Ash Dieback

Ash Dieback - Guidelines

This document is intended to help people with responsibility for Ash trees to make good decisions in response to Ash Dieback

'I have Ash trees in the garden, they are showing signs of Dieback:

What should I do?
'

Short answer

It may sound Like a silly question but are you sure it’s an ash tree?

Are you sure it is Ash Dieback? There are other conditions which cause similar symptoms. However, if your Ash trees do have Ash Dieback they are likely to fail.

If they are near property or busy places, failure could be catastrophic, and you are responsible for keeping people and property safe.

If their location means that they pose little threat to people or property when they fail
 whilst you are still responsible for that risk, it may be reasonable to leave them to decay, and to wait for the inevitable associated failures.

In most garden settings the most sensible option is to take action sooner rather than later. 

Longer answer

Ash Dieback fungus (Hymenoscyphus fraxineus) is an infection from Asia. It is not a major problem there because the native Ash trees and the fungi have evolved together. The fungus uses the leaves, and the tree’s resistance prevents the infection from invading the whole tree. Here in the UK, native Ash trees have not evolved alongside this fungus, and most of our ash tree population doesn’t have adequate resistance. 

There are exceptions: some of our tree stock appears to have resistance to the fungus.  The good news is that we can expect seeds from these trees to restock our woodland over time, and scientists are cautiously estimating that we have more resistant trees than the scant 5% predicted to survive when Ash dieback first appeared in Europe. Currently, up to 30% of trees may well have resistance to the disease.

Dieback is often spotted as the ‘twigs’ at the tips of the canopy appear dead and over time the extent of the Dieback increases. There are other possible causes of Dieback in trees including drought stress, compaction from heavy machinery, weedkillers or pathogens that damage roots which in turn can impact on canopy condition, so it may not be Ash Dieback that you observe. A small amount of Dieback is no reason to panic, but warrants investigation.

Generally, Ash Dieback enters the tree through the leaves and gradually migrates its way down the tree. It has now been recognised that the infection can establish itself in the bark at the base of the tree: it kills the bark, creating an opportunity for secondary infections (typically bark beetles and honey fungus) which will often kill the tree.

There is a lot of good information on Ash Dieback online, but make sure that what you are reading is up to date and from reputable sources.

eg, Forestry Commission, Cumbria Woodlands, Woodland Trust

What else should I consider?

Make sure that whatever you do is based on research and taking any risks into account. This may include consulting a specialist where appropriate.

Consider age or size of Tree, How old/big are the trees?


Current research suggests that very large trees, to some extent, may well cope with the infection, but the risks need to be managed.  Medium size trees, once infected are unlikely to survive.  Small trees, once infected, are likely to die quickly. (Some recent research in the UK has found arborists have had some success by pruning out dieback in large trees before it gets to established.)

Consider how established is the dieback

What season is it?
 It can be easier to observe the impact of Ash Dieback in the spring and summer months. Dead twigs and branches have no leaves or live buds.

How significant is the decline? 
Has the tree lost a small percentage of its canopy or most of it?


There is a correlation between tree health and likelihood of failure. The more advanced the decline the more likely the failure. Ash trees are known to become particularly dangerous - both through sudden failure, and by being more dangerous to fell once diseased - so early intervention is often significantly safer, and cheaper.. 

Consider the risk the tree/s present. ‘If this tree were to fail, what damage would it do?’


Trees can fail by dropping limbs or falling over. Trees falling over can ‘throw’ branches so a reasonable distance to consider when making this judgement is one and half times the tree height.  This may be a lot further than you realise.

Trees that are near or over property, roads or busy places are likely to cause harm in the event of failure. As such, you should consider the potential for harm to be significant, and you should plan intervention sooner rather than later. Possible interventions include, monitoring, pruning, felling/removal, pollarding or restricting access. 

If the tree will need to be dealt with it is safer and often significantly cheaper to do the work sooner rather than later - the longer you leave a dangerous tree, the more expensive it will be to deal with. Dead and dying ash trees are particularly unpredictable and hazardous to work on. Please don’t underestimate this. UK government guidelines are that ash trees in decline should only be dealt with by trained and experienced foresters and arborists. The risks can be very high so I would strongly suggest that unless you are an experienced professional tree worker you don’t attempt to deal with trees with Ash Dieback yourself.

Roadside trees

Roadside trees are particularly important. Ash Dieback means that tree failure is inevitable - work should be done as soon as possible after Dieback is noticed. Roadside work often needs traffic management which in turn can allow the use of cherry pickers, cranes, winches and other machines. The costs of this work can escalate quickly so it is important that the work is managed ethically with efficiency in mind. If you have roadside trees with dieback the highways agency or the council may require you to undertake work. Failing to do so will mean that their contractors will undertake the work and you will be charged. We I have found that sometimes this provides good value for money and sometimes it doesn't