Answer: It isn't unusual for newly transplanted trees and shrubs to go through a little transplant shock, until the roots become established and they adapt to their new homes. I suspect that's why the foliage turned yellow on your tree.
It's difficult to recommend how often to water your tree because soil moisture depends upon soil type and, of course, the weather. A good rule of thumb for your tree is to build a watering basin beneath it and once a week flood the basin with water, allow it to drain, then flood it a second time. This concentrates the water over the roots and allows it to trickle down, wetting the entire root mass.
Here's a simple way to check soil moisture: water your tree as usual and wait 3-4 days, then dig a hole down near the roots. If the soil is still moist 3 inches beneath the surface, you won't need to water for a few days; if it's dry, it's time to water again.
Best wishes with your mimosa!
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