When to commission advanced tree decay detection

A tree decay detection survey is worth commissioning when a visual inspection has raised a question it cannot answer, and not before. That is a narrower trigger than the equipment suppliers would like. Most trees on most sites can be assessed from the ground by a competent arboriculturist, and the decision to retain, reduce or remove follows from what they can see. Advanced investigation earns its fee in the minority of cases where a fungal fruiting body, a cavity, a lean or a history of failure means the honest answer from a visual inspection is "I cannot tell from here".

What a visual inspection establishes, and where it stops

A visual tree assessment reads the tree's own response to defects. Reaction wood around a wound, bulging at the base, bark ridges along a crack, adaptive growth above a cavity: a tree under stress lays down timber where it needs it, and a trained eye reads that as evidence about how long the defect has been present and how the tree has compensated.

What visual inspection cannot do is quantify what is left. A hollow stem with a sound outer wall may be perfectly safe. A stem with intact bark may be substantially decayed inside where a root-collar fungus has worked upwards. The visual method also struggles with decay that causes brittle fracture without obvious external symptoms, which is why Kretzschmaria deusta at the base of a mature beech makes experienced arboriculturists uneasy in a way that a bracket of Ganoderma, with its visible basal swelling and slower progression, often does not.

So the question to put to your arboriculturist is not "is the tree safe" but "what would you need to measure to move from your current level of confidence to a defensible decision".

Indicators that justify going further

Advanced investigation is proportionate when there is both a reason to doubt the structure and a target worth protecting. Both halves matter, because a decayed tree in the middle of a paddock needs no instrument.

• A fruiting body of a decay fungus at the base, on the buttress roots or on the stem, particularly where the species is associated with root or butt rot.
• A cavity or old pruning wound whose depth and extent cannot be seen or safely probed.
• A recent change in lean, soil heave on one side of the root plate, or cracked ground at the base.
• A crack or seam running vertically on the stem, or a included-bark union on a major fork over a target.
• A history of branch failure in the same tree, or failure of a neighbouring tree of the same species and age class.
• A tree proposed for retention within a development layout where the design depends on it surviving construction.

That last one is the planning case rather than the safety case, and it is where this work most often earns its keep. If the scheme keeps a tree, the council will expect the retention to be credible, and a condition report that says only "appears in fair condition" will not carry a layout built around it. The categorisation in a BS 5837:2012 survey is about quality and suitability for retention, which is a different judgement from structural condition, and the two are often reported together on the tree survey submitted with an application.

Drill resistance and sonic methods compared

Two families of technique dominate practice in the United Kingdom, and they answer different questions.

Electrical resistivity measurement is sometimes run alongside tomography to distinguish wet or active decay from old dry cavities. Where root anchorage rather than stem integrity is in question, a static load test, pulling the stem under measured force and recording root plate movement and stem strain, addresses the question the other methods cannot. Root investigation by careful excavation with an air tool is the right answer where buttress roots have been severed by earlier works.

Coring with an increment borer still has a place for identifying the decay organism, but it removes a cylinder of wood and opens a pathway, so it is a last resort on a tree you intend to keep.

Interpreting the output rather than the picture

The tomogram is seductive. Coloured zones look like fact, and clients read them as a diagnosis. They are a visualisation of sound velocity, filtered through the software's interpolation, and they need a practitioner to translate.

Three things determine whether the result supports retention. The position of the decay matters more than its volume, because a hollow centre with a sound wall behaves differently from an eccentric lesion on the tension side. The decay type matters, because white rot, brown rot and soft rot degrade different wood components at different speeds and brittle fracture failure gives no warning. And the loading matters, since crown size, sail area, exposure and the presence of a target decide the consequence of failure.

The residual wall thickness guidance that circulates in arboriculture, usually quoted as a wall of around a third of the stem radius, was offered as an indicator rather than a pass mark. Treat any report that applies it as a numerical test, without discussing loading or decay type, as incomplete.

Ask for the raw data and the reasoning behind it rather than the image alone. A report that states the method, the equipment, the height and orientation of each measurement, the operator's interpretation and a recommended re-inspection interval is a document you can rely on later.

Cost, access and timing

The fee is a day rate plus equipment, and the access cost usually exceeds the investigation cost. A highway tree needs traffic management. A stem defect eight metres up needs a climbing inspection or a platform. A root plate investigation needs an air tool, a vacuum and somewhere to put the soil. Get the access requirement priced separately so you can see what you are really paying for, in the same way you would with crown work, as covered in our note on specifying crown work so contractors price it correctly.

Timing is more flexible than with ecology, since stem investigation can be carried out year round. Two seasonal points still matter: many decay fungi fruit in autumn, so an autumn inspection may find the evidence that a February visit misses, and assessment of crown condition needs the tree in leaf.

Using the result to justify retention

Where the investigation supports retention, say so in terms the council can condition. That means a stated inspection interval, any crown reduction needed to lower the loading, and a commitment to re-inspect after construction. A retained tree inside a development needs its protection detailed and discharged properly, which is the subject of our note on discharging a tree protection condition without delay.

Where the investigation supports removal of a protected tree, the application for consent under the Town and Country Planning (Tree Preservation) (England) Regulations 2012 should attach the full report and the data. A bare assertion that a tree is dangerous is routinely refused. Note that the exemption for dead trees requires five working days' notice before the work, except where the work is urgently necessary to remove an immediate risk of serious harm, and that the burden of proving an exemption applies falls on the person who did the work.

When the council disputes your finding

Expect a tree officer to test the evidence, which is their job. The disagreements that go anywhere are about interpretation rather than the numbers.

Respond with the data, the method and the operator's competence, and offer a joint site visit before it escalates. Where the officer's own assessment is a ground-level inspection and yours is instrumented, say so plainly and politely. If consent is refused, there is a right of appeal to the Secretary of State, and the appeal turns on the quality of the evidence rather than on who sounded more certain. Where the tree sits within woodland, check how the order and felling licence regimes interact before taking either route, as set out in our piece on woodland orders and felling licences.

A tree officer who sees a thorough report, including the parts that weaken your case, will trust the next one you send. That is worth more than winning a single argument.

Enter your postcode on PlanScope to see which tree and landscape documents a site is likely to need before you commission anything.

This is general guidance, not legal advice, and decisions about tree safety and protected trees are site specific, so take advice from a suitably qualified arboriculturist and check the position with your local planning authority before relying on it.