It should be understood that plant roots and aerial portions of those same plants vary in their resistance to cold damage. Apple tree roots for instance, can be more than 60 degrees (F) more susceptible to damage or death from cold than their above ground counterparts. The reason for the difference is still under study, but some interesting information is available on the subject.
A Cornell study in 1921 shed some light. Apple and pear trees were manipulated as to have their roots under different exposures to soil, oxygen, and light. By allowing the tree roots to be exposed (above the soil), a physiological transformation of the roots occurred. The excavated roots soon took on the appearance of trunk and other above ground parts. Roots that were uncovered many months before cold temperatures arrived showed no damage at temperatures of at least 3 degrees F. Roots uncovered directly before were killed at 13 degrees F. It was unclear to the researchers if roots could eventually attain as much resistance to cold as normal aerial parts (ie- trunk and branches).
The same aforementioned study showed that trunk sections can undergo a reverse type of transformation. Fameuse trees planted were planted deep enough to have about 10 inches of scion covered with clay soil. They grew in this condition for a full season, were dug and then exposed to 3 degrees F. All tissue was killed below where the soil line existed. Control trees of same cultivar and grown with all scion tissue above soil showed no damage at the same temperatures. This shows a regression of cold hardiness resistance of the same type of tissue, when covered with soil for a certain period. There is evidence also that the more (ie- deeper) the material is buried, the less cold resistant it becomes.
The type of soil does apparently have bearing. In a 1922 study, Jonathan apples planted a foot deep in sandy soil and later excavated showed no damage to the parts in question at 2 degrees F. In the 1921 study the soil type was much heavier (higher density clay material). A similar test showed no damage on parts as low as -15 degrees F.
Older roots generally are more resistant to cold than finer, younger specimens.
This can be of practical importance in that growers can prevent any root excavation in their practices when said practice is approaching cold weather. This can happen during cultivation, raking, or mulch redistribution. It also means that older roots that have been exposed to the surface for a long period are likely not to be easily damaged by cold temperatures.