How do I tell if a tree in my garden is dangerous?

Most garden trees are perfectly safe and stay that way for decades. The ones that fail usually gave signals for a long time first, and almost all of those signals are visible from the ground to someone who knows what to look at. The legal position is worth knowing too: an owner has a duty to take reasonable care that their tree does not injure people or damage property, and 'reasonable' generally means periodic informal inspection, with professional assessment when something looks wrong.

Safety

What you'll need

• Binoculars
• A camera
• A mallet or a stick (optional)

Step by step

1. Look at the whole tree from a distance first.Stand back far enough to see the outline. You are looking for asymmetry, a lean that does not look natural, a thin or dying section of crown, and any obvious hanging or broken limbs.
2. Check for a new lean and disturbed ground.Trees that have always leaned are usually fine — they have grown reaction wood to cope. A new lean is different, and the sign to look for is at ground level: cracked or heaved soil on one side, exposed roots lifting, or a gap opening between the trunk base and the soil on the opposite side. That combination is serious and urgent.
3. Inspect the base and the root flare.The flare where the trunk widens into the roots should be visible and sound. Look for fungal brackets, soft or missing bark, cavities, ooze, sawdust from boring insects, and mounded or sunken soil. Base decay is the most dangerous kind because it affects the whole tree.
4. Look for fungal fruiting bodies.Brackets, crusts and clumps of toadstools on the trunk, the base or the ground over the roots. Some fungi are harmless recyclers of dead wood and some indicate serious internal decay in living structure. Photograph what you find rather than trying to identify it yourself.
5. Look at forks and unions.Where two stems of similar size meet, look for a tight V-shaped junction with bark trapped inside it — included bark — and for cracks running down from the fork. These unions fail in wind, and a tree with two co-dominant stems and included bark over a house is a common and genuine hazard.
6. Scan the crown for deadwood.Binoculars. Individual dead branches are normal on a mature tree. Widespread dieback across the top of the crown, thin foliage compared with previous years, or large dead limbs over a target area all warrant attention.
7. Look for hanging and broken limbs after wind.A partly detached branch caught in the canopy is the most immediate hazard on this list. Do not go under it. Do not try to pull it down. Keep everyone away and ring an arborist.
8. Check the trunk for cavities, cracks and bulges.Vertical cracks, sunken or dead strips of bark, seams, bulges where the tree has grown extra wood around a weakness, and open cavities. Trees compartmentalise decay well and a hollow trunk is not automatically dangerous, but it is a reason for a professional to look.
9. Think about the target.Risk is hazard multiplied by what is underneath. A defective tree in the middle of a field is a low priority; the same tree over a bedroom, a pavement, a play area or a power line is not. Prioritise honestly by what the tree could hit.
10. Record what you find and when.Dated photographs and a note. If a tree ever does cause damage, evidence that you inspected and acted reasonably matters, and it also lets you see change year on year.
11. Get a qualified arboriculturist for anything on this list.Look for someone with a recognised arboricultural qualification and professional membership, with public liability insurance, who will produce a written report. A tree report costs a small fraction of the consequences of getting this wrong.

Tips

• Inspect twice a year: once in full leaf, when crown dieback shows clearly, and once in winter, when the branch structure and any cracks are visible.
• Walk round after every serious storm and look up. Most failures that hurt people are hanging limbs left in place after wind.
• Ivy on a trunk does not kill a tree, but it hides everything on this checklist. Clearing a band of ivy from the base and lower trunk makes inspection possible.

Common mistakes

• Assuming a hollow tree must come down — Trees are engineered as tubes and a hollow trunk with sound outer wood can be perfectly stable. It needs assessing, not automatically felling.
• Ignoring soil cracking at the base — It is the clearest sign of root plate movement and one of the most reliable predictors of whole-tree failure. Treat it as urgent.
• Waiting to see if a hanging limb comes down on its own — It will, and there is no way of choosing when or onto what.
• Assuming a protected tree cannot be dealt with — There are exemptions for dead and genuinely dangerous trees, and councils deal with urgent cases quickly. Photograph the defect, contact them, and get professional evidence.

If it doesn't work

Questions people ask