Ash dieback

What is ash dieback?

Ash dieback is a disease which may eventually kill up to 90% of one of our most common trees, ash (fraxinus excelsior). It is caused by a fungus, hymenoscyphus fraxineus (previously known as 'chalara'), that originates from East Asia where it coexists with the native Ash tree species.

It was introduced to the UK in 2012 by an infected tree plant import and has subsequently spread rapidly through the movement of its airborne spores. First recorded in Denbighshire in 2016 it is now widespread.

There is no known cure or method for stopping its spread.

What are the symptoms?

The main symptom is dieback in the crown (top of the tree), including leaf loss and wilting, dead twigs and branches. Trees infected with the disease are more easily identified in the summer when they are in leaf and should have a healthy crown.

On younger trees dark lesions (diamond shaped) can be seen at branch junctions and at the base of the trunk. As the disease progresses it kills the bark and trunk resulting in the death of the tree.

Once affected by dieback, ash trees can be more vulnerable to secondary infection by, for example, honey fungus. This can speed up their decline.

Ultimately, as infected ash trees deteriorate and die they may fall over or shed large limbs.

For more information please see this guide from the Forestry Commission:

Forestry Commission: ash dieback (external website)

Whose responsibility is it to manage these trees?

Denbighshire County Council is responsible for trees on its land and has a legal duty to maintain the highways and ensure public safety. However, many trees next to the highway are privately owned.

Landowners have a legal duty of care under the Occupiers Liability Act (1957 and 1984) and the Health and Safety at Work act (1974) and can be held liable for any injury or damage caused by a tree in their ownership. Therefore, landowners are advised to commission regular inspections of their trees to ensure they are in a safe condition.

Where necessary, the Council has the authority, under section 154(2) of the Highways Act 1980, to serve notice on a landowner requiring them to make a dangerous tree safe.

Are the trees protected?

Before undertaking any work on trees you should check whether they are protected by law and if permission is needed from the relevant authority. Ash trees that are under a Tree Preservation Order (TPO) or within a Conservation Area will require a formal application to be submitted to the Local Planning Authority.

If a tree is considered to be in dangerous condition, it may be exempt from this procedure, however anyone proposing to undertake works to an Ash tree on this basis is advised to email trees@denbighshire.gov.uk. Find out more about TPOs and Conservation Area Trees:

Tree Preservation Orders (TPOs)

A Felling Licence (Forestry Act 1967) may be needed if you are proposing to cut down a number of trees. Additionally, if the trees are within a protected area, for example a Site of Special Scientific Interest (SSSI), you will need consent from Natural Resources Wales (NRW). More information about getting permission to fell trees can be found on the NRW website:

Natural Resources Wales: getting permission to fell trees (external website)

Are protected species present?

Trees can support species protected under law, such as nesting birds and bats. Where these may be present you should seek advice from Natural Resources Wales (NRW) or an Ecologist. A licence may be needed before any tree work can proceed.

Natural Resources Wales: getting permission to fell trees (external website)

Tree works next to the highway

Ash dieback causes the timber to become brittle, making tree felling and other cutting unpredictable and more hazardous. For this reason, it is especially important to have proper safeguards in place and for such work to be carried out by suitably qualified and experienced operatives.

To ensure the safety of the public and those conducting the work, any tree works adjacent to the highway should be have the necessary traffic management measures in place. Those carrying out the works must contact Denbighshire's Streetworks team at the planning stage by emailing: streetworks@denbighshire.gov.uk.

What is Denbighshire County Council’s response?

Denbighshire County Council’s tree team are surveying ash trees on council land and are monitoring those that are next to the highways or other areas of concern. Where council trees are felled efforts will be made to retain deadwood, which is an important habitat for many species, or to utilise the timber.

Denbighshire County Council is actively planting trees across the county as part of the Climate Change and Ecological Emergency project and has created a plant nursery to grow trees of local provenance, which will improve the biosecurity of future tree planting schemes.

Where trees in private ownership have been felled the Tree Team will be actively encouraging replacement planting.