I’ve been keeping my temperate tree species outside, packed in close together, mulched in, on the north side of a building since 2017, with few losses. I try to wait until after we’ve had a couple of hard freezes before taking the trees off the benches and putting them on the ground (to discourage rodents from factoring the location of my trees into their decisions about where to set up their winter quarters). In recent years I’ve taken to wrapping the trunks of my deciduous trees with copper mesh to prevent rodents (mainly the bunnies) from chewing on them. I don’t bother doing that with the evergreens, as the rabbits have thus far left them alone. After I put the trees on the ground and add the mulch, I water everything thoroughly. I think getting the mulch thoroughly wet helps discourage mice from hanging out in the storage area, presumably because they’d rather not be cold and wet. I’m in 6a, according to the new USDA maps (was 5b before).
FYI, you can find a wealth of information about overwintering (and other topics) by using the site’s search function, as this has been much discussed