wild forager:

What was your long-term objective when you planted semi-dwarf apple trees in containers? Do you eventually plan to set these trees in the ground?

Semi-dwarf apple trees on rootstocks such as M-7 will develop very extensive root systems that will quickly overwhelm the ability of a container to support them, even very large containers such as you have. Unless you regularly remove the trees to prune the roots, they will curl around and pack the container so full of roots that the health and vigor of the trees will be affected. If the trees doubled in size the first year, imagine what they will do the second year, and the year after that. Full dwarfing rootstocks such as M-27, or even M-26 or M-9 would have been better choices for long-term container cultivation, and even those can require some root-pruning maintenance.

I don't know what your particular 6b climate is like, but wrapping containers in burlap and plastic is not going to protect the soil in containers from freezing when night temperatures regularly fall below 20 degrees F or lower. Worse yet, the trees could be exposed to freeze/thaw cycles if daytime temperatures rise significantly.

If you don't have a shed or garage to wheel the containers into, I would say plant these trees in the ground or bury the containers.

Don Yellman, Great Falls, VA