Sunscald and frost cracks

What it is

Winter injury to bark and living tissue under it

Trees affected

Young, thin-barked trees: linden, maple, ginkgo, crabapple, redbud and others

Where in our area

Open yards and street plantings at all elevations, about 4,000 to 7,100 feet

Key sign

Sunken, cracked or peeling bark on the south or southwest side; vertical splits

Risk

Wounds that stay open invite decay and borers

High-desert winters have big swings: sun can warm a trunk past freezing in the afternoon and the temperature can drop fast at sunset. Colorado State Extension describes both injuries as results of that swing. This page covers what to look for, which trees, what an arborist checks and when pruning or removal is the answer.

What do sunscald and frost cracks look like?

Sunscald, also called southwest injury, shows on the south or southwest side of the trunk. Colorado State says the bark turns an unusual color, then cracks, becomes mushy or sloughs off, leaving a jagged wound that can take a long time to seal. Frost cracks are vertical splits in the trunk or stems; they open when the outer bark cools fast at sunset while the inside stays warm.

• Sunken or peeling bark on the south or southwest side
• Vertical splits in the trunk
• Dead bark that sloughs off
• A wound that stays open for years

Which trees and which elevations?

Young trees with thin bark are most at risk. Colorado State lists linden, maple, ginkgo, crabapple and redbud as species that benefit from winter wrap and says thick, corky-barked trees such as bur oak do not need it. The pages most likely to apply here are littleleaf linden, flowering crabapple and the maples.

Any open yard with winter sun, from Reno (4,505 feet) to Kingsbury (7,126 feet), can see the warm-then-freeze pattern, so we do not assign it to an elevation band. It is a young-tree problem more than a place problem.

Why does it happen?

Sunny winter days warm the trunk, cells lose dormancy and then a hard freeze at night kills them, Colorado State explains. Frost cracks come from the outer bark cooling quickly at sunset while the inside stays warmer.

The wound is not the end of the story. A jagged, open wound is an entry point for decay and insects, and stress also invites Cytospora canker. Colorado State rates sunscald as more serious than frost cracks.

What does an arborist check?

The arborist checks the size and depth of the wound, whether it is a smooth old scar or an open, dying patch, how much of the trunk's circumference is dead and whether decay has begun. They also check the root flare and crown for secondary stress.

• Wound size and how far around the trunk it goes
• Whether bark is loose or decay has begun
• Cracks that run deep into wood
• Crown dieback
• Fall zone

When is pruning or removal the answer?

Bark wounds are judged by the arborist on site; we do not paint or wrap trunks. Dead limbs above a wound can be pruned out.

Removal is the answer when the wound has girdled much of the trunk, when decay has weakened the trunk or when a crack goes deep into a tree that stands over a house. Many damaged young trees recover and are watched.

What does Battle Born Tree Service do about sunscald and frost cracks?

Start with a free estimate. A certified arborist assesses the tree before any work begins, and the estimate says whether the answer is pruning, watching the tree or removal. Two of our crew are ISA Certified Arborists, and credential WE-11728A belongs to one of them. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved; we do not spray, inject, fertilize or apply products. We do not plant or wrap trees.