Hope this helps:
"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.
MandevillaIt 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."
