A frost crack is a crack in the bark of a tree that appears during the winter, especially on a fairly young tree growing in a sunny location. The theory used to be that it was caused when a sudden drop in temperature caught the tree unawares, when it was not yet prepared for extreme cold. The cells of the sapwood, full of sap as the name suggests, then expand due to the cold and the bark bursts… sometimes with loud report like a rifle shot!
However, further studies, notably by American researcher Dr. Alex Shigo, have shown that the situation is more complicated than that. Although cold may trigger the most visible symptom, fissured bark, the frost crack actually results from internal injuries, generally caused by damage during transplantation or by mower hits. Indeed, when a young tree is injured, rot (caused by fungi) can set into the damaged parts, then spread up the trunk, leaving a trail of rotten wood net inside that is still invisible from the outside. The new theory, then, is that winter injury occurs when the rotten wood inside the trunk, which is full of moisture in its early stages, expands more rapidly under extreme cold, putting pressure on the bark that covers it, forcing the bark to suddenly split.
For this reason, the generally recommended treatment for preventing frost cracks – wrapping the trunk with a white or pale spiral or cloth in the winter – is not likely to be very effective. Better to choose a healthy tree and plant very carefully so as not to damage its roots or trunk. And mulch all around the trunk so there will be no need to mow at its base: damage to bark by mowers and trimmers is a major cause of fungal infections in trees. See Garden Tools Can Be a Tree’s Worst Enemy!
You can’t really treat a frost crack: once it’s there, it’s there for a long time. Don’t try to sterilize the wound or apply pruning paint or paste: these actions tend to stimulate increased development of harmful fungi!
Fortunately most frost cracks close relatively well and the trees that bear them go on to live long and healthy lives. But the wound – in the form of a straight line from bottom of the trunk to the mid-section or even higher – is often still visible even decades later. And much later, when the tree is fully mature, you may well discover it is hollow, not that a hollow trunk is necessarily a problem. (See Hollow Tree? No Need to Panic!) However, if the tree was already weak or in poor condition before the crack even started, it will likely grow weakly and unevenly, never really thriving. It’s better to remove such poor specimens while they are still young.