# Frost cracks and sunscald: why young bark splits in winter, and what a trunk wrap can and cannot do

> Why a young tree's bark splits or dies on its sunny side in winter, which trees are at risk, what a trunk wrap can and cannot prevent, and how cracks close.

Published 2026-09-25. Tags: Decay & damage, Protection & care. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/frost-cracks-and-sunscald/

Sunscald and frost cracks are two winter injuries of thin bark, and both follow a sharp drop in temperature. Sunscald kills a patch of living tissue under the bark on the south or southwest side of a young trunk. A frost crack splits a trunk lengthwise in a sudden deep cold, usually from an old wound. A white wrap guards against sunscald, and a trunk free of wounds is the best defense against frost cracks.

![Smooth gray maple bark split lengthwise by a pale, wrinkled seam, with rust-colored streaks running down](https://bluewhaletrees.com/img/articles/frost-cracks-and-sunscald-1-1000.jpg?v=397949ab "A frost crack in the bark of a sycamore maple at Stewarton, Scotland, on March 20, 2017. Maples are among the thin-barked trees most prone to winter cracks.")
*Photo: Rosser1954, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3AFrost_Crack%2C_Acer_pseudoplatanus_bark%2C_Stewarton%2C_Ayrshire%2C_Scotland.jpg))*

## Why young bark scalds and splits

The southwest side of a trunk takes the low winter sun in the warmest part of the day, and its bark can run 20 degrees or more above the air. The living cells under that bark wake from dormancy. When the sun sets or a cloud passes, the bark cools fast and freezes them. In time the bark there sinks, cracks or peels away.

In a sudden deep cold the outer wood shrinks faster than the inner, and the trunk splits lengthwise in a frost crack, sometimes with a sound like a gunshot. The split starts at a weak point: an old wound, a branch stub or a dead sprout at the base. In oaks, the likeliest start is a wound taken while the tree was small.

Trees newly planted, moved into sun or bared by pruning off low branches are at risk. So is every young tree, whatever the species, because its bark is thin:

> "Older trees are less subject to sunscald because the thicker bark that can insulate the tissue remains dormant and cold-hardy." (University of Minnesota Extension, *Protecting trees and shrubs in winter*)

The trees most at risk are thin-barked ones: maples, lindens, cherries and plums, crabapples, mountain ash, honeylocust, willows and young oaks. Blue Whale Trees sells several of these:

- [American basswood](https://bluewhaletrees.com/plants/american-basswood/index.html.md), a linden
- [American mountain ash](https://bluewhaletrees.com/plants/american-mountain-ash/index.html.md)
- [American crabapple](https://bluewhaletrees.com/plants/american-crabapple/index.html.md)
- [American plum](https://bluewhaletrees.com/plants/american-plum/index.html.md)
- The [rare and uncommon oaks](https://bluewhaletrees.com/shop/rare-oaks/index.html.md), while young

![Slender pawpaw trunk with a fresh vertical crack through its thin gray-brown bark, dry leaves around it](https://bluewhaletrees.com/img/articles/frost-cracks-and-sunscald-2-1000.jpg?v=32e81e9b "A frost crack in the thin bark of a pawpaw trunk on December 29, 2020. Thin bark insulates the living tissue under it poorly against a swing in temperature.")
*Photo: Cbarlow, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3APawpaw_frost_crack.jpg))*

## What a trunk wrap can and cannot do

A white wrap or guard reflects the sun and keeps the bark cool, as does white interior latex paint thinned half and half with water. Put a wrap on in late fall and take it off in spring once nights stay above freezing, for the first two or three winters after planting. The [care calendar](https://bluewhaletrees.com/articles/care-calendar-for-a-new-tree/index.html.md) has the dates.

A dark wrap absorbs heat. In University of Minnesota tests published in 1983, brown kraft paper wrap let wood warm and cool faster than bare wood, and reflective foil slowed the swings most. Left on into summer, a wrap keeps bark wet and can bind the trunk. Against frost cracks, Illinois Extension advises a wrap on young trees, but the Morton Arboretum does not recommend one.

[Leave the low branches on](https://bluewhaletrees.com/articles/training-a-young-tree/index.html.md) to shade a young trunk, and take them off later [at the branch collar](https://bluewhaletrees.com/articles/pruning-at-the-branch-collar/index.html.md). Water through a dry fall until the ground freezes, with [leaf litter](https://bluewhaletrees.com/articles/leaf-litter-not-wood-chips/index.html.md) or mulch over the roots.

