Stress Factor: Temperature & Weather Stress
Sunscald
Sunburn for your trees’ winter bark.
Sunscald At-a-Glance
Primary Symptom
Elongated, sunken, or cracked sections of bark that eventually peel away to reveal dead wood underneath.
Time of Year
The damage occurs in Mid-Winter, but the physical symptoms (peeling bark) often aren't noticed until Late Spring or Summer when the tree tries to expand and the dead bark finally separates.
Physical Evidence
The smoking gun for sunscald is the “Southwest Compass Rule.” Because the sun is strongest in the afternoon, the damage will almost exclusively be on the south or southwest side of the tree trunk. If the damage is appearing randomly on all sides or only on the north side, it is likely a disease or mechanical injury, not sunscald.
Sunscald Explained: Impact and Recovery
Sunscald is a physiological injury caused by rapid thermal fluctuations during winter. The science centers on the cambium (the living tissue just under the bark). On cold, sunny winter days, the sun warms the dark bark of a tree to temperatures well above freezing, “waking up” the cells from dormancy. As soon as the sun drops behind a building or the horizon, the temperature crashes back to freezing. This rapid freeze kills the newly active cells and causes the bark to contract and split away from the wood.
The Impact Scale is Localized to the trunk, but the damage can be systemic if the split girdles the tree. The Recovery Potential is Moderate; while trees can “callus over” the wound, the open split remains an entry point for wood-rotting fungi and borers for years.
Clues In Plants
This is a “young tree” disorder. Look for discolored, sunken patches on the trunk. As the season progresses, the bark will become brittle and papery, eventually falling off in large vertical strips. In severe cases, you may see “frost cracks,” which are deep, audible vertical splits that reach into the heartwood.
Managing Sunscald: Immediate and Future Steps
Immediate Action:
Do not “seal” the wound. Applying “wound paint” or tar actually traps moisture and encourages rot. Instead, use a sharp, sterilized knife to carefully trace the edges of the dead bark back to where it is firmly attached to live wood. This helps the tree grow a “callus” (wound wood) more efficiently. If the tree is newly planted and severely girdled, it may need to be replaced.
Long-Term Prevention:
The best defense is Reflective Wrapping. Wrap the trunks of young or thin-barked trees (like Maples or Apples) with white plastic guards or light-colored paper tree wrap in late autumn (November). The white color reflects the sun, keeping the bark cool and dormant. Always remove the wrap in the spring (April) to prevent moisture buildup and insect nesting.
Prime Targets and Lookalikes
Sunscald mimics Cytospora Canker and Mechanical Damage (from weed whackers). The difference: Cankers usually have visible “oozing” or fungal fruiting bodies (pimples) and are not restricted to the southwest side. Mechanical damage will have ragged edges and occur only at the base where equipment hits.
Thin-barked trees such as Maples, Lindens, Honey Locusts, and Fruit Trees (Apple/Cherry).
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Read ArticleFrequently Asked Questions
The responses provided in this FAQ are synthesized from peer-reviewed plant diagnostic studies and standardized troubleshooting protocols from university horticultural clinics. We focus on evidence-based explanations to provide clear, scientific clarity on the most common questions regarding environmental plant injury.
This is due to Lack of Canopy Shade. Older trees have thick, corky bark and a wide canopy that shades the trunk even in winter. Young trees have thin “skin” and no structural shade, making their cambium highly vulnerable to solar heating. Once a tree reaches 5–10 years of age, its bark usually thickens enough to become “self-insulating.”
While effective, it’s mostly for commercial use. White latex paint (diluted 50/50 with water) reflects the sun just like tree wrap. However, for a residential landscape, it is permanent and can be an eyesore. Tree wraps are preferred because they can be removed once the danger season has passed.
No. While winter watering prevents desiccation (needles drying out), it has no effect on sunscald. Sunscald is a temperature-regulated cell death, not a hydration issue. You cannot “water away” the thermal shock of a 40-degree temperature drop in ten minutes.
Scientific Authority
This profile is built on objective horticultural research and plant pathology data from university-led extension programs. We prioritize physiological evidence regarding environmental stress factors, nutrient availability, and cellular response to provide an unbiased assessment of each abiotic disorder.
Primary Resources
- University of Minnesota Extension: “Sunscald and Frost Cracks on Trees”
- Colorado State University Extension: “Sunscald of Fruit and Ornamental Trees”
- Tennessee State University: “Winter Injury: Sunscald”