I have one Mandevilla and two Dipladenia. I have over wintered them in my garage (worst place), a back bedroom (best place) and the basement. The plants are 2 - 4 years old.
I cut them back by about 1/2. All three get to be and are now (not in yet) 3 - 4 feet and all three are in full bloom.
The biggest problem I have is keeping the moisture up around them.
Last winter, I almost lost the Mandevilla in the garage - it died back to almost top of pot.
Thankfully - it did recover.
The basement actually works best, but I am limited for space there cause I have so much else there - brugmansias, oleander - tons of big things. But I can control the humidity there easily with a humidifier.
Good luck Vicky.
