Winter inspection limits on a tree survey

A tree survey carried out in January is perfectly usable for design. What it cannot do is tell you everything a summer inspection would, and the winter tree inspection limits need writing into the report rather than left for a consultee to point out. The survey categorises trees under BS 5837:2012, fixes stem diameters and root protection areas, and lets the layout proceed. The parts that depend on foliage, fungal fruiting bodies and full crown assessment are weaker, and an arboricultural report that pretends otherwise gets challenged.

Most schemes can live with that. The problem arises when a dormant season survey is used to justify removing a tree, or to conclude that a tree has no bat roost potential, without a caveat.

What can and cannot be seen without leaves

Leaf off conditions help with some things. The branch architecture is visible, so structural defects, included unions, cavities, old pruning wounds and deadwood are easier to see from the ground than in July. Ivy is still there, but the crown behind it reads more clearly.

What is lost is the physiological picture. Crown density, leaf size and colour, dieback in the outer crown, and the general vigour of the tree are all assessed from foliage, and none of them is available in winter. A tree in early decline can look structurally sound in February and show a thin, small-leaved crown in June.

So the categorisation under BS 5837 is at some risk in marginal cases. A tree that would be category C on its condition may be recorded as B, or the reverse, and the estimated remaining contribution in years is a weaker judgement.

Species identification in winter

Most native species can be identified from bark, bud, twig and form by an experienced arboriculturist. The difficulties are narrower than people assume, and they cluster in three places: young trees of similar genera, ornamental cultivars in garden settings, and the hybrids that occur in planted landscapes.

Where species identification affects the outcome, say so. It matters for a few practical reasons: ash and the management implications of ash dieback, poplar and willow near drains and foundations, veteran species that might trigger a habitat consideration, and any tree whose identity affects whether it is an ancient or veteran specimen. On those, an identification made from twigs in February should be flagged for confirmation in leaf.

Detecting decay and fungal signs

This is the real limitation. Many of the decay fungi that matter in arboriculture fruit in autumn, and their brackets can be gone or degraded by mid winter. A survey in February may find no fruiting bodies on a tree that produced a substantial crop of them in October.

That has two consequences. A clean winter inspection is not evidence of the absence of decay, and it should not be written as if it were. And where a tree is proposed for retention close to a new building, the inspection should note the limitation and recommend either a revisit in the autumn or a more detailed assessment using decay detection where the target value justifies it.

Basal and root decay is harder still in winter, since ground vegetation is low but soil conditions are often wet, and root plate movement is easier to miss. Where the tree sits within the influence of new foundations or drainage, that context makes the assessment more important rather than less, as our note on trees, soakaways, sewers and root ingress sets out.

Bat roost potential in the dormant season

A preliminary assessment of a tree for bat roost features can be carried out at any time of year, and winter has advantages, because cavities, splits, lifted bark and woodpecker holes are more visible without foliage.

Two cautions. Bats use trees in winter, so features are not necessarily unoccupied, and a tree assessed as having roost potential needs an assessment by a suitably licensed ecologist before any works, at the right time of year for the survey required. And an assessment from the ground with binoculars is a preliminary exercise; where a tree with high potential is to be felled, aerial inspection or emergence and re-entry surveys in the active season are the next step. Emergence surveys run in the warmer months, which is a programme constraint rather than a choice, and the bat survey page explains the sequence.

Never write that a tree has no roost potential on the strength of a winter walk-round alone. That is the conclusion most likely to be challenged by an ecological consultee, and rightly.

Caveats a survey should record

A dormant season report should state, in the limitations section rather than a footnote:

1. The date and the season of inspection, with a note on ground conditions and weather.
2. That the inspection was a ground level visual tree assessment, with the equipment used stated.
3. That foliage was absent, so crown condition, density and vigour could not be assessed.
4. That the absence of fungal fruiting bodies at the time of inspection is not evidence of the absence of decay.
5. Any tree whose species identification should be confirmed in leaf.
6. Any area of the site that could not be accessed, with the reason.
7. The period for which the survey is considered valid, and the trigger for a revisit.

Validity is worth being explicit about. Councils commonly treat arboricultural information as current for around a year, and a survey carried out two winters before the application usually attracts a request for an update. Where the design has moved in the meantime, an update is needed anyway.

When a summer revisit is needed

Not every scheme needs one. Where the survey supports a layout that retains trees at a comfortable distance and removes nothing of consequence, the winter survey plus honest caveats is enough.

A revisit in leaf is worth programming where:

• a tree proposed for removal is category B or better and the justification rests on its condition
• a retained tree sits close enough to new construction that its tolerance of the works matters
• a tree has features suggesting decay that could not be assessed in winter
• the site contains ash, or any species whose management is driven by a disease the survey could not evaluate
• the council's own arboricultural officer asks for one, which happens more often than not where removals are proposed

Build the revisit into the programme rather than treating it as a contingency. The cost of a second visit is small against a validation delay or an objection from the tree officer.

Programming around the limitation

Winter is a good time to survey because it is the quiet season for arboricultural consultants and because the survey output, the tree constraints plan, is what the designer needs to start. Do that work in the dormant season by all means.

Then sequence the rest. Detailed condition work, decay investigation and any ecological survey that depends on the active season go later, and the application either waits for them or is submitted with the limitation stated and the further work offered by condition. Councils accept the second approach where the removals are uncontroversial and resist it where they are not.

Where tree work is to be specified from a winter survey, the specification itself needs care, as our piece on specifying crown work so contractors price it correctly explains. And if the survey supports a protection scheme, the method statement and protection plan can be prepared from it, which is what the tree survey page covers in more detail.

One honest point. Some arboricultural reports carry a generic limitations page that says nothing about the actual survey conditions. A tree officer reading a February survey with a boilerplate caveat treats the whole report with more suspicion than one that states plainly which three trees need a summer look.

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