When the trunk can't adjust to this differential fast enough, it cracks. The cracks are most frequent near the lower part of the trunk where growth has been uneven or may extend up the entire length of the trunk and follow one or more of the main branches, or they may be quite short. The south side of the trunk where circumferential growth had been rapid, seems the most susceptible. The cracks usually draw together as the weather warms the next day and will heal when the cambium becomes active in spring. However, this spot is always weaker than the rest of the tree and is subject to future cracking.
Young trees seem particularly prone to frost cracks but they can occur on trees of any age.
There is nothing you can do about it but wait. When the weather warms, the crack will close, often within several hours after it splits. In fact, many cracks may go unnoticed. Extreme cold usually does not kill lawngrass, but it can if there are some contributing factors. Young plants are more sensitive to cold than mature grass plants.