Winter kill happens when the weather warms up too fast during the winter before spring. In sunny areas, this fools the tree into thinking it’s spring and sap travels up into the trunk as the sun warms the roots and trunk. This watery spring time sap flash freezes after the sun goes back down or the weather drops below freezing again. This causes vertical fissures or cracks under the bark in the sap tissue, invisible at first. Once the tree tries to grow in the spring these cracks start to show on the outside of the tree. Simple damage is the crack itself, severe damage is loss of tops or limbs above the damage as the scarring slows or stops sap flow to that region. Common victims are balsam firs, younger maples, and fruit trees. A good way to prevent this is keeping sun off of the trunks of trees from ground level up to about 4 feet up with dark fabric held away from trunks with fence posts. Trunk stays cool and shaded.

This is different than wind/winter burn on conifers.