Answer: Prune in the early spring taking the previous year's growth back to five or six buds. If you do this on an annual basis you can keep the tree smaller than its usual mature size. If you wait until it is oversized and then try to reduce the height and width you can damage the tree and it will be short lived. Mimosa's don't like to be trimmed so if you do it in the early spring, just as the buds begin to swell on the branches, it won't be as harmful to your tree.
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