Goodness, a Mimosa should be fine with almost anything you do to it. It could be the roots got too cold in the pot over winter, and that's what killed it.
It's this tree, right?
You can put them in the ground anytime - they are actually hard to kill (trust me I know, I've hacked at one for almost two years now trying to kill it - it wants to be IN my foundation) They are an invasive species in some parts of the country (like here!).