Mature trees are remarkably resilient. They shrug off storms, droughts and the occasional careless mower, and many will outlive the people who planted them. But when a tree does start to decline, it almost always leaves clues. Learning to spot them gives you time to act – whether that means improving growing conditions, arranging remedial work or, in the worst case, planning a safe removal.
Here are nine signs worth looking for on a regular walk around the garden. None of them is automatically a death sentence, but each one deserves a closer look.
1. Fungal brackets or mushrooms at the base
Fungi are the visible fruiting bodies of an organism that is mostly hidden inside wood or soil. Shelf-like brackets on the trunk, or clusters of honey-coloured toadstools around the base in autumn, can indicate decay in the roots or heartwood. Some fungi are harmless decomposers of dead material; others, such as honey fungus and certain bracket species, can seriously weaken a tree. Photograph anything you find and have it identified.
2. Dead or hanging branches
A few dead twigs inside the crown are normal. Large dead limbs, particularly over paths, patios or roofs, are not. Branches that have snapped but remain lodged in the crown (sometimes called “hangers”) are especially hazardous because they can fall without warning in the next gust of wind.
3. A thinning or patchy crown
Compare the canopy with neighbouring trees of the same species. Smaller leaves, sparse foliage, dieback from the tips of branches or early autumn colour in mid-summer all suggest the tree is under stress. Common causes include root damage, drought, compacted soil and disease.
4. Cracks and splits in the trunk
Vertical cracks, deep seams or splits where two main stems meet are signs of structural weakness. Pay particular attention to tight V-shaped unions between co-dominant stems, where bark can become trapped between them and prevent a strong joint forming.
5. Cavities and soft wood
Holes in the trunk are not always a problem – hollow trees can stand for centuries – but the amount of sound wood remaining around a cavity matters. If wood crumbles when pressed, or a cavity is growing year on year, an arborist can assess how much strength is left.
6. A new or increasing lean
Many trees grow at an angle towards the light and are perfectly stable. What should concern you is a change. If a tree that used to stand straight now leans, or an existing lean has become more pronounced after a storm, the root plate may have shifted.
7. Heaving soil or exposed, damaged roots
Look at the ground on the opposite side of any lean. Cracked or raised soil can indicate that roots are lifting. Damage from trenching, driveway construction or changes in ground level within the root zone is one of the most common causes of decline in urban and suburban trees, often appearing years after the work was done.
8. Bark loss, cankers and weeping
Areas where bark has fallen away to reveal bare wood, sunken lesions (cankers) or dark fluid oozing from the trunk can all point to disease. Bleeding cankers on horse chestnuts are a familiar example. On ash, diamond-shaped lesions around side shoots and blackened, wilting leaves are classic signs of ash dieback, which has affected huge numbers of ash trees across the UK.
9. Pests in large numbers
Most insects on a tree are harmless or beneficial. Heavy infestations, sawdust-like frass around holes, or tell-tale D-shaped exit holes in bark are different. Report suspected new or notifiable pests to the relevant plant health authority; early reports help protect trees nationally.
What to do if you spot a warning sign
Start by recording what you see. Date-stamped photographs taken from the same spot each season make it far easier to judge whether a problem is static or getting worse. Then consider the target: a declining tree in the middle of a field is a very different risk from one leaning over a children’s play area or a public footpath.
- Low risk, low target: monitor, improve soil conditions with mulch, avoid compaction and water young trees in dry spells.
- Visible defect near people or property: ask a qualified arborist for an inspection. Look for recognised qualifications, appropriate insurance and work carried out in line with the British Standard for tree work (BS 3998).
- Immediate danger, such as a split limb over a road: keep people clear and seek professional help promptly.
Prevention beats cure
The healthiest trees are those whose roots are left alone. Keep heavy vehicles, skips and building materials off the root zone, resist the urge to pile soil against the trunk, and replace close-mown grass under the canopy with a ring of organic mulch. These small habits do more for long-term tree health than almost any treatment applied after the fact.