A tree may appear healthy on the outside while experiencing significant internal decay, leading to a hollow core. This condition compromises the tree’s structural integrity, increasing the risk of sudden failure, especially during high winds or storms. Recognizing and assessing this hidden damage is fundamental to property safety and tree health management. This article explores practical methods for identifying internal wood decay and determining when a professional arborist is needed to evaluate the risk a tree may pose.
External and Visual Clues of Decay
The first step in assessing a tree’s internal condition involves passive observation of its exterior and canopy. Fungal growth on the trunk or near the base is a strong indicator of internal rot. Look specifically for mushrooms, conks, or bracket fungi, which are the reproductive bodies of the fungus and only appear after the infection is well-established inside the tree.
The protective bark layer can also display clear warning signs. Deep vertical cracks, seams, or large open wounds on the trunk provide entry points for decay-causing organisms like fungi and insects. Open cavities or holes, often found near the base or where a branch broke off, allow for a visual inspection of the wood inside, which may appear soft, crumbly, or discolored.
Signs of decay also manifest in the tree’s crown, indicating a disruption in the flow of water and nutrients. Sparse foliage, premature leaf yellowing, or excessive dieback in the upper canopy can be a symptom of severe root or trunk decay. When a tree’s internal structure is compromised, its ability to sustain healthy growth is diminished, leading to a general decline.
Acoustic and Physical Testing Methods
Once visual clues suggest potential hollowness, active, hands-on tests can provide a better indication of the extent of the void. The most common do-it-yourself method is sounding, which involves systematically tapping the trunk with a rubber mallet or the back of a hatchet handle. A solid, healthy trunk will produce a low, dull thud, while a hollow section will emit a higher-pitched, drum-like echo.
This sounding test should be performed at various heights around the trunk’s circumference to map out the potential area of decay. If a visible opening, crack, or cavity exists, a stiff wire or a long screwdriver can be used for careful probing. Gently push the probe into the opening to gauge the depth and width of the hollow space, using caution not to damage any surrounding healthy wood.
The base of the tree and the visible root structure, known as the root flare, should also be examined for softness or movement. If the wood feels spongy when pressed or if the soil around the base is cracked or heaving, it suggests possible butt or root rot, which severely affects the tree’s stability. The presence of fungal bodies directly on the root flare is a serious sign of this root-level decay.
Knowing When Professional Assessment is Needed
Homeowner assessments provide an initial indication of decay, but they cannot accurately determine the precise depth, wall thickness, or overall structural stability of the tree. Decay often occurs high up in the trunk or deep within the core, making it invisible to surface-level tests. Any tree displaying significant signs of decay that is located near a house, driveway, or other high-value target warrants an immediate professional inspection.
Certified arborists use specialized diagnostic tools to accurately measure the extent of internal defects.
Specialized Diagnostic Tools
A resistograph is a micro-drill that bores a small hole into the trunk, graphing the resistance encountered to precisely map the density of the wood and the width of any cavity.
For a non-invasive, more comprehensive assessment, sonic tomography uses sound waves to create a cross-sectional image of the trunk, revealing the exact size and location of the hollow area. If the decay is estimated to exceed about 30% of the trunk’s diameter, the tree’s structural capacity may be significantly compromised, requiring expert intervention and a detailed risk analysis.