Ash dieback and its effect on retention decisions

Ash dieback changes a retention decision from a question about amenity to a question about how many years the tree has left, and the answer is usually fewer than the ten years that BS 5837 uses as its retention threshold. That matters because a heavily affected ash falls into category U, outside the design process, while the same tree is still protected by a Tree Preservation Order or by conservation area status until the council says otherwise. The disease does not remove the consent requirement.

How the disease progresses and what it means for retention

Hymenoscyphus fraxineus, the fungus behind ash dieback, was first confirmed in Britain in 2012 and is now widespread across England. It kills through progressive crown dieback over several seasons, and then through basal lesions at the collar which open the tree to secondary infection by honey fungus and other decay organisms.

The retention consequence follows from the failure mode rather than the appearance. Affected ash becomes brittle, and it fails suddenly rather than gradually. Whole-tree collapse and the shedding of large limbs happen without the warning signs a decaying tree normally gives, and the wood becomes unsafe to climb, which means the felling window for conventional climbing arboriculture closes while the tree is still standing.

That produces a practical sequencing point that catches owners out. A tree that could be dismantled by a climbing team this winter may need a mechanical platform or a harvester next winter, at several times the cost. Where removal is inevitable, doing it earlier is usually cheaper and safer.

A small proportion of ash shows tolerance and retains a healthy crown while neighbours decline. Those individuals matter for the species' long-term recovery and are worth retaining where the target beneath them allows it.

Categorising an affected ash in a survey

BS 5837:2012 sorts trees into categories A, B, C and U, with category U covering trees that cannot realistically be retained as living trees in the context of the current land use for longer than ten years.

An ash with extensive crown dieback and basal lesions is a category U tree, and categorising it as C to preserve a landscape argument is a mistake that will be found. An ash with light crown dieback and no basal lesion may still be category B or C, with a note on its condition and a recommendation for re-inspection.

Three things the survey should record for every ash, beyond the usual data:

1. An estimate of crown dieback as a proportion of the crown, taken in full leaf, since a winter assessment cannot distinguish dieback from dormancy.
2. Presence or absence of basal lesions at the collar, which is the feature that changes the risk profile most.
3. A stated re-inspection interval, because the condition of an affected ash population changes materially within a single growing season.

Date the assessment prominently. An arboricultural report on ash written two years ago describes a different population from the one on site, and an officer is entitled to ask for an update. Set the assessment within the wider Arboricultural Impact Assessment rather than treating the ash separately.

Protected trees and the exemption question

This is where owners most often act unlawfully in good faith.

The Town and Country Planning (Tree Preservation) (England) Regulations 2012 exempt cutting down a tree that is dead, and works that are urgently necessary to remove an immediate risk of serious harm. A dying tree is not covered. The word "dying" was in earlier exemptions and is not in the 2012 Regulations, so an ash with 60 per cent crown dieback is protected and needs consent unless it presents an immediate risk.

Two procedural points go with that. Removal of a dead protected tree requires five working days' notice to the authority before the work, except in an emergency where notice is given as soon as practicable afterwards. And where a protected tree is removed under either exemption, section 206 of the Town and Country Planning Act 1990 imposes a duty to plant a replacement tree of appropriate size and species at the same place as soon as reasonably practicable.

For a tree in a conservation area that is not covered by an order, the route is a section 211 notice giving the authority six weeks to consider making an order, with the same dead tree exemption and notice period.

If the tree really is an immediate risk, act and document: dated photographs of the defect, the arboriculturist's written opinion, and a record of the notification. The exemption is judged on the facts at the time, and a file assembled afterwards is worth much less than one assembled on the day.

Where the felling involves volume rather than individual trees, a Forestry Commission felling licence may also be needed, since felling more than five cubic metres in a calendar quarter, of which no more than two may be sold, falls outside the exemption. Our note on woodland orders, felling licences and how they interact covers the overlap.

Risk management along highways and footpaths

Ash lining a road or a public footpath is the highest priority in any affected population, because the occupancy beneath it is continuous and the failure mode is sudden.

The landowner's duty here is the ordinary duty of care to those who may be affected, and a local highway authority has powers to require the removal of a dangerous tree overhanging a highway. In practice the sequence is an inspection of every ash within falling distance of the carriageway or footway, a prioritised removal programme, and a record of both.

Two practical points. Removal beside a highway needs traffic management, which adds cost and lead time and is worth programming across a whole frontage rather than tree by tree. And the inspection interval for roadside ash should be annual at minimum while the disease is active in the population, because a tree assessed as safe in one season can be a different proposition in the next.

Where an affected ash stands over a development site, the occupancy question is the same one set out in our note on tree risk and target zones, and the layout is what changes it.

Retaining standing deadwood where safe

Not every affected ash needs to come down to ground level, and this is where a decent submission distinguishes itself.

Standing deadwood is valuable habitat for saproxylic invertebrates, for hole-nesting birds and for bats, and ash is a good host. Where the target beneath is low, in a field corner, a woodland interior, a hedgerow away from a path, a monolith left at a safe height retains most of that habitat value.

The technique is to reduce the tree to a stable stem, sometimes with short limb stubs retained, at a height that cannot reach anything if it fails. It has to be done while the wood is still sound enough to work, which returns to the timing point.

Where the tree is protected, monolithing is still works to a protected tree and needs consent, and it is usually easier to obtain than felling because the authority keeps the habitat and loses only the crown. Set it out as the proposal rather than as a concession.

Replacement species and diversity

Ash is generally excluded from new planting schedules, and replacing it like for like is not an option worth arguing for.

Species choice depends on the site rather than on a list. Field maple, hornbeam, small-leaved lime and oak occupy similar niches on drier soils, alder and willow on wet ground, birch as a fast-establishing pioneer where quick screening is needed. Native species of local provenance are what an ecologist will specify for hedgerow and woodland replacement.

The harder lesson from ash dieback is about concentration rather than species. Replacing an ash monoculture with a maple monoculture repeats the mistake, and several authorities now condition a cap on the proportion of any single species in a replacement planting mix. Accept that willingly: it is the right answer and it is easier to agree at the outset than to vary later. The stock size and establishment requirements that go with it are covered in our note on tree planting conditions: species, stock size and establishment.

Where the loss affects a habitat unit, the effect runs through the statutory metric, and a Biodiversity Net Gain Assessment has to account for both the condition of the existing habitat and the replacement. That work is for a competent person under the Environment Act 2021, with the biodiversity gain plan approved by the local planning authority.

Explaining the loss in a submission

Councils are familiar with ash dieback and they are not suspicious of it. What they are suspicious of is an application that uses the disease to justify clearing trees that were never affected.

So evidence each tree individually. A tree schedule with the category, the dieback estimate, the basal lesion observation and the survey date. Photographs of the affected trees, taken in leaf. A plan distinguishing ash from other species and affected ash from unaffected. And a short statement of the retention decision for each tree, including the monoliths and the tolerant individuals you propose to keep.

Then the replacement: species, numbers, sizes, positions and the management period, cross-referenced to the Landscape Strategy.

An application that keeps two tolerant ash and three monoliths, and explains why, reads very differently from one that removes every ash on the site and cites the disease.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This article is general guidance, not legal advice; councils usually expect a suitably qualified arboricultural author, so professional review is strongly advised, and consent requirements for protected trees should be confirmed with your local planning authority before any work.

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