Answer: You may be able to train the rose of sharon to a tree shape, although with several trunks. It does this to certain degree naturally with age if left unpruned. As it grows you would remove some of the lower branches, little by little, beginning at the base and working up. You can also cut it back very short every spring, or cut it back to the main trunk if needed, to control the height. However, I am not sure you will be able to keep it really small -- it is normally a very large shrub. The time to prune rose of sharon is now, before it begins to grow for the year. It blooms on new wood, so do not prune it during the summer as you will be removing the flowering wood.
Butterfly bushes really are bushes, very dense and twiggy so I do not think you would be able to train it like a tree. If you have a dwarf variety it will stay a bit smaller naturally. This plant is also pruned as hard as desired in early spring. It too blooms on new growth of the season, so you should not prune during the growing season.
Good luck with your project!
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