

Mandevilla are not hardy in our area, USDA Zone 6, so you have two options if you would like to over-winter your vine. The first option is to bring it into your home. If you have enough space and a bright sunny window then move the container inside once the weather starts to cool down.

It is always a good idea to prune it back before you transfer it into your home. Cut the vine back so that it is a comfortable size for your home–by half is fine–less or more will also work. It will grow slowly during the winter months. You do not need to encourage growth at this time of year by fertilizing the plant, just water it when it begins to dry out.

Sometime in February, give the plant another good pruning and begin to fertilize it once a month. Mandevilla flowers on new growth. By pruning and fertilizing the plant, you are working to give it an early start so that when you place it outside in May or June it will take off and soon be covered with flowers. If you are growing a mandevilla in your home, it is a good idea to lower your thermostat to somewhere in the 60s or low 70s, otherwise it may get too dry.

The second option for over-winter your mandevilla is to allow it go dormant. Keep the vine outside until it gets nice and cool and then move it into a cool garage or basement that maintains a winter temperature above freezing, around 50?F is ideal. In this scenario you should cut the plant back hard, to about 12 inches. Occasionally give it water so that it doesn’t dry out, but essentially leave it alone. Bring it inside when spring is just around the corner and let it start to grow. Then set it outside and enjoy this beautiful plant throughout the summer. Good luck!"

Courtesy of NYBG's Plant Information Service