Understanding and Preventing Frost Cracks in Your Trees

The Cold Truth: Understanding and Preventing Frost Cracks in Your Trees

As ISA-Certified Arborists, we know that winter on the Prairies can be harsh, and not just for us! Rapid temperature swings and deep freezes are a defining characteristic of our climate, and they pose a unique threat to your yard’s greatest assets: your trees. One of the most common signs of winter stress is the sight of frost cracks, long vertical splits that can appear suddenly on a tree’s trunk. While alarming, understanding why they happen is the first step in successful winter tree care.

What Are Frost Cracks?

Frost cracks are long vertical splits that appear on tree trunks during winter due to rapid temperature swings. Often heard as a sharp, sudden cracking sound on a cold night, these splits typically run down the side of the trunk and can sometimes be several feet long. They are a direct result of physics—specifically, the expansion and contraction of wood under extreme temperature differences.

The Science of the Split: How Frost Cracks Form

A frost crack is not just random damage; it’s a specific process caused by the difference in temperature between the inner and outer layers of the tree, especially when sunshine is involved.

1. Sun warms the southwest trunk face by day. During a clear winter day, the dark bark on the southwest side of the tree (where sun exposure is greatest) absorbs solar radiation and warms up significantly, causing the outer wood and bark to expand.
2. Night temperatures plunge, rapidly contracting bark. Once the sun sets, or a cold front moves in, the temperature can drop drastically. The rapidly warmed outer bark and wood quickly cool and contract.
3. Internal wood cools slowly, causing stress and splitting. The heartwood and internal layers of the tree remain relatively warm and stable. This difference in cooling rates—rapid contraction on the outside vs. slow, stable cooling on the inside—creates immense tension, ultimately leading to a sudden, audible split.

Trees Most at Risk

While any tree can be susceptible to frost cracking, certain species with thinner bark, faster growth rates, or previous injury are more prone to this winter injury.

• Linden
• Maple
• Fruit trees (such as apple and cherry)
• Poplar and Aspen

Young trees with smooth bark are also particularly vulnerable, as their protective layer is not as thick or developed as mature specimens.

Spotting the Warning Signs

Identifying a frost crack is usually straightforward, but sometimes the signs can be subtle. Keep an eye out for:

• Vertical splits: A long, straight crack running up and down the trunk.
• Peeling bark: The edges of the crack may show lifted or peeling bark.
• Cracks widening on warm days: Although caused by cold, you may notice the split being most pronounced during the day when the temperature gradient is at its peak.
• Dark staining: Older, healed cracks often have a ridge of callous wood but may also show dark staining where water has entered, indicating previous damage.

 Are Frost Cracks Dangerous?

The good news is that most trees are incredibly resilient. Frost cracks are often survivable, and a healthy tree will immediately begin forming a protective layer of callous wood to seal the wound.

However, the injury is not without risk. These cracks may reopen yearly at the same weak point, causing repeated damage. More importantly, an open crack creates a direct entry point for fungal spores, pests, and bacteria, which can allow decay to take hold in the heartwood or compromise stability over many years. A professional assessment is always recommended for large, deep, or repeated cracks.

 Arborist-Approved Prevention Strategies

Taking proactive steps in the late fall and early winter can significantly reduce the risk of frost cracking in your valuable trees.

• Mulch to regulate soil temps: A 3- to 4-inch layer of wood mulch over the root zone helps maintain consistent soil moisture and temperature, reducing overall tree stress.
• Avoid winter pruning on sensitive species: Pruning creates wounds that are slow to heal in cold weather, adding another vulnerability. Stick to dormant season pruning on appropriate species only.
• Plant away from southwest exposure: If possible, consider planting susceptible species where they will be naturally shaded from intense afternoon sun in the winter.
• Maintain overall tree health: A tree that is properly watered, fed, and free of summer stress is much better equipped to manage winter injury.

With some areas of Canada experiencing colder temperatures than typical or with large fluctuations in temperature, frost cracks or “exploding trees” has become a hot topic.  Simon Peacock from Green Drop Trees in Winnipeg commented within this CBC article.

By understanding the mechanics of a frost crack and implementing these simple preventative measures, you can ensure your trees are ready to face the deepest freeze and thaw cycles the season has to offer. To talk to a Green Drop Trees arborist about your specific issues, contact us.