Overwintering Brugmansia in Bubble Buckets

I do not have a greenhouse here in Missouri, so in order to overwinter my brugmansia, I use 5 gallon buckets of water with an aquarium bubbler in the water. The brugs are cut off just above the ground and most leaves removed, leaving just a trunk and a few leaves at the top. I have also added a few drops of Superthrive to this bucket. The buckets are kept inside the garage for the winter. It is an unheated garage, but the temp stays above freezing at all times in the garage.