You step outside the morning after a cold snap in January and stop cold. A vertical crack runs down the trunk of your maple or cherry, splitting the bark from somewhere high up and descending several feet. The exposed wood is pale and raw. Your stomach drops. It looks catastrophic. In most cases, it’s not.
What you’re witnessing is a frost crack, also called radial checking or frost checking. It’s a mechanical stress injury caused by rapid temperature change, not disease or pest damage. Understanding what’s happening and what to do next will help you decide whether your tree needs intervention or simply time.
The Physics Behind the Split
Here’s what actually happened inside your tree: the air temperature plummeted 20, 30, or 40 degrees in a matter of hours. The outer bark contracted rapidly. The inner wood, insulated by that bark, contracted much more slowly. The differential stress between the cold outside and the warmer inside created tremendous tension, and something had to give. The result is a vertical split, usually running the full length of the trunk or a major branch.
The crack almost always appears on the south or southwest side of the tree. Why? That side experiences the most solar warming during the day in winter, which deacclimated the bark before the cold snap hit. When the temperature dropped suddenly, the stress was greatest on that sun-exposed side.
This is thermal shock, pure and simple. No fungus is involved at the moment of cracking. No insect caused this. The tree’s own wood failed under mechanical strain.
Which Trees Are Vulnerable
Frost cracks affect thin-barked species almost exclusively. The bark acts as insulation. When it’s thin, that insulation is poor, and the temperature gradient inside the wood becomes steep and dangerous.
In Western Washington, the species most prone to frost cracks include:
- Maples: Japanese maple, red maple, and sugar maple are particularly susceptible, especially young trees under 15 years old.
- Cherries: Both ornamental and fruit-bearing varieties crack readily.
- Other thin-barked trees: American beech, linden, and London plane are common victims in our region.
Thick-barked species are largely immune. Douglas-fir, western redcedar, oak, and ash rarely develop frost cracks, even in severe cold. The bark does its job.
What to Do: Often Nothing
Here is the hardest advice to follow: do nothing.
The tree will repair this crack itself. Within weeks, callus tissue (specialized healing wood) will begin rolling over the crack margins from both sides. This callus growth will gradually cover the exposed wood. Over years, the crack will be largely hidden by this rolled callus, though you may still see it if you look closely. The tree is compartmentalizing the injury and moving on.
Do not fill the crack with concrete, foam, wound dressing, or wood putty. These old practices trap moisture inside the crack. Trapped moisture creates ideal conditions for decay fungi to establish themselves. You will cause more harm than the frost crack ever did. The tree heals best when you let air and light reach the exposed wood and allow callus to form freely.
Do not prune branches near the crack or remove the tree unless it has failed structurally. Pruning creates additional wounds and stress.
Leave it alone.
The Frost Rib Phenomenon
Some trees, particularly maples and cherries in exposed sites, crack again the following winter or the winter after that. This is called a frost rib. Each time the crack reopens and heals, callus tissue builds up in a ridge beside the previous crack. Over many years, the trunk develops a washboard texture with ridges of callus tissue.
A frost rib is not a sign of failure. It is evidence that your tree has been managing this stress through repeated healing. Thousands of frost-ribbed trees in Western Washington are structurally sound and will live for decades more.
When You Should Worry
Call a certified arborist if:
- The crack extends completely through the trunk wall and you can see light through it, suggesting possible structural failure.
- The tree has developed a pronounced lean since the crack appeared, indicating the trunk may be compromised.
- Decay fungi have colonized the crack margins. Look for conks (shelf fungi), mushrooms, or staining that suggests rot is actively progressing.
A certified arborist can assess whether the tree is a hazard and whether removal or structural support is warranted.
Prevention for Young Trees
If you have a young maple, cherry, or other thin-barked tree in an exposed location, you can reduce frost crack risk by wrapping the trunk with tree wrap from November through March during the tree’s first 2 to 3 winters. The wrap insulates the bark and prevents the steepest temperature gradients from forming.
Once the bark thickens as the tree matures, wrapping becomes less critical.
Also consider site and species selection. If you live in a wind-exposed location or a frost pocket where cold air drains and pools, choose thicker-barked species like oak, Douglas-fir, or ash. Reserve thin-barked maples and cherries for sheltered sites.
Why Western Washington Matters
The Midwest and Northeast often experience sustained deep freezes in December and January, with weeks of subzero temperatures. Those conditions do cause frost cracks, but they’re almost predictable.
Western Washington is different. Our winters are mild and wet. We rarely see sustained freezes. Instead, we get sudden cold snaps where the temperature drops from 45°F to 20°F or below in 24 hours, often after a warm spell that brought trees out of dormancy. The 2024 January freeze is a textbook example. That kind of rapid thermal shock is precisely what triggers frost cracks in our climate.
It’s not the absolute cold that breaks trees here. It’s the speed of the change.
Sources
- Shigo, Alex L. “Modern Arboriculture.” Shigo and Trees, Associates, 1991.
- Burns, Russell M., and Barbara H. Honkala, eds. “Silvics of North America: Vol. 1. Conifers.” U.S. Department of Agriculture, Forest Service Agricultural Handbook 654, 1990.
- International Society of Arboriculture. “Best Management Practices: Tree and Shrub Pruning.” ISA, 2010.
- University of New Hampshire Cooperative Extension. “Frost Crack and Winter Damage.” UNHCE, 2018.