M ost dangerous trees give warning long before they fail. The problem is that the signs are easy to miss if you don’t know what you’re looking at, and the dramatic-looking ones (a hollow trunk, a lean) are often less urgent than the subtle ones (a fresh bracket fungus, a hairline crack at a stem fork). This is a homeowner’s checklist of the ten signs that matter most, with the right response for each.
Why visual inspection matters
Under the Occupiers’ Liability Acts 1957 and 1984, any landowner with a tree close to a path, road, building or boundary has a duty to take reasonable steps to make sure it doesn’t hurt anyone. The courts have been consistent: reasonable means a proportionate system of inspection, not professional surveys every month. For most domestic trees, the starting point is a confident walk-around once or twice a year, by the owner, looking for changes.
What a homeowner cannot do is assess internal decay, root-plate stability or wood quality. That requires equipment — a resistance drill, a sonic tomograph, sometimes a root-zone excavation — and the training to interpret what they show. What a homeowner can do is notice that the tree has changed since last year, and call someone competent before that change becomes a failure.
The ten signs below are what an arboricultural consultant looks at first on any inspection. None of them are a death sentence on their own. Several together, on the same tree, are a different matter.
Signs on the trunk
Cracks are the most important thing to look for, and the easiest to miss. Vertical cracks running for more than a foot or two along the trunk indicate the wood is splitting under load. Spiralling cracks, where a line of bark separation winds around the stem, are worse — they usually mean the tree has rotated under wind load and the fibres are tearing internally. A fresh crack at a fork between two co-dominant stems is the classic precursor to a major limb failure.
Hollows and cavities are less alarming than they look. A tree can carry a significant internal cavity for years, provided the remaining shell of sound wood is thick enough. The rule of thumb arborists use is that the residual wall thickness should be at least one-third of the radius. Less than that, and the tree starts behaving like a hollow tube under load.
Fungal brackets on the trunk are the sign most worth knowing by name. Ganoderma applanatum (a flat, woody, brown-and-cream bracket, often near the base) indicates white rot of the heartwood. Inonotus hispidus (shaggy, orange when fresh, blackening with age, higher up the stem) is common on ash, beech and London plane and causes brittle fracture. Meripilus giganteus (a large rosette of overlapping cream-brown fronds at the base) is the classic indicator of root and butt rot in beech — a tree with Meripilus is a tree to take seriously. Armillaria, or honey fungus, fruits in autumn as clusters of honey-coloured mushrooms at the base; the black bootlace rhizomorphs under the bark do the actual damage.
Signs at the base and roots
Look at the ground around the trunk. Recent heave — cracking, doming or lifting of the soil on one side, particularly opposite the direction of any lean — means the root plate has started to move. That is the single most urgent finding on any tree. A tree with a moving root plate is hours or days from failing in the next bit of wind.
Exposed roots are common where soil has eroded or been removed, and not necessarily dangerous. What matters is whether the root collar (the flare where the trunk meets the roots) has been damaged or buried. A buried root collar, often the result of soil being heaped up during landscaping, causes the bark to rot and the structural anchorage to weaken over years.
Recent work in the root protection area is one of the most under-appreciated risks. The root protection area (RPA) is a circle around the trunk with a radius of twelve times the diameter of the trunk at breast height. Excavation, paving, compaction or trenching inside that zone — even years before — routinely lead to failures of otherwise healthy-looking trees. If a driveway, extension or service trench has gone in near a mature tree in the last decade, that tree warrants a closer look.
Co-dominant stems with included bark are the other base-level concern. Where two stems of similar size fork from the same point and the bark has grown inward at the union (rather than forming a smooth U-shaped collar), that fork is mechanically weak and likely to split. Common in lime, sycamore and many garden cherries.
Signs in the crown
Dieback in the crown — bare twigs at the canopy edges, sparse leaves, dead wood scattered through the upper branches — is the visible end of a process that started elsewhere. The roots, the trunk or the conducting tissue have all stopped supporting the outer crown. By the time the crown is visibly thinning, the underlying problem is usually well established.
Unexplained limb drop is the most unnerving sign. Branches falling without storm or obvious cause, on still summer days, is a phenomenon arborists call summer branch drop. It’s real, well-documented, and particularly common in mature oak (especially Quercus robur), beech, horse chestnut and some poplars. The mechanism isn’t fully understood, but moisture stress, internal cracking and rapid temperature changes all seem to play a role. A tree that has dropped a major limb in still weather should be inspected before the next storm.
Finally, any tree that has been through a major storm in the last twelve months and hasn’t been looked at since deserves a check. Storm damage often leaves hanging limbs, partial breaks at major unions and root-plate movement that isn’t obvious from the ground.
When to act — three categories
Slight concern: the tree has a feature you noticed and want explained, but it’s not changing visibly and there’s no obvious target underneath it (no path, no shed, no neighbour). Action: book a written tree-condition survey at your convenience, typically within two to three months. Cost: £120–£300 depending on the tree.
Moderate concern: the tree shows two or more of the signs above, or one significant sign (a fresh bracket fungus, an unexplained lean, a fork with included bark over a building). Action: get a contractor out for a closer assessment within a couple of weeks, and don’t park or sit underneath it in the meantime.
Acute concern: visible lean that wasn’t there before, fresh root heave, a major crack that’s opened in the last storm, large fungal brackets at the base, a partially detached limb hung up in the canopy. Action: treat as immediately dangerous. Cordon the area, keep people and vehicles away, and ring an emergency tree surgeon. Most contractors in our network can attend within hours.
What a tree-condition survey covers
A proper survey is a written document, not a verbal opinion. It identifies each tree by species and approximate age, records the relevant dimensions (height, diameter at breast height, crown spread), describes the visible defects, assesses the targets within falling distance, and gives a recommendation — monitor, remedial work, fell — with a timeframe.
For a domestic survey, ask the surveyor what qualifications they hold. The relevant credentials in the UK are LANTRA Professional Tree Inspection (PTI), the Royal Forestry Society’s Certificate in Arboriculture, or membership of the Arboricultural Association as a Registered Consultant. A contractor who only holds practical chainsaw tickets (CS30, CS31) is qualified to do the work, but not necessarily to write the report.
The report should be valid for two to three years on a stable tree, less on a tree with active decay. Keep copies for your insurance file. If something later goes wrong, having a recent inspection on record is the difference between a contested claim and a covered one.