I havent buried the graft at camp. Probably should have. Antonovka, dolgo, and B118 are about as cold hardy as they get. Not sure if graft failures are from the rootstock itself is not cold hardy enough.
I thought to get "seedling" trees from rootstock, you got to bury them almost a foot in the graft. Just an inch or two keep the graft union warmer.
Several have survived -35 deg F so far. PRobably got 25 on anty, 3 or 4 dolgo, 5 or 6 siberian crab, and only 1 kerr on B118 to try out. Everything is 3 years old or less, so hard to tell. Been weary of using M111 up at camp. PRefer m111 at home due to heavy clay as well as I dont want to be too high up in the air to well prune or spray the entire tree.