Ash Dieback

What is Ash dieback?

Ash dieback is a fungal disease that affects Ash trees across the UK and Europe. It is thought to have arrived in the UK around 20 years ago from fungal spores carried by wind from the continent as well as from infected tree planting stock imported from mainland Europe. The Ash dieback fungus was known as Chlalara fraxinea but its scientific name is now Hymenoscyphus fraxineus.

The fungus is thought to have originated in Asia and their native Ash have evolved to live with this fungus. In mainland Europe however, our Ash trees are succumbing to the infection and up to 90% of Ash trees have been killed by the disease in some areas. It is now found throughout Wales.

Disease in Ash trees results from the fungus getting into the leaves and twigs, then causing damage to the essential water and food transport systems under the bark. The leaves wither and turn black and lesions (dead or dysfunctional patches) appear on the bark.

The loss of leaves and damage to the basic functioning of the tree will lead to a gradual dieback in the crown, sometimes showing as bronze-tinged twigs in the winter canopy. The trees become brittle and branches can snap off. In woodland situations this may not be a problem but in some places the trees need to be managed appropriately. Severely affected Ash trees are at risk of collapsing, presenting an immediate danger to the surrounding area.

Only Ash trees (Fraxinus species) are affected by this fungal disease. The Woodland Trust have a useful page on their website to help you identify Ash trees. 

You are not legally required to take any particular action if you own infected Ash trees, however, the normal legal duty of care applies - see below. Natural Resources Wales may serve you with a Statutory Plant Health Notice (SPHN) requiring action. This, though, is very unlikely in South Wales as the disease is now widespread.

With the exceptions of felling for public safety (under a duty of care) or timber production, Natural Resources Wales advise a general presumption against felling living Ash trees, whether infected or not. This is because there is good evidence that a small proportion will be able to tolerate H. fraxineus infection. There is also the possibility that a proportion of Ash trees can become diseased, but then recover to good health. These, too, would be valuable for forestry research, although it is still too early to know whether there are such trees in the British Ash population.

How do I recognise Ash dieback?

Ash trees are usually quite late to leaf up compared with our other British natives. So, it is from June onwards that the conspicuous blackening leaves are most noticeable. For very young trees this, and the diamond shaped lesions on the twigs or stem are the key features of early infection.

The disease causes the tree to die back from the edge of its canopy. In mature trees, the first sign of the disease is bare, dead twigs at the top of the tree and ends of the branches. As the disease progresses and the number and length of dead branches increases, the tree responds by growing new leaves closer to the main branches and trunk giving the tree a clumpy ‘pom-pom’ look. Eventually the tree will look increasingly bare and dead. (Remember that in the autumn and winter clumps of sometimes dark-coloured Ash keys (seeds) are retained on the trees after the leaves have fallen. This is quite normal, but from a distance they can be mistaken for the blackened leaves.)

Ash dieback can also lead to obvious discolouration, cracking and death of the bark at the base of the trunk. An inspection should take note of fungal fruiting bodies and black “shoelace” features, all of which indicate the tree has been colonised by a secondary fungus, hastening the decline of the tree. Ash trees with these symptoms have a higher risk of sudden death and collapse, so should be a priority for safety works if in a location which poses a risk to public safety.

The Woodland Trust has a useful section on their website, with photographs clearly showing the symptoms to look out for.

Whilst it is likely that many of our Ash trees will succumb to this fungus, there is hope and signs that some trees show a level of resistance. In order to find the resistant trees, it is important to retain as many Ash trees as possible where this is safe. Indeed, Ash trees which do not display any symptoms of the disease acquire an increased value in terms of public amenity and environmental diversity.

What is Caerphilly County Borough Council doing to manage this problem?

The Council is directly responsible for all the trees growing on land that it owns or holds in the county borough. This ‘estate’ is very diverse and includes urban parks, country parks, housing areas, highway land, cemeteries and care homes etc. Each site has its own requirements, opportunities and constraints in relation to trees. As of 2016 the current estimated number of trees is 260,000 though this number is expected to rise as wider-reaching surveys are undertaken in the future.

The Council, as the Highway Authority, also assesses trees that are within falling distance of the Highway. These trees may not be the council’s responsibility to manage and so the landowner is made aware of any danger and asked to take appropriate action for the safety of the highway users. The Highway Authority has powers under the Highways Act 1980 to serve notice on owners to abate the nuisance or foreseeable hazard to the highway. This will include Ash trees in classes 2-4 (see below).

