Why Is the Bark Splitting on My Tree in Flagstaff, AZ?

Finding a long crack running up the trunk of a tree can be alarming, especially when the tree otherwise appears healthy. In Flagstaff, one possible explanation is winter injury caused by dramatic temperature changes. Strong Arizona sunshine can warm one side of a tree during the day even when the surrounding air is cold. When temperatures drop rapidly later in the day or overnight, the sudden change can damage the bark and underlying tissue. The result may be discolored bark, a sunken area, peeling bark, or a long vertical split known as a frost crack.

What Is a Frost Crack?

A frost crack is a vertical split that develops in the trunk when temperature changes create stress within the wood. During a sunny winter day, portions of the trunk can warm and expand. When the temperature suddenly drops, the outer portion of the tree cools and contracts faster than the interior wood. That difference can create enough stress for the wood to split. Frost cracks can range from relatively small openings to deep cracks extending a considerable distance along the trunk.

Why Does This Happen in Flagstaff?

Flagstaff's combination of elevation, sunshine, freezing nights, snow, and changing winter temperatures can create conditions where bark experiences substantial temperature swings. A local Flagstaff gardening resource describes sunscald occurring when smooth-barked trees are exposed to intense sunlight and extreme winter temperature fluctuations. This means a sunny winter afternoon isn't necessarily harmless to a tree just because the air temperature remains cool.

What Is Sunscald?

Sunscald, sometimes called southwest injury, occurs when bark and the living tissue underneath it are damaged by rapid temperature changes. University of Arizona Cooperative Extension explains that it commonly appears on the south or west side of a trunk, where bark receives strong sunlight. Warm daytime temperatures can activate sensitive tissue beneath the bark, followed by freezing temperatures at night that damage or kill those cells. Early symptoms may include reddish or brown discoloration, while more advanced damage can produce sunken, splitting, or peeling bark.

Is Sunscald the Same as a Frost Crack?

They're related but aren't exactly the same thing. Sunscald primarily involves injury to living tissue beneath the bark, while a frost crack is an actual split in the trunk caused by temperature-related stress within the wood. However, the two problems can occur together. University of Arizona Extension notes that sunscald can lead to vertical frost cracks, which is why a homeowner may initially notice discoloration or damaged bark and later discover a larger vertical opening.

What Does Winter Bark Damage Look Like?

Look for long vertical cracks, reddish-brown discoloration, sunken sections of bark, peeling bark, or exposed wood underneath. Damage frequently appears on the south or southwest-facing portion of the trunk because that's where winter sunlight can create some of the strongest temperature fluctuations. Older frost cracks may also develop raised ridges of new tissue along their edges as the tree attempts to close the wound.

Does a Crack Mean My Tree Is Dying?

Not necessarily. A tree may continue living for many years with a frost crack, especially when the remaining trunk and canopy are healthy. The concern is that an open wound can provide an opportunity for additional problems. University of Arizona Extension notes that frost cracks can provide entry points for decay organisms and canker pathogens. A crack should therefore be evaluated along with the overall condition of the tree rather than assuming either that the tree must immediately be removed or that the damage can safely be ignored.

Can a Frost Crack Close on Its Own?

Trees don't heal wounds the same way human skin does, but they can grow new tissue around damaged areas and gradually seal them off. You may see callus tissue developing along the sides of an older crack. However, frost cracks can reopen during future winters when similar temperature stresses occur. The Morton Arboretum notes that once a frost crack develops, it may reappear annually. That's one reason an older tree may have a noticeable vertical seam or ridge running along its trunk.

Should I Fill or Seal the Crack?

Don't automatically fill a trunk crack with caulk, concrete, expanding foam, or another household material. Modern tree care generally focuses on allowing trees to compartmentalize wounds naturally while addressing factors that could place additional stress on the tree. If the crack is deep, expanding, associated with decay, or located on a large tree near a house, driveway, play area, or other frequently used space, having the tree professionally evaluated is safer than attempting a DIY repair.

Are Young Trees More Vulnerable?

Young and thin-barked trees can be particularly vulnerable to sunscald because their bark provides less protection against dramatic temperature fluctuations. Recently planted trees can also be more susceptible because they're still becoming established in their new environment. Paying attention to trunk condition during a tree's early years can help homeowners identify winter damage before it becomes extensive.

Can Tree Wrap Help Prevent Sunscald?

For susceptible young or thin-barked trees, properly used tree wrap or a light-colored tree guard may help protect the trunk from winter sunlight and temperature fluctuations. University of Arizona Extension recommends wrapping the trunks of young trees with thin bark during the winter months, while protective materials should not simply be left around the trunk indefinitely. Proper installation and seasonal removal matter because anything wrapped around a growing tree can create additional problems if it's forgotten.

Can Watering Help Protect Trees During Winter?

Maintaining adequate moisture can help trees handle environmental stress, although watering won't repair a frost crack that's already present. Winter conditions can still be dry in Northern Arizona even when temperatures are cold. An Extension response concerning ponderosa pines recommends supplemental winter watering during dry periods when temperatures allow, along with mulch to moderate changes in soil moisture and temperature. The appropriate watering schedule depends on the tree species, age, soil, weather, and amount of natural precipitation.

When Should I Be Concerned About a Crack?

A small older crack on an otherwise healthy tree may be much less concerning than a deep opening accompanied by decay, mushrooms, loose bark, canopy decline, dead branches, or significant movement in the trunk. Location also matters. A structurally compromised tree beside a home, driveway, sidewalk, or frequently occupied area presents a different level of concern than the same tree standing far from people and structures. Arizona Cooperative Extension recommends trained tree professionals for assessing tree health and risk, particularly when large trees or potentially hazardous conditions are involved.

Don't Ignore a New Crack in Your Tree

A split trunk doesn't automatically mean a tree has to come down, but it's worth understanding why the damage occurred. Flagstaff's intense sunshine and freezing winter temperatures can create bark injuries that homeowners in warmer parts of Arizona may rarely encounter. Checking the size and location of the crack, looking for additional signs of decline, and monitoring whether the damage changes can help determine whether the tree simply needs observation or should be professionally evaluated.

Need Help With a Damaged Tree in Flagstaff, AZ?

Flagstaff Tree Pros provides tree trimming, pruning, tree removal, stump grinding, and other professional tree services throughout Flagstaff and surrounding Northern Arizona communities. If you've discovered a deep crack, peeling bark, dead branches, storm damage, or another concerning change in one of your trees, contact Flagstaff Tree Pros today to request a free estimate and determine the safest next step for your property.