A tree trunk is the spine of the whole tree, so any split, hole, hollow, or cavity in it gets noticed fast and worries most homeowners. The good news is that not every trunk problem is a death sentence, and not every one is harmless either. The arborists at Cut Rite Tree Service put together this complete guide to tree trunk problems: why trunks split, what causes large holes, when a hollow trunk is still safe, whether you should fill a cavity, how to care for a damaged trunk the right way, and the real reason people paint tree trunks white here in the Central Valley.
Cut Rite Tree Service is a Fresno tree service rated 4.8 stars across more than 218 Google reviews, fully licensed under CSLB #1015044. We diagnose and treat trunk damage on trees across Fresno, Clovis, Madera, and the surrounding Central Valley. If you are worried about a tree, call us at (559) 320-7723 for a free estimate.
Why Tree Trunks Split
Tree trunk splitting is exceedingly common and can significantly reduce a tree’s structural integrity. Some trees even fall because of a severe trunk split, which is a serious hazard to your home, your vehicles, and your family. A split usually comes down to one of a few causes, and several of them are made worse by the Central Valley climate.
Frost Cracks
When temperatures swing from a sunny afternoon to a cold night, the outer layer of the trunk cools and contracts faster than the wood inside it. That tension can split the bark and wood in a long vertical crack, most often on the south or southwest side of the tree. Fresno winters are mild, but our sharp day-to-night temperature swings still produce frost cracks on thin-barked trees like maples and fruit trees.
Sunscald
Sunscald is one of the biggest culprits in the Central Valley. Intense, direct sun heats up the bark, especially on young trees with thin bark and not much foliage to shade the trunk. The heated tissue can be killed off, and as it dries and contracts it cracks open, leaving the trunk vulnerable. This is exactly the problem that white trunk paint is meant to prevent, which we cover further down.
Included Bark and Weak Forks
When two trunks or a trunk and a heavy limb grow tightly together in a narrow V, bark gets pinched between them instead of solid wood bonding the union. This is called included bark, and it makes a weak fork that splits apart under the weight of the canopy or in a windstorm. Proper structural pruning while a tree is young is the best defense, which is one reason routine tree trimming and pruning in Fresno pays off long term.
Storms, Wind, and Heavy Loads
Valley windstorms put tremendous force on a tree, and a sudden gust can crack a trunk or tear a major limb away, taking a strip of trunk with it. The weight of an overgrown, unbalanced canopy adds to the strain. Lightning and the load of fruit on a heavily bearing tree can do the same. Any wound left behind becomes an entry point for the decay that leads to holes and hollows.
Rapid Growth
Fast-growing species, or any tree pushed with heavy water and fertilizer, can grow wood faster than the bark can stretch to cover it. The result is vertical splitting along the trunk. Steady, moderate growth produces stronger, denser wood than a tree that is forced to shoot up quickly.
What Causes Large Holes in Tree Trunks
It is understandable to feel alarmed when a large hole appears in a tree you love. Holes in a trunk almost always trace back to one of four causes, and each one tells you something different about how worried you should be.
Decay and Old Wounds
Most large holes start as a wound. A broken limb, an old pruning cut, a lawnmower or string-trimmer strike, or a storm injury breaks the protective bark. Once the wood underneath is exposed, fungi move in and break it down. Over years that decay pocket opens up into a visible hole. The original wound may be long forgotten, but the cavity it caused keeps growing.
Insects and Pests
If you see bugs scuttling around your tree, many species feed on the wood itself. Bark beetles are small pests that bore through the bark and leave tiny holes, tunneling inside to lay eggs and feeding on the cambium layer that moves water and nutrients up the tree. Left alone too long, they can kill the host, and many of them carry fungal spores that spread to nearby trees. Wood-boring beetles leave larger oval or D-shaped exit holes, and carpenter ants will hollow out wood that is already softened by decay. Pests are often a sign that the tree was already stressed or wounded.
Woodpeckers
Neat rows of small holes, or one larger cavity, are often the work of woodpeckers. They are usually drilling for insects living in the wood, so woodpecker damage is frequently a clue that the tree already has a beetle or grub problem under the bark. A nesting cavity is bigger and rounder. The birds rarely kill a healthy tree on their own, but they are pointing at a deeper issue worth investigating.
