Arborist Blog

Frost Cracks Explained

March 2, 2025

An ISA Board Certified Master Arborist explains frost cracks.

Here in the frozen North, we pride ourselves on our hardiness and ability to embrace the winter. Whether indoors or out, we have all developed adaptations that help us get through the cold and dark. Our native trees and shrubs are no different, having evolved over tens of thousands of years to survive in this sometimes-harsh environment. Unfortunately, the same can’t always be said for some of the non-native plants we humans have introduced to Minnesota.

Frost cracks explained.

If you were willing to brave the cold earlier this month and went for a walk in the woods, you may have heard what you thought was the crack of a rifle going off. More than likely you were in a place where shooting is illegal, which then begs the question: what was that sound? When frost cracks open on particularly cold days, they can do so with a bang. Even if you weren’t around to hear it, you may have noticed cracks opening on the trunks of trees. Some trees are more susceptible than others; frost cracks are most common on non-native linden species and certain maple cultivated varieties.

How do frost cracks form?

How frost cracks form is often misunderstood, so here’s a step-by-step:

The trunk is wounded. This could be from hail, animals, people, or sunscald. Anything that damages the cambium underneath the bark.
The cambium around the wound site develops callus tissue and woundwood, which grows from the wound margins to cover the wound. The faster the tree is able to seal the wound, the less likely it is for a crack to form later.
In cold weather the following winter, the outer sapwood freezes and shrinks faster than the inner sapwood and heartwood. This creates tension in the outer sapwood. Woundwood expands and contracts at different rates than normal sapwood, which can focus the tension at wound sites.
If enough tension is present, the outer sapwood layer will split, often quite loudly.
The following spring, as the wood warms and expands, the crack will begin to close. The tree will again attempt to seal over the wound with woundwood and may even be able to fully seal the crack.
This crack can still re-open anytime it gets cold enough. Whether this is the next winter or not for several years, the underlying wound is present. If it’s been several years since the crack last opened, it will require extremely cold conditions. This is what was present earlier this month and led to so many old cracks opening.

While frost cracks can be dramatic if you notice a “new” crack on your tree, it’s important not to panic. Although the cracks may have appeared suddenly, they’ve often been present for decades, opening when conditions are right. Trees have developed means of dealing with wounds, and the cracks are not necessarily a structural risk. There’s generally not anything we can do to address the crack directly, but steps to improve the tree’s overall vigor can help the tree help itself. If you are concerned about your tree, contact an ISA Certified Arborist or ISA Board Certified Master Arborist and have them assess the crack and the tree’s condition.