You walk out in March and there is a vertical split running up the trunk of your young maple, right on the side that gets afternoon sun. It was not there in November. Nobody hit it with a mower or a car. So what happened?
A frost crack tree trunk injury like this is one of the more common calls we get every spring across Boise and the Treasure Valley, and it has almost nothing to do with anything you did wrong. It is a climate problem, specific to how our winters swing between freezing nights and sunny, mild days, and it hits certain species and certain trunk orientations far more than others.
If you found a crack or a patch of dead, sunken bark on a tree this winter, do not panic and do not reach for tar or wound sealant. Send us a photo through our contact form or give us a call, and we can usually tell from a picture whether it is cosmetic or something that needs attention.
What a Frost Crack Actually Is
A frost crack forms when the outer wood and bark of a trunk cool and contract faster than the wood underneath during a rapid temperature drop, typically on a clear, cold winter night after a warm, sunny day. The outer layer contracts, the inner wood does not keep pace, and the tension splits the trunk vertically, often with an audible crack (some old-timers call these “frost seams” or say they heard the tree “shooting” in the night).
Frost cracks almost always run vertically and tend to reopen in the same spot year after year once a tree has cracked once, because the wound tissue that forms around the old crack is denser and expands and contracts differently than the surrounding healthy wood.
Sunscald: A Related but Different Injury
Sunscald is the other major form of winter trunk damage, and people often use the two terms interchangeably even though the mechanism is a little different. Sunscald happens on the southwest side of a trunk, where winter sun during the day warms bark tissue enough to bring it out of dormancy. When temperatures drop sharply after sunset, that reactivated tissue freezes and dies.
The result is not usually a crack. It is a sunken, discolored, often cracked patch of dead bark, typically on the lower trunk, facing southwest. Over time the dead bark can slough off entirely, exposing bare wood underneath.
Both problems come from the same root cause: rapid temperature swings acting on trunk tissue that has not built up the insulation or bark thickness to handle it.
Why the Southwest Side of the Trunk Takes the Damage
This is the detail that helps people recognize the pattern once they know it. In the Treasure Valley, winter days are often clear and sunny even when air temperatures stay near or below freezing. The sun tracks across the southern sky and hits the south and southwest face of a trunk hardest in the afternoon, when it is also lowest in the sky and shining almost directly onto vertical bark.
That afternoon sun can warm bark tissue on the southwest side by 20 degrees or more above the surrounding air temperature. Then the sun sets, and that same tissue can drop 40 or 50 degrees within an hour or two as evening cold sets in. Few living tissues handle that kind of swing without damage, and young, thin bark handles it worse than old, thick bark.
Which Trees Are Most at Risk
Species with thin, smooth bark are far more vulnerable than species with thick, corky, or deeply furrowed bark. In our area, the highest-risk trees include:
- Young maples, especially newly planted or under 6 inches in trunk diameter
- Fruit trees (apple, cherry, plum) in their first several years
- Honeylocust, especially younger specimens
- Linden
- Any recently transplanted tree, regardless of species, since transplant stress reduces bark’s cold hardiness for the first few years
Mature trees with thick bark, and species that naturally have rough or platy bark like oak or mature cottonwood, rarely develop frost cracks or sunscald. This is one of several reasons young trees need more winter protection than established ones, alongside the drought vulnerability we cover in our guide to winter tree care in Boise.
Frost Crack vs Structural Crack: Why the Difference Matters
Not every crack in a trunk is a frost crack. We have a separate guide on structural cracks in tree trunks that covers the kind of cracking associated with internal decay, included bark at branch unions, or mechanical stress, all of which are a different and often more serious problem than winter injury.
| Feature | Frost Crack / Winter Injury | Structural Crack |
|---|---|---|
| Location | Trunk, often southwest face | Anywhere: trunk, branch unions, crotches |
| Season noticed | Appears or worsens in late winter/early spring | Can appear or worsen any time, often after storms |
| Shape | Vertical, running with the grain | Can be vertical, horizontal, or spiral |
| Underlying cause | Rapid freeze-thaw temperature swings | Decay, included bark, storm stress, old wounds |
| Immediate risk | Usually low, mostly a healing/entry-point concern | Can indicate active failure risk |
| Typical fix | Time, protection from future injury, monitoring | Cabling, bracing, or removal depending on severity |
If the crack you are looking at appeared after a windstorm rather than over winter, or if it is accompanied by a lean, cracking sounds, or visible movement at the base, that points toward a structural issue rather than winter injury, and it is worth a same-week assessment rather than a wait-and-see approach.
Can a Frost Crack or Sunscald Wound Heal?
Trees do not heal wounds the way people do. They compartmentalize, walling off damaged tissue and growing new wood around the wound over years, a process arborists call CODIT (compartmentalization of decay in trees). A frost crack can close at the surface as callus tissue grows over the split from both edges, but the crack itself does not disappear from the wood underneath, and the site typically stays a point of weakness.
Sunscald wounds heal similarly, with callus tissue growing in from the edges of the dead patch over several years. A small sunscald wound on an otherwise healthy, vigorous tree often closes up within three to five years. A large wound, or one on a tree that is already stressed by drought or poor planting, may never fully close and can become an entry point for wood-decay fungi or boring insects.
