Decay and cavities: what the standards and agencies describe

This page describes what published sources say about one condition in trees. It gives no verdict on any tree and is not an assessment. The sources come from the United States and the United Kingdom. The parent page, structural defects in trees, compares the conditions that have pages here.

What decay and a cavity are

A Washington State University Extension article says nearly all wood decay in living trees is caused by fungi, which reach the wood through a wound or a natural opening such as a branch stub, an open knot or a dead branch. The University of Florida publication describes decay as the breakdown of wood by micro-organisms or extreme environmental exposure and says it can affect any part of a tree. It describes cavities as often being sites of past injuries or pruning cuts and a point of entry for decay fungi. The National Tree Safety Group glossary defines hollowing as the process in which decay organisms progressively remove the inner heartwood of the trunk or major limbs.

Signs the sources list

Source	Signs of decay and cavities it names
US Forest Service brochure	Wood that is soft, punky or crumbly, or a cavity where wood is missing. Mushrooms, conks and brackets on root flares, stems or branches are called indicators of advanced decay. The brochure says the tree usually decays from the inside out while sound wood is added on the outside.
University of Florida	Conks and other fungal fruiting bodies, cavities, carpenter ants and animal nesting holes. The publication says decay can be hard to detect and harder to judge once found.
University of Kentucky	Rotten wood that is soft, mushy or crumbling or missing, loose bark, and piles of sawdust or crumbling wood at the base. The leaflet says small amounts of decay do not necessarily mean a tree is hazardous.
ISA Trees Are Good leaflet	Item F of A to H: asymmetric shapes that may come from reaction wood, fungal fruiting bodies and cavities. The checklist also asks about cavities, rotten wood and mushrooms at the base or under the tree.
Washington State University	Mushrooms in the soil near the base and conks on trunks or branches. The article says a conk generally means rot several feet below and above the fruit, and that the absence of fruiting bodies does not mean a tree is free of decay.

What the sources add as limits

The Forest Service brochure says the presence of decay by itself does not indicate that a tree is hazardous. It gives one measure in its list of reasons to act: sound wood thinner than 1 inch for every 6 inches of diameter at any point on the stem. It adds that trees with a sound outer shell may be relatively safe, depending on the ratio of sound to decayed wood and on other defects, and that judging a decaying tree is usually left to trained arborists. Illinois Extension says decaying trees are more likely to fail and that arborists are trained to evaluate them. The Washington State article says field identification of the cause of decay can be challenging, that where conks or mushrooms are evident further inspection is needed to find the full extent, and that in its final stages decay shows as either white rot or brown rot. The two rots are compared on white, brown and soft rot compared.

The UK view: decay is normal

The National Tree Safety Group guidance (2024) takes a different starting point. It says dead branches, cavities and decay are normal features of trees and that a tree can wall off decay and grow new wood around it. It says old trees can have trunks hollow throughout their length with no detriment to their health when the outer living sapwood is not unduly damaged. It also says hollowing does not necessarily weaken old trees where the height to diameter ratio is sufficiently low and the capacity for adaptive growth remains, and that old hollow trees are often found still standing after storms while nearby solid-stemmed younger trees are uprooted. The same guidance lists advanced decay among the signs that might indicate a structural defect, although not necessarily a hazardous one, and says combinations of defects can raise their importance.

Where to read on

The fungi that cause decay are listed, one page each, on wood decay fungi on trees. For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials.