Answer: The best time to transplant a deciduous tree in southeast Texas is late fall or early winter after the leaves are shed. Your recent transplant is going to have to deal with summer with no time to develop a root system.
The good news is that it is a mimosa, and just might make it! Keep it moist, but not too wet. Don't let grass and weeds compete with it. If it is still alive in July, you may be home free. Starting next winter, mulch it with compost or manure to encourage vigorous growth. Resist the urge to push it along with fertilizer this year as it just doesn't have a good root system yet. A little fertilizing is alright in a month or two, but keep it light.
If this one doesn't make it, bake a cake and go visit that neighbor again this fall. I'll bet they have plenty more mimosa seedlings!
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