Hollow Tree Trunks: Is a Hollow Tree Dangerous?
The first question most people ask when they find a hollow tree trunk is simple: is it safe? It is a fair question, but the answer surprises people. A hollow trunk does not automatically mean a tree is about to fall. Trees become hollow for a few reasons, including age, where the core gradually decays over many years as a natural part of the tree’s lifecycle, and disease or injury, where insect damage, fungal infection, or an old scar weakens the core and opens up a hollow space.
Here is the part that catches people off guard. A tree carries most of its structural strength in the outer layers of the trunk, not the center. Picture a steel pipe, which is hollow yet remarkably strong. As long as a solid, healthy wall of wood remains around the outside, a hollow tree can keep standing safely for many years. Whether a specific hollow tree is dangerous depends on a few factors.
- Size and location of the hollow. A small hollow in a limb is far less concerning than a large one low on the main trunk. The general guideline arborists use is the amount of sound wood remaining around the opening. When the solid shell gets too thin relative to the trunk’s diameter, the risk of failure climbs.
- Remaining healthy wood. A tree with a generous wall of solid wood and a full, vigorous canopy can continue to grow and thrive for years despite a hollow center.
- Surrounding conditions. A hollow tree standing in an open field away from anything is a different situation than the same tree leaning over your roof, your driveway, or a sidewalk. The target underneath matters as much as the defect itself.
Because judging the remaining wall thickness and the tree’s overall health takes a trained eye, a hollow trunk is the right time to have a professional take a look. Our arborists can assess whether a hollow tree on your Fresno property is structurally sound or whether it has crossed the line into a hazard.
Tree Cavities: Should You Fill Them?
A cavity is an open pocket of decay in the trunk or a fork, and cavities in trees can be incredibly dangerous or completely harmless. Some resilient trees develop deep cavities and still thrive, but that is not always the case. A cavity near the base of the main trunk or at a major union is more concerning than one out on a limb, because it sits where the tree carries the most load. Large cavities also expose the inner heartwood and sapwood to water, UV radiation, and disease.
Now the big myth. For generations, people filled tree cavities with concrete, expanding foam, gravel, or bricks, believing it would support the tree and keep water out. Modern arborist guidance says you usually should not fill a cavity. Filler does not bond to living wood, so the tree keeps flexing while the rigid fill does not, which actually grinds away at the surrounding wood and speeds up decay. Concrete and foam also trap moisture against the inner wood, creating the perfect damp environment for the fungi you were trying to stop. Filling a cavity also hides the very decay an arborist needs to see and monitor.
The better approach is to leave the cavity open so it can drain and dry, and let the tree do what it does naturally, which is wall off the decay and form its own callus around the edges. If a cavity is collecting debris, it can be gently cleaned out by hand without gouging into the living wood. The real job is monitoring the tree over time and managing the canopy weight above the defect, not plugging the hole.
How to Repair and Care for a Damaged Tree Trunk
Trees are good at self-healing and are hardy, dependable things that survive for decades on their own. Technically, a tree never heals a wound the way skin closes a cut. Instead it seals the wound off, walling away the damaged area and growing new wood over it. Your job is to support that natural process and avoid the old-fashioned mistakes that interfere with it.
Make Clean Cuts and Clean Up Torn Bark
When a limb tears away or bark is left ragged around a wound, carefully trim the loose, torn bark back to where it is still firmly attached, using a clean, sharp knife. A smooth, clean edge lets the tree form callus and seal the wound much faster than a jagged tear. Do not cut into healthy bark beyond what is necessary, and never carve the wound into a larger shape than it already is.
Do Not Paint the Wound
This is the single most important repair tip, and it is the opposite of what most people expect. Do not paint a fresh wound or pruning cut with tar, pruning sealer, or wound paint. Decades of research show those products do not prevent decay and often trap moisture and disease against the wood, slowing the tree’s own sealing process. An open, clean wound exposed to air heals over more reliably than a sealed one. The white trunk paint we discuss below is a completely different thing and is never applied to an open wound.
