A tree can look perfectly healthy from the lawn, with a full green canopy and plenty of new growth, while its base is quietly rotting. Root and butt decay is one of the most common reasons apparently sound trees fall over in storms, and it is also one of the most overlooked, because the evidence sits at ground level, often hidden by mulch, long grass or garden beds.
Learning to read the base of a trunk takes a few minutes and costs nothing.
Why the base matters so much
The lowest metre of a tree carries the greatest mechanical load. Every gust that pushes on the canopy is transferred down through the trunk and into the root plate, and the forces are highest right at the root flare where the trunk widens into the main roots. A small pocket of decay there is far more significant than the same pocket higher up a branch.
Fungi that cause butt rot typically enter through wounds: mower and line trimmer damage, old pruning cuts, roots severed during trenching or driveway work, or bark crushed by vehicles. Waterlogged soil and buried root collars also encourage infection.
Signs to look for
Walk all the way around the tree, clear away leaves and mulch from the base, and look closely.
Fungal fruiting bodies
Mushrooms, brackets and crusts at the base are the most obvious warning. They are the reproductive structures of a fungus that is already established inside the wood. Common examples in New Zealand gardens include:
- Bracket fungi such as Ganoderma, which form hard, shelf-like brackets, often brown on top with a pale underside. They can persist for years.
- Honey fungus (Armillaria), which produces clusters of honey-coloured mushrooms around the base in autumn, along with white fan-shaped sheets under the bark.
- Soft, short-lived mushrooms that appear after rain and vanish within days. Some are harmless decomposers of buried wood; others indicate root decay.
Not every mushroom near a tree is a problem, but anything growing directly from the trunk, the root flare or main roots deserves attention.
Other clues
- Cavities, soft or crumbly wood, or wood that sounds hollow when tapped with a rubber mallet.
- Bark that is cracked, sunken, loose or weeping dark fluid.
- Swellings or bulges at the base, which can be the tree adding wood around an internal weakness.
- Soil heaving or cracking on one side, or a new lean.
What happens during a professional assessment
When warning signs appear, a qualified arborist can carry out a visual tree assessment and, if needed, more detailed testing. Tools range from a simple sounding mallet and probe to resistance drills that measure how much sound wood remains, and sonic tomography that maps decay across a cross-section of the trunk.
The key question is not whether decay exists, since many old trees carry some, but how much sound wood remains and whether that is enough for the tree’s size and exposure. A tree with a thick shell of healthy wood around a hollow core can be structurally adequate. One with decay spreading through the root flare on the windward side may not be.
The assessment should also consider what lies within falling distance. A decayed tree in an open paddock is a very different risk from the same tree overhanging a bedroom. If branches already reach over the house, this checklist for overhanging branches and roof damage is a helpful companion.
Options once decay is confirmed
Depending on the findings, recommendations usually fall into a few categories:
- Monitor: record the decay and reinspect every one to two years.
- Reduce load: shorten the crown or remove heavy limbs so less force reaches the weakened base.
- Change the target: move a play area, parking spot or path out of the fall zone.
- Remove: where too little sound wood remains, felling is often the only sensible option.
There is no spray or injection that reliably cures established wood decay.
Felling a decayed tree is particularly hazardous, because the wood may fail unpredictably during cutting. It is not a job for a weekend chainsaw. Guidance on safe tree work and chainsaw use is available from WorkSafe New Zealand, and it makes clear why compromised trees need trained operators. Check the district plan as well, since protected trees still need consent for removal unless an emergency provision applies.
Preventing decay in the first place
Most butt rot starts with avoidable damage. Keep mowers and trimmers away from the trunk with a mulched ring around one metre wide. Avoid piling soil or mulch against the bark. Plan trenches and paving well outside the root zone, and water deeply in drought rather than letting stressed trees decline. When pruning, cut correctly at the branch collar so wounds can close.
Frequently asked questions
Should I knock off fungal brackets when I see them? Removing them does not stop decay inside the tree. Leave them in place and photograph them so an assessor can identify the fungus.
Can a hollow tree still be safe? Sometimes. Trees naturally hollow with age, and safety depends on the thickness of the remaining sound wall relative to the trunk diameter.
How much does a decay assessment cost? A basic visual assessment may cost a few hundred NZD. Resistance drilling or tomography adds cost and is usually reserved for large or high-value trees.