I have never got a good intuition about threat of frost for these trees in my nursery. I have many seedlings in pots or air prune boxes, 6-12" soil depth w mulch on top. I am in zone 5 where we have already had 24 hours below freezing, and though it is warming up again, longer periods of frost are on their way. I am looking for information on what hazards cold and frost poses for these trees if any, under what conditions, and what to do about it.
Are baby trees at danger from frost if they are not being moved until after thaw? As in, can air prune boxes hang out through a cold spell or even winter, or should I surround/insulate them with mulch or hurry to heel everyone into ground? I know one way frost hurts trees is due to roots cracking and snapping if moved when frozen, but if these seedlings are not moved until above freezign temps, what is the risk?
Is it enough to cover pots in mulch, on top and sides, to insulate? Or do the trees need to be in the ground, heeled in? Below is an example of a somewhat 'insulated' stash of potted trees and stratifying seeds.
I realize it is a spectrum with uncertainty. When exactly soil freezes through including roots, versus just some frost on a pot exterior or the first inch of soil, is not precisely answerable. General insights would help me prioritize risk mitigation.
Based on my past Q&A (During a warm-spell in early winter, can dormant hardwoods survive planting? Even if frozen temps are a day away?) and experience since then, I see it is relatively safe to plant trees out in the cold as long as the soil is not frozen at root depth. It usually takes until January or February for the top inch to freeze fully, so I have months still to plant or heel in seedlings. What I'm unsure of is leaving seedlings in air prune boxes or heeled into pots.