Answer: These trees may or may not be hardy for you; if you obtained them from a local garden, they should have better cold hardiness than trees that originated in a warmer climate. They are hardy into USDA winter hardiness zone 6, though some survive in zone 5 in sheltered spots. If possible, plant them in a spot with protection from winter winds. They require full sun and well drained soil. Mulch generously in late fall after the ground has frozen.
Sometimes in northern areas these trees are frozen back to the ground in a hard winter, but will regrow and bloom the following summer. Be very patient if this happens before deciding they are dead, it can take into late June before they show signs of life.
Normally these trees come into growth and leaf out very late in the spring, once the soil has warmed. Wait to plant them outside until the weather has settled and the soil has warmed up. This would be several weeks after your last frost.
Since they have been indoors and are very tender as a result, you will need to acclimate them to being outside to avoid shocking them. The hardening off process will take a week or two. Set them outside in a sheltered location and expose them to gradually longer periods of direct sun. Begin with a little early morning sun and work up to a half day or more of sun prior to planting them in the ground.
Good luck with your mimosa trees!
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