Support the Tree’s Health
A tree recovering from trunk damage benefits from the basics done well: deep, consistent watering during our long Fresno dry season, a ring of mulch over the root zone kept away from the trunk itself, and protection from any further injury by lawnmowers and string trimmers. In some cases a properly chosen fertilizer can supplement growth, matched to the species and the soil’s nitrogen, phosphorus, and potassium levels. A vigorous, well-watered tree seals its wounds far faster than a stressed one.
Why Do People Paint Tree Trunks White?
You have probably seen orchard trees and young trees with their lower trunks painted bright white and wondered why. The practice goes back to ancient times, but the modern reason is straightforward: the white paint prevents sunscald, the same trunk-splitting problem we covered earlier. Young trees and citrus have thin bark and little foliage to shade the trunk, so intense Central Valley sun can heat the bark, kill the tissue, and crack it open. A coat of white reflects the sunlight and keeps the bark temperature steady, protecting the tree through its most delicate years.
This is why the white-trunk look is everywhere in Central Valley orchards. Citrus, almond, and other young fruit trees are especially prone to sunscald in our climate, and growers protect entire rows this way. The paint can also help discourage certain boring insects and reduce frost cracking. The key detail is that it must be a watered-down white latex paint or a traditional lime-based whitewash, applied only to the trunk. Never use oil-based paint, and never confuse this protective whitewash with the wound paint we warned against, since the two serve opposite purposes.
When Trunk Damage Means the Tree Is a Hazard
Plenty of trunk problems can be managed, but some mean a tree has become a genuine hazard and removal is the safe call. Lean toward having a professional evaluate the tree, and toward removal, when you see any of the following.
- A deep vertical split that runs a long way up the trunk, or a crack you can see flexing and opening when the wind blows.
- A hollow or cavity where the remaining shell of solid wood is too thin to support the canopy, often roughly a third or less of the trunk’s radius.
- Mushrooms or fungal conks growing on the trunk or at the base, which signal active internal decay.
- A split or large cavity right at the base or at a major fork carrying the weight of the whole tree.
- A tree that has started to lean, or one with cracked or heaving soil on one side of the root zone.
- Any of the above on a tree positioned over your house, driveway, power lines, or a place where people gather.
When a tree reaches that point, taking it down safely is a job for a licensed, insured crew, not a weekend project. Our Fresno tree removal team has the equipment and training to remove a compromised tree without damaging your property. If the tree can still be saved, our tree trimming and pruning crews can reduce the canopy weight over a defect and improve the tree’s structure so it stays standing.
Frequently Asked Questions
Is a hollow tree trunk dangerous?
Not always. A tree holds most of its strength in the outer wall of the trunk, like a hollow pipe, so a hollow tree with a solid, healthy outer shell and a full canopy can stand safely for years. It becomes dangerous when the remaining wall of wood gets too thin or when the tree sits over a home, driveway, or walkway. An arborist can assess the wall thickness and tell you for sure.
Should I fill a cavity in my tree with concrete or foam?
No. Modern arborist guidance is that you usually should not fill a tree cavity. Concrete, foam, and gravel do not bond to the living wood, they trap moisture against it, and they speed up decay while hiding it from view. Leave the cavity open to drain and dry, and have the tree monitored over time instead.
Should I paint a tree wound after pruning or storm damage?
No. Wound paint and pruning sealer do not prevent decay and often trap moisture and disease against the wood, slowing the tree’s own sealing process. Make a clean cut, trim any torn bark back to where it is firmly attached, and let the open wound seal naturally in the air. White whitewash on a trunk to prevent sunscald is a separate practice and is never applied to an open wound.
Can a split tree trunk be saved?
Sometimes. A minor, shallow split on an otherwise healthy tree often seals on its own with good care, and certain forks can be supported with professional cabling or bracing. A deep split that runs far up the trunk, one that flexes in the wind, or one at the base usually means the tree is a hazard and should be removed. Have an arborist inspect it before deciding.
Talk to a Fresno Arborist About Your Tree’s Trunk
Splits, holes, hollows, and cavities all tell a story about a tree’s health, and the right response ranges from simple care to safe removal. If a trunk problem on your property has you concerned, the licensed arborists at Cut Rite Tree Service will inspect the tree, tell you honestly whether it is sound or a hazard, and lay out your options.
Call Cut Rite Tree Service at (559) 320-7723 for a free estimate, and let our team take a look at your tree today.
