Potted apple trees over winter

I have about 15 potted apple trees that I grafted last spring. I am wondering if I need to do anything to protect them over winter.
I am in zone 6a

I don’t know if it will help reassure you or not, but I have around 100 from last spring, plus a sizable number from the previous year sitting in pots. Outdoors. Under trees in part shade. I lost a few to drought and a few (not too many) to too much rain (root rot) during the months since grafting…and if I lose any this winter I also expect it to be from excess moisture. Cold weather itself has not killed any in the past couple years, and before that I only lost some on M111 roots from late freezing after they had put out little leaves and begun to grow.

B9, B118, G890 account for most but I have experimented grafting to a half dozen other rootstocks.

If we should get one of them 30 below zero winters that I’ve seen 4 or 5 times in my 60-something years…then it might not be a good report in spring. But normal zone 6 they can handle if they are dormant. I’d expect most to be fine even in zone 4 conditions…especially if there was some snow covering them a little at the time of the cold period. I’d also add, local garden centers do not put apple trees inside in hoop houses but rather leave them outside. Some may or may not put a little straw on the top of the pots. I don’t, but some have a few leaves nature provided.

We don’t seem to get lows during the winter we used to but in S.NY (Z6) nurseries often surround potted trees with wood chips to make sure tender roots are not killed. Bubble wrap around the pots probably also helps, but wood chips store water that is very temp. stabilizing. Snow is a great insulator but sometimes extreme cold comes without it.

I have fruit trees in pots in my nursery that I keep half buried in the soil which also works well if you don’t mind roots growing out of pots- which I don’t. .