Answer: How sad! Unfortunately, the news isn't good. Such rapid dieoff is probably due to mimosa wilt disease, caused by a soil-borne fungus (Fusarium oxysporum var. perniciosum). It can kill a mature tree in a single season. Unfortunately, there's no cure for this problem -- even systemic fungicides don't overcome the disease. You might try growing it in a large pot instead. Sink the pot into the ground for the winter to insulate the roots against the cold. Then pull it out again in early spring to keep the fungus from invading the pot. Just don't put the pot in the soil where the current mimosas are growing.
Another tree that might take the place of wilt-susceptible mimosa is honey locust. It also has delicate leaves and grows. Some varieties branch into very interesting shapes. I hope this helps!
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