FAQ

Wych elm (Ulmus glabra) is native to the uplands and the north and west of the United Kingdom. Most other elms, including English elm, are now thought to have been introduced by man. Spanish-led studies have established that English elm reached this country from the Iberian peninsula, to where it had been brought from central Italy for use as a vine prop. Other field elms probably came from northern France and Germany. That accounts for all except hybrids between Wych elm and field elm which probably arose here.

That is an impossible question to answer. There is not even agreement on how many elm species are present. Some would say there are two – wych elm and field elm (of which English elm is a distinctive strain), plus interspecific hybrids between them. Others count as many as twelve species. Most would accept that the following strains of elm can reliably be identified: wych elm, field elm (also referred to as narrow-leaved or smooth leaved elm), English elm, Dutch elm, Huntingdon elm, Plot elm, Jersey elm, Cornish elm, and Goodyer’s elm.

Wych elm and field elm have broad ranges on the continent, stretching into Asia. English elm occurs sporadically in central Italy, Spain and Portugal, and has been planted locally in many other parts of the world (notably the Azores and Australia). Dutch elm, Jersey elm and Cornish elm can be found on the near continent. Plot elm, Huntingdon elm and Goodyer’s elm are not found elsewhere.

No. “Dutch” or “x hollandica” is the conventional designation for any hybrid between wych elm and field elm.

All the elms which have traditionally grown in this country are susceptible to elm disease, Dutch elm included. The disease acquired the name “Dutch” because it was first systematically studied by scientists in the Netherlands.

Yes, most notably in the elm conservation area centred on Brighton, where there are good examples of all the main types and of many lesser known elms. The wych elm can still be found in parts of its range. A few field elms still exist, chiefly in the eastern counties of England and notably on the Abbots Ripton estate in Huntingdonshire where conservation measures were practised for many years. Some specimens of Huntingdon elm, Jersey elm and possibly Cornish elm remain. The English elm has been eliminated as a mature tree from its entire range beyond the Brighton area.

Probably the much-planted resistant hybrid “Sapporo Autumn Gold” if rounded in shape, or possibly the older Dutch hybrids “Lobel” or “Dodoens” if more upright..

Elm disease is a fungus (Ophiostoma novo-ulmi) which affects the tree’s vascular system, causing it to produce a gummy substance (“tyloses”). The result is that the sap flow is cut off and the tree dies, at least down to ground level.

The fungal spores are spread by a small winged beetle, Scolytus, which feeds in the crotches of elm twigs from April to early October. The beetle breeds under the bark of dying or recently killed elms. The new generation of Scolytus emerges the following spring, covered in fungal spores, and infects more trees by feeding. The disease can also travel through the linked root systems of adjacent trees, though this may be less common than is supposed.

To put it simply, East Asia, but this subject is still being intensively studied and is one area in which the major contribution to global knowledge has been made by UK scientists.

The catastrophic outbreak of the disease in the second half of the twentieth century resulted from the import of unbarked and infected rock elm logs from Canada. However, elm disease in a less virulent form was present earlier in the twentieth century and killed several million trees. There is thought to have been an outbreak in the nineteenth century, and it has been suggested that much earlier falls in elm pollen deposits could have resulted from elm disease epidemics.

Outbreaks of a less aggressive form of the disease have petered out, though even they did widespread damage. The current form of the disease, which is the result of mutation, is highly aggressive, and there is no reason to think that it will simply fade away.

The beetle population does plummet at the end of a severe outbreak. However, all that is needed for the population to be sustained at a low level is a small amount of recently dead wood, and this can be of surprisingly small diameter to support over-wintering. The cycle of tree death and regrowth from the roots will not march in step across the elm’s entire range, so that some Scolytus can always maintain itself outside a devastated area, and some can always re-enter an area in which there is new elm growth. There, the population will gradually increase again.

That is a function of the beetle population cycle and the elm regrowth cycle interlocking. After a disease flare-up, Scolytus will locally die out, though some will move on. The elm regrowth has a few years of low infection rates as a result, and appears substantially healthy. But the Scolytus population is steadily rising again. Soon there will be an optimal amount of dead wood for over-wintering, the beetle population will explode, and all the elms will be infected.

