Frost Cracks and Winter Sun on Thin-Barked Trees

It surprises many homeowners that some of the worst damage a tree suffers in the Chicago area happens not in the heat of summer but on a clear, cold winter day. The culprit is the sun, which on a bright winter afternoon warms the bark on the south and southwest side of a tree, only for the temperature to plunge again when the sun sets or a cloud passes. That rapid warming and cooling stresses the bark and the wood just beneath it, and on young or thin-barked trees it can cause the bark to split, an injury known as a frost crack, or to die in patches, known as sunscald. Understanding this winter phenomenon is key to protecting vulnerable trees through our cold months.

Frost cracks form from the stress of temperature swings within the trunk. When the sun heats one side of the trunk on a cold day, that side expands while the frozen interior does not, and the resulting stress can cause a sudden vertical split in the bark and wood, sometimes with an audible crack. These cracks typically appear on the south or southwest side of the trunk, the side that gets the most winter sun, and once a tree has cracked in a spot, it often reopens in the same place in following winters, creating a persistent wound that can invite decay and insects over time.

Sunscald is the related injury where the winter sun damages the bark tissue itself. The warmth of the low winter sun can bring the cells in the bark out of dormancy on a mild afternoon, only for them to be killed when the temperature drops sharply afterward. The result is dead, sunken, or cracked patches of bark, again concentrated on the south and southwest side of the trunk. Like frost cracks, sunscald creates a wound that compromises the tree's protective bark layer and can become an entry point for problems, and on a young tree it can girdle and seriously harm the trunk.

The trees most at risk share a common trait: thin, smooth bark that offers little insulation. Young, newly planted trees are especially vulnerable because their bark is thin and their trunks are fully exposed, and certain species with naturally smooth, thin bark remain susceptible even as they mature. Trees that were recently pruned to expose a previously shaded trunk, or that stand alone in an open, sunny spot with no shade on their south side, are also at higher risk. Knowing whether a tree fits this profile tells you whether it needs winter protection.

Prevention is straightforward and effective. The classic protection is to wrap the trunks of vulnerable young or thin-barked trees with a light-colored tree wrap for the winter, which shades the bark and moderates the temperature swings, then to remove it in spring so it does not girdle the growing trunk or trap moisture. Keeping young trees healthy and well established, and being thoughtful about not suddenly exposing a long-shaded trunk to full winter sun, also reduces the risk. The wrap is a small, seasonal step that prevents a wound that can otherwise trouble a tree for years.

For homeowners in the Chicago suburbs, frost cracks and sunscald are a real winter risk to young and thin-barked trees, and one that is largely preventable with a bit of foresight. If a tree on your property has a vertical split or dead bark patches on its south-facing side, winter sun is the likely cause, and it will tend to recur without protection. Wrapping vulnerable trunks through the coldest months, keeping young trees healthy, and getting advice on any tree showing repeated winter injury protects the bark that protects the tree. It is a simple safeguard against a damage most homeowners never see coming.