From the ground up 20 inches the trunk developed a half inch split. How can I save my mimosa?
It,developed during this season of heavy rains.
It is difficult to offer suggestions without a photo and more information. Some splitting of tree bark occurs from frost cracks or drying, some are structural defects that allow splitting apart of two co-dominant stems. Some are so severe that they cannot be saved, some are repairable.
Assuming that you have a mimosa tree with co-dominant stems that have started to split apart under over-burdened weight of foliage and rainwater load: you may be able to prevent further splitting by pruning both sides to reduce weight and wind resistance. In combination with pruning for weight reduction, you can install a cable support between the two sides. If it's a large tree this may be best done by a tree service.
Sometimes we bolt the two split sides together through the split, again it should be done in combination with pruning and cabling. There is no way to weld the two split surfaces together, just holding them from further splitting is the best you can hope for.