Internal tree rot occurs when wood decay fungi break down the tree’s structural tissue, often starting in the heartwood. This decay severely compromises the tree’s stability, creating a significant safety and property risk, even when the tree appears healthy externally. The process typically begins when fungal spores enter through wounds, such as broken branches, mechanical damage, or large pruning cuts. This rot is concerning because the tree may maintain a full, green canopy while its central support structure is slowly eaten away.
External Visual Indicators of Internal Decay
The most definitive visual sign of internal rot is the presence of fungal fruiting bodies, known as conks, mushrooms, or brackets, growing on the trunk or around the base of the tree. These structures are the reproductive organs of the wood-decaying fungus and indicate that the organism is well-established inside the wood tissue. Decay fungi often enter through the root collar or major wounds, making the trunk base a primary area for finding these growths.
Other signs of internal weakness can manifest in the canopy and trunk. A thinning or sparse canopy, especially with dead branches near the top, may signal extensive root or lower trunk decay disrupting the flow of water and nutrients. Look for vertical cracks or long seams in the trunk bark, which can indicate internal wood failure or structural weakness. Similarly, unhealed wounds that weep sap excessively suggest that the tree’s natural defense mechanism, known as compartmentalization, is failing to contain the infection.
Physical and Auditory Tests for Hidden Rot
A basic, non-invasive method to check for hidden decay is the “sounding” test, which involves tapping the trunk with a rubber mallet or the back of a hatchet handle. Sound wood produces a high-pitched, solid sound when struck. Conversely, a hollow or decayed section will return a low-pitched, dull thud or a drum-like resonance. This change in acoustic tone allows identification of potential pockets of decay just beneath the bark surface.
For areas where a cavity or wound is already visible, a simple probing test can help gauge the extent of the decay. Use a long, thin, blunt object, like a stiff wire or a screwdriver, to carefully probe the soft or “punky” wood inside the opening. Measuring the depth of the easily penetrated wood provides a rough estimate of the remaining sound wood shell thickness. Another test is to gently check for excessive root plate movement by pushing on the trunk on a calm day, as movement where the trunk meets the soil suggests significant root rot.
Assessing Structural Risk and Imminent Failure
The presence of rot does not always mean a tree is an immediate danger, but certain conditions elevate the risk of structural failure. A tree that has developed a significant, recent lean, particularly following a storm or heavy rain, suggests the root plate or lower trunk is compromised and can no longer support the weight. Heaving or raised soil on one side of the tree, especially opposite the direction of the lean, is a strong indicator of root failure and imminent uprooting.
Decay that creates a cavity encompassing more than one-third of the trunk’s diameter in the lower section significantly reduces the tree’s ability to resist wind loads. Decay visible high up in the main stem or in major scaffold limbs also poses a substantial risk because the sheer mass of the branch can cause catastrophic failure. The most vulnerable area for failure is often the base of the tree, between the root flare and the first major branch, where extensive decay is highly destabilizing.
When to Call an Arborist and Advanced Diagnostics
Professional intervention becomes necessary when the tree is located near a high-value target, such as a house, driveway, or public area, or if the decay appears extensive. Arborists certified in tree risk assessment possess the expertise to evaluate the severity of the decay in relation to the tree species and its surroundings. They combine visual inspection with advanced diagnostic tools that measure the exact extent of internal decay without causing significant injury to the tree. These specialized tools provide a clearer picture of the tree’s structural integrity.
Resistograph
A Resistograph is a micro-drill that penetrates the wood, recording the resistance encountered in real-time to create a graph showing the wood’s density. Areas of low resistance on the graph indicate decay or a hollow cavity.
Sonic Tomography
Sonic tomography is a non-invasive technique that uses sound waves, measuring the speed at which they travel through the trunk. This generates a color-coded map of the internal structure. Sound wood transmits the wave quickly, while decayed or hollow areas slow the wave down, revealing the precise location and size of the defect.
Based on the diagnostic results, an arborist can recommend mitigation options, such as pruning to reduce crown weight or installing cables and bracing rods to support weak limbs or trunks. If the remaining shell of sound wood is insufficient to support the tree, or if the decay is too aggressive, the only safe option may be removal. These advanced assessments provide the data needed to make an informed decision between retaining the tree and eliminating a hazard.