Holes in trees, which can range from pin-sized insect tunnels to large, irregular cavities, are common in urban and natural landscapes. These openings are evidence of a past or ongoing biological process, such as a wound or decay. The danger a hole poses is not inherent to the hole itself, but is determined by its origin, size, location, and the tree’s ability to respond to the damage. A tree’s safety is judged by how much structural wood remains sound beneath the bark layer.
What Creates Holes in Trees
The size, shape, and pattern of a hole often diagnose the agent responsible for its creation. Wood-boring insects, such as bark beetles or clearwing moth larvae, create small, circular entry or exit holes in random patterns. The presence of frass (a fine, sawdust-like material) or sap tubes near these holes confirms an active insect infestation.
Birds, particularly sapsuckers, leave small, shallow holes arranged in neat horizontal and vertical rows. They drill to access the tree’s sap, which attracts insects they consume, but extensive drilling can sometimes girdle a thin-barked tree.
Larger, irregular holes or cavities are typically the result of natural decay, where wood-rotting fungi gain entry through an old wound. Fungi, such as heart rot, consume the non-living wood in the tree’s interior, often following a broken branch or improper pruning cut.
Mechanical damage from human activity, such as a lawnmower striking the base or construction equipment scraping the bark, creates an initial wound. This damage breaks the tree’s protective outer layer, serving as an entry point for decay organisms to colonize the inner wood.
Determining Structural Risk and Safety
A tree with a hole becomes a structural concern when the cavity compromises the sound wood needed to support the tree against wind and gravity. Arborists use the “critical wall thickness” ratio to assess this risk, comparing the remaining sound wood shell thickness to the trunk’s radius.
The traditional guideline suggests a safety factor is reduced when the remaining wall thickness is less than 30% of the radius. However, mature trees often maintain stability even with less wood, compensating by growing new wood strategically to reinforce the trunk.
The location of the hole is an important factor in determining the risk of tree failure. Cavities lower on the trunk, especially at the root collar or base, are more serious than those higher up in the canopy. Decay at the base compromises the tree’s anchor point, increasing the chance of the entire tree uprooting in high winds.
The appearance of fungal fruiting bodies, such as mushrooms or conks, on the trunk or base indicates advanced internal decay. These structures are the reproductive parts of the fungus and confirm extensive wood rot inside the tree.
A tree’s ability to compartmentalize the damage also plays a role in its long-term safety. Some species form strong barriers to contain the spread of decay, allowing them to remain structurally sound for decades despite an internal cavity.
Protecting the Tree and When to Seek Professional Help
When a hole is discovered, the best initial action is to monitor the tree for signs of change, such as an increase in size or the development of new symptoms like branch dieback. Current arboricultural science advises against filling tree cavities with materials like concrete, foam, or tar.
These materials do not bond with the wood, can trap moisture, and interfere with the tree’s natural defense mechanism to seal off the damaged area.
A certified arborist should be contacted immediately if a tree shows signs of instability, such as a sudden or increasing lean. Professional intervention is necessary if fungal bodies are present, if the hole is low on the trunk, or if the tree is dropping large limbs.
Since an arborist can use advanced tools to measure the exact thickness of the remaining sound wood, they provide the most accurate assessment of the tree’s stability. This evaluation is important when a tree with a hole is located near a high-value target, like a home, driveway, or frequently used area.