Frost cracks in tree trunks: what extension services describe

This page describes what the named extension services say. It does not diagnose a tree, say whether any cracked trunk is safe or unsafe, or tell anyone what to do with one. Where the sources disagree, both views are given.

What a frost crack is

South Dakota State University and the South Dakota Department of Agriculture (forest health update of December 2-9, 2015, John Ball) describe frost cracks as deep, longitudinal cracks on the lower trunk, almost always on the southwest side. They say that side can swing 30 to 50°F or more between a sunny winter day and a clear winter evening. They list the same species as for frost cankers, and add oak and walnut.

The University of Delaware Cooperative Extension fact sheet Winter Injury and Frost Cracking (Nancy Gregory, February 2026) shows a red maple with bark splitting. It says the south-facing side of the tree has more pronounced temperature changes than the rest, and that freezing and thawing of that bark by day and re-freezing at night leads to cracking. It names thin-barked trees such as maple and cherry as more prone.

Where the sources differ on the cause

Source	What it says starts the crack
South Dakota (2015)	A wound to the trunk, for example from a grass whip, a lawn mower or improper pruning, which leaves a structural weakness. Winter sun followed by a rapid evening temperature drop then ruptures the crack to the surface. It says the origin is not frost or cold as such.
Delaware (2026)	Temperature swings on the sunny side of the trunk, freezing and thawing of bark and wood.
Arkansas (2019)	Calls winter sunscald or frost cracking a result of rapid temperature fluctuations. Researchers suggest frozen tissue on the south or southwest side, warmed by sun, thaws and then refreezes rapidly. The sheet reports Minnesota research linking the tissue death to moisture stress.
Utah (2011)	Uses the name southwest winter injury for the day-night swing that damages cells inside the bark, and lists cracking among the results.

Whether the sources call it harmful

Delaware says that many times the tree can heal or compensate on the cracked side, and that the area should be watched for signs of insect damage or disease as time passes. Utah says sunscald injury may cause significant stress and gives pests and diseases a way into the damaged area, and that damaged areas may eventually heal. Arkansas says very large sunscald wounds can contribute to tree death and are a likely site for a future trunk break, and that insects in the wound fissure are not of great concern. This page does not weigh these against one another or against a particular tree.

What the sources list for prevention

Delaware lists not painting or covering the area, because the tree should begin to form callus around the split, avoiding late-fall fertilizing because new growth splits more readily, and keeping a tree watered in dry spells. Utah says damage should not be painted or filled with paint or tar. South Dakota names adequate water during the growing season and avoiding trunk wounds as the long-term means, and says the benefit of wrapping is uncertain because wrap may not prevent temperature swings and can trap moisture and pests if left on. Arkansas covers wrapping for summer sunscald with light-coloured materials.

Look-alikes the sources name

South Dakota separates frost cracks from frost cankers. A canker is dead, discolored bark that sloughs off over the cambium, while a crack is a deep split that starts from a wound. Arkansas notes that early sunscald can look like a canker with small vertical cracks. The sibling page on sunscald covers that side, and the page on weather, soil and chemical damage lists the others.