Signs of Internal Decay — How Hollow Is Your Tree, Really?

Trees compartmentalize decay — they wall off rotted areas with chemical barriers to keep the rot from spreading. This works incredibly well, which is why a tree can live for decades with a major internal cavity. The problem is purely structural: a hollow trunk has less wood resisting wind load and gravity, and at some critical ratio it can no longer hold up the canopy.

The rule of thumb arborists use is the 1/3 rule: if more than one-third of the trunk's radius is hollow at any cross-section, the tree's strength is significantly compromised. Once the hollow exceeds 70% of the diameter — meaning the remaining shell is less than 15% on each side — failure is essentially a question of when, not if.

External clues are your best diagnostic. A conk (the woody bracket fungus you see on dead snags) growing out of a living trunk is the smoking gun — it means the decay is already extensive enough to produce a fruiting body, which usually requires years of internal rot. The species of conk also tells us what kind of rot: white rot weakens the trunk gradually, brown rot turns wood into powder and weakens it dramatically.

Carpenter ants are not the cause of decay — they're a symptom. Ants colonize wood that's already softened by rot. If you see a steady column of carpenter ants going in and out of a trunk wound, the decay column inside that trunk is significant. Same with sawdust piles: that's frass being pushed out as insects tunnel through compromised wood.

We confirm decay with two tools. A simple mallet test (tapping the trunk with a hard rubber mallet) tells us whether the wood is solid or hollow — solid wood gives a dull thud, hollow wood gives a drum-like echo. For high-stakes trees, we use a Resistograph or sonic tomograph that drills a tiny hole and graphs the wood density across the cross-section. That's the difference between an opinion and a measurement.