Is this you?
- ▸A visible cavity, hole, or opening in the trunk
- ▸Drum-like sound when you tap the trunk with a hard object
- ▸Carpenter ants or wood-boring insects active at one trunk location
- ▸Bark falling off in sheets near the cavity
- ▸Old pruning wound or fire scar that became a cavity
- ▸Mushrooms or conks growing from the trunk itself
Hollow trunks are normal — older trees almost always have some heartwood decay because heartwood is dead wood the tree no longer uses for active transport. The structural strength of a trunk lives in the outer wood and the bark; as long as that shell is intact and thick enough, the tree can be remarkably hollow inside and still be safe.
The standard arborist threshold is the t/R ratio — wall thickness (t) divided by trunk radius (R). When t/R drops below 0.30 (meaning the solid shell is less than 30% of the trunk radius), the tree is considered significantly compromised. Below 0.20, the tree is at high risk of failure under normal wind loading. We measure this with diagnostic tools, but you can do a rough version yourself.
The mallet test is the simplest at-home version. Tap the trunk every 6 inches around the circumference with a rubber mallet or the back of a hatchet. Solid wood gives a sharp, dull thud. Hollow wood gives a deep, drum-like echo. Map the hollow zones — if more than a third of the trunk circumference sounds hollow, it's time for professional diagnostics.
For high-value trees we use sonic tomography or a Resistograph. Sonic tomography sends sound pulses across the trunk between sensors and graphs the wood density across the cross-section — no drilling, no damage, a color map of solid and hollow zones. The Resistograph drills a tiny 1/8-inch hole and graphs the wood's resistance, giving a profile of solid vs decayed wood along that line. Both are non-destructive and give us actual measurements.
What we do with the data: if t/R is above 0.30 and the tree has no other defects, we usually recommend monitoring with annual re-measurement. Between 0.20 and 0.30, we look at canopy size, lean, target, and wind exposure to make the call. Below 0.20, we typically recommend removal — especially over any target. Hollow trees fail suddenly and without warning, which is why we don't push the math.
Spokane / North Idaho Context
Spokane's most common hollow species are mature cottonwood (extremely fast heartwood decay), willow, and old-growth Ponderosa with fire scars from past wildfires. If you live in Browne's Addition, South Perry, or any older Spokane neighborhood with 80+ year-old cottonwoods, get the big ones mallet-tested every spring.
What to do right now
- 1Do the mallet test annually on every large tree near a structure.
- 2Measure visible cavities — opening height, width, depth — and photograph annually to track growth.
- 3Never fill or seal a cavity — it traps moisture and accelerates decay.
- 4Keep cavities open to drain; never plug with foam, concrete, or wood blocks.
- 5Call (509) 204-4333 for Resistograph or sonic tomography on high-stakes trees.
What it costs: Diagnostic Resistograph testing: $150–$300 per tree. Sonic tomography (for very high-value trees): $300–$700. Removal of a hollow tree: $1,000–$4,500.
Frequently Asked Questions
Can a hollow tree be reinforced or filled?
No. Foam, concrete, and other fillers do nothing for structural strength and accelerate decay by trapping moisture. Cabling can support cracked unions but doesn't address hollow trunks.
Are wildlife snags safe to leave?
If they're nowhere near a target, yes — dead and hollow trees are valuable wildlife habitat. We routinely cut hazardous trees back to a 15–20 foot wildlife snag instead of removing them entirely when the location allows.
How long do hollow trees typically stand?
Hugely variable. Some hollow trees stand for decades. Others fail suddenly in the next windstorm. That unpredictability is why we measure and don't guess.
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