Most people here either bury their trees, put them in a greenhouse, or layer with inches of thick mulch, such as bark dust, and put against a building to protect. I use coconut/coir fiber. I like it because it is clean and easy to sculpt around the pots like clay when moistened. In winter we don't want to water the dormant trees as much, and we have to protect the roots from the heavy rain and ice build-up. The mulch wicks up water from both the inside and outside, so the tree won't ever sit in a water puddle, or dry out if the mulch stays damp. Does that make sense? We get heavy rain and freezing winters.