A few decades ago I planted several hundred Poncirus seedlings in open, cultivated soil that was tilled during the summer. 100 % of these froze out in the 1st winter. I attributed the mortality to freeze/thaw cycling. I've grown many Poncirus seedlings since and haven't lost many, if any. My thinking is that naturally consolidating soil with a mulch cover, even if it's only black poly film helps to protect against the soil freezing and thawing damage. there may have been other factors at play, but in late April the bark had freeze damage to the point of sloughing off of the lower stems. Additionally, the trees were no longer anchored in the newly thawed soil, easily pulling out by hand. It appears the combination of repeated honeycombing, (freezing followed by thawing soil) was lethal to the seedlings.