## How a crack closes, and why it reopens

Leave a crack alone. Paint, tar and fillers only get in the way of [compartmentalization](https://bluewhaletrees.com/articles/glossary/index.html.md), the tree's own response to a wound. New wood grows in from the edges the next summer and seals the crack, though the wood inside stays split for life. A hard winter can open it again:

> "Nevertheless, more often than not, the callus tissue breaks open again in the following winter, until in a relatively mild winter the new wood may have sufficient strength to resist recracking." (Hans Kubler, *Origin of Frost Cracks in Stems of Trees*, 1987)

Each summer's extra growth along the seam builds a raised ridge, a frost rib, visible for years. An open crack lets in insects and decay and may shorten a tree's life, but [decay in a trunk is not a death sentence](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md).

## Common questions

### Should I wrap my tree trunk for winter?

Wrap a young, thin-barked tree in white from late fall to spring for its first two or three winters. An older tree with thick, rough bark rarely needs one.

### Why is my tree's bark peeling on one side?

Peeling patches on the southwest side of a young, smooth trunk are usually sunscald from the winter before. Trim the loose dead bark back to live tissue, leave it unpainted and wrap the trunk next winter.

### Can a tree survive a frost crack?

Usually: new wood seals the crack each summer, and a tree can live with one even if it reopens. Have an arborist check one that keeps reopening or runs into a hollow on a big tree near a house.

## Sources

- [Radial shakes and "frost cracks" in living oak trees](https://research.fs.usda.gov/treesearch/15026): USDA Forest Service, Research Paper NE-478 (Heinz Butin and Alex Shigo, 1981): oak frost cracks starting at wounds, branch stubs and dead basal sprouts, most often wounds taken while the tree was small, and cracks that close with callus and reopen years later.
- [Origin of Frost Cracks in Stems of Trees](https://auf.isa-arbor.com/content/13/4/93): International Society of Arboriculture, Journal of Arboriculture 13 (Hans Kubler, 1987): the outer wood cooling and shrinking first, old injuries as the weak point, the crack's closing, reopening and frost rib, and insects and decay entering it.
- [Materials for Potential Use in Sunscald Prevention](https://auf.isa-arbor.com/content/9/2/35): International Society of Arboriculture, Journal of Arboriculture 9 (Margaret Litzow and Harold Pellett, 1983): sunny-side bark far warmer than the air, kraft paper wrap changing temperature faster than bare wood, reflective foil slowing it most, wet trunks under wraps and bark marked by wraps left into June, and painted trunks staying cooler.
- [Protecting trees and shrubs in winter](https://extension.umn.edu/garden-and-home/yard-and-garden/lawns-and-landscapes-in-minnesota/protecting-trees-and-shrubs-in-winter): University of Minnesota Extension: sunscald killing active tissue, the trees at risk, white wraps rather than dark ones, how many winters to wrap, and trimming a sunscald wound without paint.
- [Sunscald Injury or Southwest Winter Injury on Deciduous Trees](https://extension.usu.edu/forestry/publications/utah-forest-facts/021-sunscald-injury-or-southwest-winter-injury-on-deciduous-trees): Utah State University Extension: why the southwest side warms most, oaks and lindens among the young trees at risk, the seasons and years to wrap, white interior latex paint thinned with water, and shading the trunk.
- [Exploding trees? A closer look at frost cracking](https://extension.illinois.edu/blogs/good-growing/2026-01-30-exploding-trees-closer-look-frost-cracking): Illinois Extension: the gunshot sound, thin-barked trees and young trees of every species, a white wrap against frost cracks, water before the ground freezes, and paint and tar getting in the way of compartmentalization.
- [Frost Cracks and Exploding Trees](https://yardandgarden.extension.iastate.edu/article/2026/01/frost-cracks-and-exploding-trees): Iowa State University Extension and Outreach (Jeff Iles, 2026): cracks at old wounds and stubs, cracks that close in summer and reopen, and trees that live with them.
- [Help Plants Handle Winter](https://mortonarb.org/blog/help-plants-handle-winter/): The Morton Arboretum: no wrapping against frost cracks, and water and mulch instead.
- [Winter injury to trees and shrubs](https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/winter-injury-to-trees-and-shrubs/): The Morton Arboretum: sun-side bark as much as 20 degrees above the air, the outer wood shrinking faster than the inner, and maples among the trees most prone to frost cracks.