Watering when the soil is dry is correct. Dry soil or overwatering (soggy soil) can both cause such problems. In a container it is much more prone to either soil moisture extreme. You can transfer it to the ground and should do so as soon as possible to give it time to settle in and grow more roots before summer arrives. Continue to water the are of the root ball and just beyond for the first summer season. When you take it out of the pot if the roots are circling a lot it may have trouble in the long run as circling roots may end up strangleing the tree over time as the roots and trunk grow larger.
Hope this helps. Good luck with your mimosa and stop in to the Monrovia website any time we can help with plant questions.
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