Q: I have a five-year old mandevilla growing in a large container. The first year I got the plant I heard somewhere to not cut it back but to simply put it into a protected place for the winter. I keep mine in an enclosed garage. It has new three-foot long shoots on it, all at the top. Should I prune the plant back?
A: Mandevilla is not difficult to keep alive from year to year if you have a place to keep it above freezing. Vines can be pruned away from their trellis in October and the root system dropped into a large pot.
I prune mine to 24″ before the first frost and put it in a moderately heated sunroom for winter. It loses 50% of its leaves but does fine when I bring it back out in late April.
Your garage was warm enough to support new growth. I think it will look odd to have the sprouts coming so high on the vine, so prune the main vine back to ten inches high before you plant it when the ground is warm.