I have overwintered my dipladenia/ mandevilla for 4 consecutive winters (zone 3) The first year I kept it in my attached insulated garage which stays about 50 F; it went dormant and lost all its leaves. In the spring when it started to leaf out I began watering slowly until the days got warmer and increased the watering to support more new foliage. Once the weather was warm and reliable it went outdoors for the summer. It bloomed but quite late in the season. After that I found a place right in front of a south facing patio door and it bloomed until about December and sporadically over the winter.
Since then I have brought it indoors to that same sunny window and it has done very well. It still didn't bloom for me untill quite late in the season but I didn't cut it back as
@pirl suggested so I think I will try that this spring to try to encourage earlier bloom.
I'm assuming Minneapolis is about zone 5 or 6? It will not survive remaining outdoors.
Hope this helps! They are beautiful plants and well worth saving.
