I've gotten into the habit of wintering my trees rough, meaning the majority get no temperature protection, at all. In the process, I've learned which species are truly zone 5 hardy, and I have learned even quality pots, like signed Bigei pots (read more than $150 each) are not 100% winter hardy if the shape doesn't let the root ball rise with the expanding soil as it freezes. I lost a nice Bigei bag pot due to freezing. It had a freehand signature. Not just a chop. Bigei is a master of pottery, his pots are fully vitrified. It don't mater, if the shape doesn't let the soil mass rise, the pot will crack.
Best pots the walls slope outwards as they go up. This way, the soil mass just floats up as it freezes.
All trees with good pots go into my frost free well house.
Everything in plastic and properly outward sloping walled pots, they stay on the ground outside for winter.