I have this beautiful Mandevilla in a pot on a sunny spot on my front porch. Just looking at it makes me think of summertime. I am trying to decide if it's worth my time to bring this plant in for the winter or just replace it next year. I found this info at Plant-Care.com, so I thought I would share it with you.
As the nights begin to cool off the Mandevilla will slow down. That is the time to give them some food to harden them up for the long winter.
Try to use a liquid fertilizer with a high middle number e.g. 10-56-14, (Lightly) since you can cause fertilizer burn. We are not trying to promote growth but to toughen the plant up. Don’t go over board with the fertilizer; let the plant stay outside as long as possible. It should be able to handle high 40’s for brief periods.
After about 3 weeks (if you can hold out that long weather permitting) prune the plant back. Not just tipping the plant but cutting it down to about 12 inches above the soil line. I would also treat this vining plant for any possible pests problems since it will be going indoors for the winter.
Next we want to begin slowing up on the water and let the plant run on the dry side. Remember that we have given the plant some fertilizer and removed a lot of growth so the requirements for water should drop.
When you bring the plant inside place it in as much light as possible. You are looking for maintenance not growth. You’ll probably get some quick new growth, just try to maintain it. Some people place the plant in a clear plastic bag and over wintered their Mandevilla in their heated garage.
Try to maintain the plant on the dry side throughout the winter.
When spring comes you can move the plant outside. Expect the growth produced during the winter to be burned off!!!
Although, we cannot guarantee the above method will work you, it is a guideline that has worked for others you can try.