Ash is an essential feature of British woodlands and produces high quality timber. It has the most advanced breeding programme but is now seriously threatened by ash dieback. We are now working to locate individuals that are tolerant to the disease and understand how this is inherited.

About ash

Ash (Fraxinus excelsior L.) is a highly valued timber and wildlife tree. It is a fast growing species, associated with several forest types. It has a broad distribution throughout Europe and occurs in widely different climates of both maritime and continental influence. It occupies large areas, owing to its presence in both the early and the mature stages of succession. Shade tolerant when young, ash becomes a strong light demander at around seven to eight years old. It rarely forms stands of any great extent, occurring more commonly as a component of mixed broadleaved woodland. Its drought tolerance and frost sensitivity make ash a species likely to be favoured in the short term by climate change, and indeed until recently had been expanding in range in Europe.

However, ash health and survival is currently seriously compromised by ash dieback caused by the fungus Hymenoscyphus fraxineus (asexual anamorph Chalara fraxinea) that has the potential to kill all but a few resistant trees. Moreover, the emerald ash borer beetle Agrilus planipennis, a serious pest of ash species in N. America, has reached Europe (though not yet the British Isles) and poses an equally if not more serious long-term threat to ash.


Interesting fact: ash is a dioecious species and shows a continuum of gender from pure males, through several stages of hermaphrodite, to pure females.

An ash plus tree

Why ash is important

The attraction of ash is that it is native, produces valuable timber on a relatively short rotation, and grows well on suitable sites over much of lowland Britain, although the best ash stands are found on base rich, freely draining but moist soils. It is susceptible to forking, partly due to the ash bud moth (Prays fraxinella), but increasing evidence points to early flushing and subsequent late frost damage as being a significant cause. It is therefore important to plant the right provenances of ash to prevent poor form.


Interesting fact: Ash is the most common hedgerow tree in Britain.

Ash flooring

Our work with ash

Ash was the first species we began work on in 1993, and had the most advanced breeding programme, with the first tested seed for a broadleaved species available in 2012.  Unfortunately, this was the year that ash dieback first arrived in the UK on imported seedlings. Since then, we have shifted our focus on ash from timber traits to tree health and were funded by Defra for five years through The Living Ash Project.  This project, run in partnership with Forest Research, Earth Trust and Sylva Foundation aimed to identify trees with tolerance to the disease.  Using existing trials incorporating over 40,000 trees and citizen science, we identified 412 putatively tolerant trees (about 1%, as reported in other countries as highly tolerant) which were planted on the public forest estate in Hampshire in 2019. We also used seed material from our breeding programme to establish new progeny trials to investigate the heritability of tolerance.

We are now embarking on a second phase of work with ash, again funded by Defra, to test the degree of tolerance in our selections.  Working with Fera, we are screening our selections with liquid chromatography mass spectrometry (LC-MS – a type of chemical fingerprint) to quantify tolerance of individuals selected.  We are also working with Forest Research to carry out controlled inoculations as another means to check tolerance.  Kew are researching vegetative propagation techniques so that we can avoid grafting (and thus potentially the use of non-tolerant rootstocks) in future when we need to bulk up tolerant material. This project runs from March 2020 – 2025.