You could put them in a cold frame if you do not want them to be in the house. Or you could dig out the bottom of the cold frame, fill it with cow/horse manure, layer it with soil, and then have a hotbed. If you get the tree acclimated then it will do just fine. It appears that Honey locusts are hardy to -40 so it seems that you don't need to be all too worried. But since it is young a cold frame may benefit the plant.
How trees survive the cold
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