We took our own mini artificial tree when we sailed the magic a few Christmas' ago. it was 2ft maximum and sat ontop Of the steamer trunk just fine....I dressed it with battery operated LED lights and small Disney baubles, it was cute. I didn't know the ship sold trees, but I would have been disappointed to have bought and then not been able to bring it off the ship.
Check the gov't websites first. Depending where you are coming from fruits, plants, food, etc. may not be able to come in. I know people who have had flowers in their cabin were taken away the last night by the room stewards.
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