The Council will not become involved with disputes between private landowners regarding tree health issues and duty of care (unless the tree concerned is a foreseeable hazard to an adjacent public right of way, highway, or other place of lawful public access).

The Council employs professional staff, suitably qualified and experienced in arboriculture (the care and management of trees). To assist with the management of the tree stock, and to meet the Council’s duty of care, a computerised tree management database is used. The recorded information includes details of a tree’s location, species, age and useful life expectancy, condition, recommendations for any necessary works, and the value of an individual tree as an amenity / environmental asset.

Tree inspections are undertaken across the county borough but especially in those areas where trees could pose the greatest risk to people or property. Other management issues are also identified and addressed as far as possible. Pro-active or ‘planned’ inspections are based on a ‘zoned’ approach according to the site usage, visitor/user frequency, and associated risk assessment.

We aim to inspect HIGHER USE zones every two years. These include:

  • transport routes – A-roads & B-roads
  • schools
  • housing estates (communal areas) and OAPs/sheltered housing provision
  • municipal parks
  • cemeteries

We aim to inspect MEDIUM USE zones every four years. These include:

  • country parks
  • libraries
  • public open space with large mature trees
  • playgrounds
  • social services establishments
  • car parks
  • Council-held corporate sites
  • Monmouthshire & Brecon Canal

For LOWER USE zones our inspections are based on expediency:

  • All other sites
  • Transport routes - C-class and unclassified roads
  • Public Rights of Way

We categorise the disease in Ash trees using a system of Class 1 to Class 4 (see graphic below). Based on this simple visual method for identifying potentially hazardous Ash trees, our Tree Inspectors, Highway Inspectors and Grounds Maintenance staff can take appropriate action on both planned and ad hoc inspections of council trees to ensure that the trees are managed appropriately.

Depending on more detailed inspection of the tree and the tree location, management options may include crown reduction pruning or pollarding to make the tree safe, rather than felling it at ground level. Ash trees support a wide range of associated biodiversity, such as lichens, bryophytes and animals that breed, roost and shelter in the trees. Standing deadwood is a very valuable habitat but it must be where it is safe to retain it. Veteran trees are also of significant wildlife value and considered irreplaceable assets, especially those that are very old or considered ‘ancient’ trees.
Disease in ash trees

What do I need to do?

If you have trees on your land, especially Ash trees showing symptoms of Ash dieback, that could potentially fall on neighbouring land, roads, right of way or property, it is important that the trees are assessed by a suitably qualified and experienced arboriculturist (a list of some local reputable practitioners is available on our Tree Management FAQs page of the website) to establish their health and the level of risk they pose. The risk presented by a tree in poor health or with dead or dying branches depends on its location as well as its condition. Trees in poor condition close to areas of high use (for example, main roads) will pose a much higher risk than trees in fields, hedgerows and woodlands away from houses and public rights of way.

As trees are valuable for wildlife, then consideration should be given to reducing the risk from the tree by pruning work, rather than felling it entirely, if this is viewed as a safe option. Where there are no targets for the tree to damage, then you are not required to fell the tree, even if it has Ash Dieback, (unless you are served with a Statutory Plant Health Notice from Natural Resources Wales directing you to do so).

Under the Occupiers’ Liability Acts 1957 and 1984 and the Health and Safety at Work Act 1974, landowners have a legal requirement to ensure that ALL trees on their land are maintained to a safe standard and do not pose a risk to the public. If a tree with obvious defects causes damage to people or property when it falls, or its branches fall, it is highly likely that the landowner could be legally liable for such damage. It is important that landowners take appropriate action to assess the risk posed by all trees on their land, including trees in poor condition along boundaries.

If pruning or dismantling work is seen as the best option (for reasons of reducing risk from the tree) then a suitably qualified arborist (tree surgeon) should be employed to do the work. They will need to have valid public liability insurance. A reliable arborist will not be offended if you ask to check their insurance certificates. There is a list of some suitably qualified local arborists on our list here, and the Arboricultural Association also has details of tree contractors that work to the highest safety and professional standards.

Some sources say that Ash dieback causes structural change to the timber. Trees may become unpredictable and are sometimes unsafe to fell from ground level. It is recommended that only suitably experienced, qualified and equipped tree surgeons or contractors fell or dismantle severely infected trees.

