Sunscald and Frost Cracks: Winter Trunk Injury on Young Trees

By Margaret Ellison, Tree & Shrub Health. Last updated: July 22, 2026

Every spring, Rochester homeowners walk out to a young maple or backyard cherry and find a patch of sunken, papery bark or a clean vertical crack running up the trunk. It almost always faces the same way: toward the afternoon sun on the south or southwest side. That direction is the tell. This is winter injury, not a disease or a pest, and it traces to how Upstate New York's bright, cold winter days treat thin bark. The good news is that the fix is cheap and seasonal, and knowing the cause points straight to it.

Why does the southwest side of a young tree trunk crack in winter?

Winter sun heats the dormant bark on the south and southwest sides of a young trunk, coaxing that tissue out of dormancy. When the sun drops behind the horizon and the temperature crashes, the newly active cells freeze and die. Kansas State Research and Extension notes the southwest side of a trunk can run 40 degrees warmer than shaded bark, which drives that damaging swing.

In Rochester the setup is common from December through March: clear, sunny afternoons that warm dark bark, followed by a hard overnight freeze. Low winter sun sits directly on the south and southwest faces of a trunk, and snow cover reflects even more light onto the bark. Older trees with thick, corky, fissured bark insulate the living tissue underneath. Young and smooth-barked trees do not, which is why the injury clusters on recently planted specimens and always on the sunniest side.

What is the difference between sunscald and frost cracks?

Sunscald and frost cracks share the same freeze-thaw cause but look different. Sunscald appears as sunken, dried, or discolored bark, often an elongated dead strip or canker on the sunny side. A frost crack is a long vertical split in the wood itself. According to The Morton Arboretum, a frost crack forms when a sun-warmed trunk expands and then, on a sudden temperature drop, the outer wood contracts faster than the inner wood until it splits.

The two often show up on the same tree and even in the same spot, which is why they get confused. Both concentrate on the south and southwest side, and both favor young, thin-barked trunks. The practical distinction is depth: sunscald kills a patch of bark and the cambium beneath it, while a frost crack opens the wood and can reopen in later winters along the same line. For a broader look at every cause of split bark, our guide on bark splitting and cracking on Rochester trees covers the full list.

Which Rochester trees are most at risk?

Young, recently planted, and thin-barked trees carry the most risk in Monroe County. Maples, cherries and other fruit trees, honeylocust, and lindens top the list because their smooth, thin bark heats fast and holds little insulation. The Morton Arboretum names maples, crabapples, fruit trees, and lindens among the species most prone to frost cracks, along with plane trees, poplars, birches, and willows.

Two other situations raise the odds sharply. A tree moved from a shaded nursery row into an open, sunny yard has bark that was never conditioned to direct winter sun. And any trunk newly exposed after a neighboring tree comes down or a limb is removed suddenly takes full afternoon light on bark that used to be shaded. Choosing species and sites suited to our climate helps from the start, which is part of why our list of the best trees to plant for Rochester's Zone 5b to 6a winters weighs winter hardiness so heavily.

How do you prevent winter trunk injury on young trees?

Wrap the trunk of a young or thin-barked tree with a light-colored tree wrap or a white plastic guard each fall, running from the ground up to the first branches, then remove it every spring. University of Minnesota Extension recommends white because it reflects sun and keeps the bark temperature constant, and it warns against dark wrap, which absorbs heat and makes the problem worse.

Timing matters. K-State Research and Extension advises putting light-colored wrap on in November and taking it off the following March, and leaving wrap on year-round is a mistake because it traps moisture and can harbor insects. University of Minnesota Extension suggests newly planted trees be wrapped for two winters or longer, and thin-barked species for five winters or longer, until the bark thickens enough to protect itself. Beyond wrapping, three habits reduce risk: site young trees where they are not baking against a south-facing wall or reflective pavement, keep them watered through fall until the ground freezes, and skip late-season high-nitrogen feeding that pushes tender growth which will not harden off before winter. Steady, season-long vigor is the real defense, which is what a plant health care program is built to maintain.

What should you do if the bark is already damaged?

Leave the wound open and do not paint or seal it, since dressings trap moisture and slow the tree's own compartmentalization. Keep the tree vigorous so it can wall off the damage: water it during dry spells and maintain proper mulch over the roots, the same recovery steps The Morton Arboretum recommends. Watch the wound margins over the next few seasons for loose bark, decay, or wood-boring insects moving into the opening.

Small sunscald patches on an otherwise healthy young tree often callus over on their own. A large wound, one that wraps more than a third of the way around the trunk, or a frost crack that reopens each winter is worth a professional look, because it can become an entry point for decay fungi and borers that a certified arborist can catch early. Mound mulch as a flat ring, never piled against the bark, since a mulch volcano can itself rot the trunk base and undo the recovery you are trying to support.

FAQ

Will a sunscald wound or frost crack heal on its own?

Often, yes, if the tree is otherwise healthy and the wound is small. Trees do not heal in the human sense but seal damage off by growing callus tissue over the edges. A large wound, or a frost crack that reopens each winter along the same line, may never fully close and should be monitored for decay and insects.

Do older, established trees get sunscald and frost cracks?

Much less often. Mature trees develop thick, corky, fissured bark that insulates the living tissue underneath from rapid temperature swings. Sunscald and frost cracks overwhelmingly affect young, recently planted, and thin-barked trees, which is why prevention focuses on a tree's first several winters.

When do I put a tree wrap on and when do I take it off?

Apply light-colored wrap or a white guard in late fall, around November in Upstate New York, and remove it in spring, around March. Wrapping from the ground up to the first branches protects the whole vulnerable stretch. Never leave wrap on through the growing season, because it traps moisture and can harbor insects and disease.

Can I just paint the trunk white instead of wrapping it?

On fruit trees, a coat of diluted white latex paint (never oil-based) is a long-standing orchard method that reflects winter sun much like a white guard. It is mostly used in commercial orchards. For a typical landscape maple, cherry, or honeylocust, a removable light-colored wrap or white guard is the simpler, more common choice and leaves no lasting mark.

Sources

• Monster Tree Service of Rochester
• University of Minnesota Extension: Protecting trees and shrubs in winter
• The Morton Arboretum: Winter injury to trees and shrubs
• K-State Research and Extension: Preventing winter sunscald on trees
• Cornell Cooperative Extension