This is why the response to fresh winter trunk damage matters. Do not paint the wound with tar, asphalt-based sealant, or “wound paint.” Research from the University of Idaho Extension and similar land-grant extension programs has shown for decades that sealants trap moisture against the wound and can slow healing rather than help it. Bare wood exposed to air, kept clean of debris, heals better on its own.
Should You Wrap Your Tree’s Trunk?
Tree wrap (the paper or plastic spiral wrap sold at most nurseries) is a legitimate preventive tool, but it is frequently misused, and misuse causes its own problems.
When wrapping helps: young, thin-barked trees (maple, fruit trees, linden) in their first two to three winters after planting, especially if they are in an exposed, south- or west-facing location with no afternoon shade from a building or larger tree.
When wrapping is unnecessary: mature trees, thick-barked species, or trees already shaded from winter afternoon sun by a structure or neighboring tree.
The mistake that causes damage: leaving the wrap on year-round or forgetting to remove it in spring. Wrap left on through summer traps moisture against the bark, encourages insect activity underneath, and can actually cause the girdling and constriction problems we cover in our guide to girdling roots, just applied to the trunk instead of the roots. Put wrap on in late October or November and take it off by April, every single year, for as long as the tree needs the protection.
A Meridian Scenario: Two Trees, Two Outcomes
A homeowner in a newer Meridian subdivision planted a row of three young maples along a south-facing driveway in the fall. None were wrapped. By the following spring, all three had vertical cracks on the southwest side of the trunk, and one had a sunscald patch nearly a foot long. We assessed all three: two were minor enough to monitor and protect going forward with proper wrapping, and one had a crack severe enough that it had also invited borer activity, and that tree needed removal and replacement at a cost of about $420, on the low end of typical removal pricing given how young and small the tree still was.
Contrast that with a similar planting in a North End yard where the homeowner wrapped three new fruit trees every fall for their first three winters, removing the wrap each spring. None developed frost cracks or sunscald, and by year four the bark had thickened enough that wrapping was no longer necessary.
When Winter Trunk Damage Signals a Bigger Problem
Most frost cracks and sunscald wounds are a nuisance, not an emergency, but a few situations warrant a closer look from an arborist rather than a wait-and-see approach:
- The crack is wide enough to see into the wood, or you can see movement at the crack when the tree sways
- Fungal growth (mushrooms, conks, or shelf fungi) appears at or near the wound
- The tree shows other signs of decline: thin canopy, dieback, or the bark peeling in areas unrelated to the winter wound
- The damaged tree is large enough that a trunk failure would hit a house, driveway, or area where people walk or park
Do not attempt to “clean up” a large trunk wound yourself with a chisel or knife to remove loose bark. Trimming back to “clean wood” is a myth that causes more damage than it prevents; let the tree’s own compartmentalization process do the work, and let an arborist advise if intervention is actually needed.
Preventing Frost Cracks and Sunscald Going Forward
A few habits reduce the odds significantly for young or thin-barked trees:
- Wrap vulnerable trees each fall and remove the wrap each spring, without fail, for the first three to five years
- Plant new trees at the right time of year so they have a full growing season to establish before their first winter
- Avoid heavy pruning in late summer or fall, since it can stimulate late growth that has not hardened off before winter
- Keep new trees well watered through fall so they enter winter in good condition, since a well-hydrated tree tolerates temperature swings better than a stressed one
Frequently Asked Questions
Will a frost crack kill my tree? Rarely on its own. A frost crack is a wound, not a death sentence, and most trees compartmentalize the damage and continue growing normally. The bigger risk is a secondary problem like decay or borer activity entering through the wound over time.
Can I fill or seal a frost crack with caulk or wound dressing? No. Leave the wound open to air. Sealants and fillers trap moisture and can promote decay rather than prevent it. The tree’s own callus tissue is the only “repair” it needs.
Does frost cracking mean I planted the wrong species? Not necessarily. Even good species choices for Boise, like maple and honeylocust, are vulnerable while young and thin-barked. It is more about age, bark thickness, and sun exposure than species selection alone.
How do I know if a trunk wound is frost damage or something more serious like a disease? Frost cracks and sunscald have a specific pattern: vertical orientation, southwest-facing location, and appearance in late winter or early spring. Damage in unusual locations, at unusual times of year, or with oozing, discoloration, or fungal growth is worth a professional look.
Should I wrap a mature tree’s trunk in winter? Usually not needed. Mature trees have bark thick enough to buffer normal temperature swings. Wrapping is most useful for trees under a few inches in trunk diameter or within their first two to three winters after planting.
Can sunscald happen on the east or north side of a tree? It is uncommon but not impossible, usually only in unusual site conditions with reflected heat, such as near a light-colored wall or pavement. The southwest face is by far the most common location in our climate.
What’s the difference between frost cracks and normal bark texture on an older tree? Older trees naturally develop ridges, furrows, and texture as bark matures, which is different from a clean vertical split with exposed wood underneath. If you can see bare wood inside a crack, that is trunk injury, not normal aging.
Get a Professional Opinion Before You Do Anything to the Wound
Winter trunk damage looks alarming, and the instinct to “fix” it with sealant or aggressive trimming is understandable but usually counterproductive. Most frost cracks and sunscald wounds just need time, protection from repeat damage, and a healthy tree behind them to close the wound naturally.
If you are not sure whether what you are looking at is minor winter injury or something that needs intervention, our ISA-certified arborists can take a look and give you a straight answer, free of charge.
Call (509) 351-8404 or request your free consultation online.