What is my Duty of Care?

Private landowners have a duty of care under common law to ensure they do all that is reasonably practicable to prevent injury or damage to neighbours or their property. They also have a duty of care towards visitors to their land, including trespassers, under the Occupiers Liability Acts. The Highways Act also requires landowners to ensure their trees do not endanger people on roads and footpaths.

Businesses have additional obligations under the Health and Safety at Work Act to ensure their workplaces are safe.

For single householders your duty of care may require you to regularly check the condition of your Ash trees (and any other trees), or to engage a suitably qualified professional to do this for you. Taking photographs of the tree at various times of the year, each year, will help give you a better understanding of how any disease is progressing. Comparing the photos will instantly show if there has been some decline in tree health or whether now is the time to take urgent action.  

For larger landowners and businesses, you (or a professional on your behalf) will need to:

  1. Identify how many Ash trees you have

  2. Assess their current condition

  3. Identify where affected trees pose a risk, such as adjacent to roads or schools

  4. Make safe any hazardous trees based on sound Arboricultural advice from a professional Arboricultural consultant (see also Felling licences)

  5. Monitor on an annual basis – photographs are a very useful visual aid to assessing decline

The optimum time to survey for Ash dieback is between the end of June and September as the dieback will be most apparent while the tree is in ‘full leaf’. The optimum time to fell trees if necessary is from September to February inclusive as this avoids the bird nesting season.

Please note, Ash dieback causes structural change to the timber. Trees may become unpredictable and may be unsafe to fell from ground level. It is recommended that only suitably experienced, qualified and equipped tree surgeons or contractors fell or dismantle severely infected trees.

Tree Preservation Orders and Ash Dieback

A Tree Preservation Order is placed on a tree (or group of trees) to protect those that make a significant impact on their local surroundings. It is an order made by a local planning authority which makes it an offence to cut down, top, lop, uproot, wilfully damage or wilfully destroy a tree without the planning authority’s permission.  Please refer to Caerphilly County Borough’s Tree Management Frequently Asked Questions for more information, including trees in Conservation Area which are also protected from felling.

Even if the Ash tree is showing symptoms of Dieback, an application for consent to undertake pruning work will be required. Each tree will be assessed on its own merits.  There is no charge for submitting an application for TPO work.

Under an exemption in the legislation any protected tree that is dead, dying or dangerous can be removed without the need to submit an application. However, unless the tree is imminently dangerous, please give five days’ notice so that we can arrange any inspections needed. This is in your interests – you could be prosecuted if you have carried out unauthorised work or have used the exemption without good reason.

We strongly advise that removal (or any other emergency work) is undertaken by reputable and suitably insured tree surgery contractors. A list of contractors is available on our Tree Management FAQs page. Alternatively, email to request a copy.  If you think your protected Ash tree is dead, dying or dangerous, please email us at and include photographs of the tree from various angles so that we can assess the tree’s condition, as part of the 5 days’ notice requirement.

Remember that you are not obliged to fell the dead or dying tree to ground level if there are no vulnerable targets (roads, paths, property). You should consider retaining the tree where possible, as veteran trees are a valuable, irreplaceable habitat.  

Tree felling licences and Ash Dieback

Depending on the volume of timber that is to be felled, you may require a felling licence to remove Ash trees on your land, even if they are infected with Ash dieback. Information on felling licensing can be found within the guidance provided by Natural Resources Wales - Tree Felling: Getting Permission. Below is an extract from the document. 

“If your trees become infected by H. fraxineus then you will need to consider public safety and monitor your trees, particularly in areas with high levels of public access. NRW issues felling licences to fell growing trees, and there is no exemption under the Forestry Act 1967 to fell diseased trees.”

“Felling necessary for the prevention of danger or the prevention or abatement of a nuisance.

This exemption will only apply if there is a real rather than a perceived danger, or a nuisance as recognised in law. You may be required to provide evidence that the trees presented a danger, for example through an accredited arboriculturist’s report or photographic evidence. A diseased tree is not necessarily dangerous. We strongly recommend that you contact us (Natural Resources Wales) if you are considering felling a tree or trees you consider to be dangerous. We may be able to give you advice that would minimise the danger without any felling. You may be prosecuted for illegal felling if it is shown that the tree or trees did not present a real or immediate danger, or they did not present a nuisance as recognised in law”

Tree Felling: Getting permission

Protected species legislation

Nesting birds and their eggs, chicks and nests are legally protected. Undertake tree felling and pruning work outside the bird nesting season (1st March to 31st August) wherever possible.

