I thought someone would give you an answer before now, but here goes. Up until last year, I always left the largest trees in their pots, pulled them up under the eaves of the house or into the garage. They loose all their leaves, receive no water and they do fine. Last year for the first time, we placed 6 of the very large trees straight into the ground (no pot) and dug them up for storage in November. We left most of the dirt around the root ball and sat them on cardboard or plastic in the garage. Same thing, no water, very little light and come spring they all began to grow again.

I believe your large trees will do fine sitting in cardboard boxes, it keeps the roots off of the cold ground/garage floor.

Hope this helps.

Karen
in Houston!