I live in Michigan, too. Way up in the Upper Peninsula. My butterfly bushes ("Pink Delight' & "White Profusion') die to the ground every winter. Some winters I prune it back before the worst of winter, others I have left the 'wood' standing. Either way, I cut it back to about 12 inches before the leaves sprout in
spring. I get about 5.5-6 feet of growth in one season. If you want to obtain a particular shape, I would think that you can trim the branches early in the season. (They
flower on new wood.) I have propagated mine by taking cuttings. They seem to root very easily. I have even just stuck a broken branch into the ground and it started growth. I deadhead a lot, as it keeps the
flowers coming longer. Good luck!
Robin