Before felling any mature trees, these should be checked for potential to support roosting bats. Bat roosts are protected even when not occupied. You will require a licence from Natural Resources Wales to destroy a resting place or breeding site of any species of bat. For more information see Natural Resources Wales - Bat Licensing. Natural Resources Wales (NRW) grants licences so you can work within the law.

How should I manage my woodland?

If you have any roads or footpaths through or next to your woodland, you should inspect any trees that are within falling distance of them as a priority. Similarly, if you have neighbours who would be harmed from trees that may become a hazard, then these should be inspected as a priority. If any trees are found to be dead, then action will need to be taken to safeguard any targets.

Always take photographs of the tree (before you take any action) for you records, as Natural Resources Wales will need to know if there have been any breaches of regulations relating to felling licences (see section above). It may not be necessary to fell such trees to ground level, but they should be reduced (shortened) in height and/or spread to avoid any vulnerable targets.  Retaining the remainder of the tree is important for wildlife, as many species rely on veteran trees and decaying wood.  

Woodlands with Ash Dieback need management and will require input from Natural Resources Wales. The following link may provide some guidance.

Chalara manual - 2. Managing ash trees and woodland, including logs and firewood

Information gathering for the CCBC database

To help us manage the large amount of information we receive about Ash trees in poor condition we need help from you. Below we have listed some of the information that will help us take the best course of action. We cannot respond to all correspondence relating to Ash queries, but be assured that we will be assessing any trees you tell us about, and will be contacting owners where this is necessary, in order to keep people and property safe from harm of falling trees or branches.

To help with our admin it would be really helpful if you could please put the following heading into your email:
This concerns an Ash tree

In no particular order, the following information could be put in your email to help us:

  • Have you checked that the tree is an Ash tree? Mountain Ash / Rowan are not in the same family as Common Ash and so are not affected by Ash dieback. Take a look at the Woodland Trust web site to help you confirm that the species is correct
  • Where is the tree? This would include street name, post code and any local features such as a bus stop, numbered streetlight etc. Or you could send a simple sketch plan
  • Send a photograph of the whole tree and the parts that are believed to be affected.
  • What girth is the tree, in centimetres? Put a tape measure around the trunk at about shoulder height. If not possible to measure, is it too big to hug with your fingers touching? Your outstretched arms are roughly the same as your height!
  • Does it have one or more stems (trunks) from ground level?
  • Are there areas of the canopy with less leaves or smaller leaves than the remainder of the tree?
  • Has the tree shed any large branches in the last 12 months? (Of diameter at least 1 inch/25mm or greater)
  • Is the canopy very gappy with significantly less leaves than normal in July/August? Or even completely bare?
  • Does it have epicormic growth [relatively small, young shoots, growing profusely] from the trunk or main branches? Don’t worry if you are unsure – the photo will help us
  • Date of photograph (remember that Ash trees rarely have all their leaves before May – that is normal for the species)
  • Does the tree have lesions at the base or on the main trunk? Again, don’t worry if you don’t know, but if something looks odd please take a photo for us
  • Are there any fungal fruiting bodies (mushrooms or brackets) on or around the base or trunk of the tree?
  • Are there bootlace/shoelace type strands around the base of the tree or visible beneath any loose or missing bark?
  • Looking at the chart below (and if you feel confident to assess it) which Class do you think it falls into? Class 1, 2, 3 or 4? (Or can’t tell? It takes a while to get your eye in!)
  • Do you think it is a council tree or privately owned?
  • Contact details – in case we have more questions (we will not be sharing your data outside of the organisation, nor necessarily giving replies or acknowledgements due to time constraints for Arboricultural staff)
  • If you are a Housing tenant, please submit your report to the Area Housing Office
  • Any other information you think we may find useful? This could include dead branches, fungi on or around the tree, telephone wire or electric wires through the branches/canopy, missing bark, finger sized holes into the trunk, emerging beetles or grubs, builders’ rubble or fly tipping around the base of the tree
  • Do you know of recent excavation, trenching, or construction work, beneath the extent of the tree’s canopy? Are/were there any vehicles, storage containers, or other heavy installation placed beneath the tree’s canopy in recent months?
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