Adam_MA

Only 6 inches deep with the cold frame? I think in zone 5 you would want to go much deeper than that. Last fall I dug a cold frame under my deck. It butted against 2 of my foundation walls so it got some heat from that but I still went down about 36" then filled in with 4" of gravel for drainage. I think for a cold frame to be most effective you should go down to the frost line. I would at very least go down to 24" and then add a couple inches of gravel. Also once you get snow and the temp is staying around freezing you can fill your cold frame with the snow all the way up to (and possibly around) the branches of the tree. This will keep the temp more even and prevent the freeze/thaw cycle that does damage.

More importantly than anything though you need to find out exactly what species and variety of maple this tree is and make sure it is hardy to your zone.

I hope this helps a bit

Adam