Answer: It is not really normal,no. A healthy mimosa does not die back at all during the winter and continues to grow larger each summer. It does leaf out a bit late compared to other deciduous plants, but not in July! When this tree suffers cold damage during the winter it will die back sometimes all the way to the ground. Then it regrows from the roots if it can gather the energy to do so. It sounds like this is what is happening to yours. Since it has done it for four years now I suspect the planting site is just too cold for it. I'm sorry.
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