What matters about a cavity
The question is not how big the hole is but how much sound wood surrounds it. A tree with a cavity and a thick, even wall of healthy wood can stand for many years. A tree with a cavity, a thin wall, cracks running from it, or a lean can fail suddenly. Where the cavity sits matters too: a hollow in the lower trunk or the root flare carries more risk than one high in a limb that could be pruned off.
Cavities often start where a limb broke off or a cut was left badly, and where woundwood failed to close over. That is another reason correct cuts matter.
Why it matters in Northern Nevada
Mature cottonwoods, willows and old Siberian elms along Carson Valley, the Truckee Meadows and older neighbourhoods often have cavities, and some also have another job: birds and small animals use them. Whether to keep a tree for that reason is a legitimate question, and it depends on what the tree would hit.
Never put anything in the cavity expecting to stop decay. Filling a cavity with concrete or foam is an old practice that modern arboricultural guidance does not recommend, because it does not strengthen the tree.
What an arborist does about it
The arborist looks at the shape of the opening, tests the wall, checks the rest of the trunk and the roots, and asks what is below. Options include leaving the tree alone and re-checking, reducing the weight of limbs that load the weak section, cabling in the right cases, or removal. Because trees can be hollow on the inside while looking sound, we recommend a certified arborist's assessment before anyone relies on a hollow tree standing over a house.
Sources
- International Society of Arboriculture, Trees Are Good: Recognizing Tree Risk (brochure) (read October 17, 2026)