In theory it could be, provided every single fragment of dead or dying elm wood could be destroyed in the course of a single winter. That is of course completely impracticable. An alternative, again theoretical, is to attenuate the aggressiveness of the fungus to make it less good at killing trees. Unfortunately, a less effective fungus will lose out to a more effective one through natural selection.

In effect, the answer to this is the same as to the question above. Sanitation, which is the removal of all breeding sites (felling and burning affected trees) is a theoretically valid means of disease control. The problem with it is that it can never be carried out sufficiently rigorously over a sufficiently large area to be truly effective. A scolytus beetle can travel 20 miles or more on a strong wind. A recent statistical analysis of the main disease outbreak concludes nothing could have been done to stop it.

By rigorous sanitation, probably in combination with partial geographical isolation and wind patterns which tended to route Scolytus elsewhere. The fact that the elm population of Brighton is very mixed probably also helped. Brighton is only a qualified success, however, even though it is a unique one. A great many elms, particularly English elms, have been lost there, and the enclave is under constant threat.

Protection by fungicidal injection is possible, but laborious, costly and damaging to the tree. It provides protection for only two years before needing to be repeated, and is only suitable for larger trees. Various more sophisticated inoculation protocols have been developed, but the general view is that they are not effective. In theory a tree which has only the very beginnings of infection can be cured by pruning out the affected part together with about six times as much healthy growth below it. However, two factors militate against this working. One is that where there is visible wilting, there are probably a great many as yet unseen feeding wounds all over the crown. The second is that any damage to an elm, pruning included, chemically advertises it to all scolytus in the vicinity.

Some have a level of natural resistance. Wych elm is very susceptible, but is not preferred for feeding and can therefore escape. English elm is both very susceptible and much preferred for feeding. Huntingdon elm and some field elms have a degree of resistance. They may escape infection for years, but then succumb quickly to it, or they may die only very slowly from it. A very few seem to be able to tolerate it, though often much damaged by it. Some field elms are unaffected by disease, but this may merely indicate isolation, luck, or “senile escape syndrome”. French studies indicate that virtually no traditional strains of elm assessed have reliable field resistance.

English elm will certainly not, because it never reproduces sexually. Every new generation is genetically identical to the previous growth, and development by mutation is therefore impossible. Field elm rarely reproduces sexually in this country. Wych elm shows little genetic variation. The natural development of resistance seems highly unlikely.

In theory, yes. Transgenic English elm was successfully produced by Professor Kevan Gartland at the University of Abertay in 2004. It is not clear whether it was resistant, because no trials are reported to have taken place. Experiments along these lines appear to have been abandoned in a climate of national opinion unfavourable to genetic modification.

What is usually meant is that a surviving elm has been found (almost invariably a field elm – never an English elm), and its finder assumes or hopes that it has survived because it is resistant. But an elm may avoid the disease for several reasons other than inherent resistance. Field elms are markedly less susceptible to infection when mature than they are when young because their pattern of growth changes as they develop. Young clones of such trees may well be very attractive for feeding, and fare far less well than those who popularise or plant them would hope. An elm should not be claimed as resistant unless that has been proven by inoculation trials.

Yes. There are now many hybrid elms which have been bred to resist the disease and scientifically proven to do so, though none can accurately be described as immune.

The great majority of them are crosses between the traditional elms and elm species from elsewhere in the world which have an elevated natural resistance to the disease. Of the latter, the main resources used were Ulmus pumila (the Siberian elm), Ulmus japonica (the Japanese elm), Ulmus wallichiana (the Himalayan elm), and Ulmus chenmoui (the Chenmou elm). Another component in some of the breeding programmes was Ulmus glabra “Exoniensis”, a small wych elm unique among traditional strains in having a significant level of disease resistance.

There were elm breeding programmes in Holland, Italy, and the United States. The Dutch programme, carried on for 70 years, was essentially a horticulturally-driven project. The Italian programme was part of a broader study of many aspects of the elm , and used the full range of elm species. One of its aims was to produce trees which would thrive in Mediterranean climates. US elm breeding was a response to the disastrous loss of some 100,000,000 American elms, and a number of programmes ran at various locations in the States.

There is a great deal that could be said in answer to this. For one thing, a significant number of our most prominent trees in this country are not native (planes, the chestnuts, cedars, larches to name but a few) but seem not to have done any harm. Far from it – the diversification of our tree stocks is a guard against just the sort of devastation wrought upon the (non-native) English Elm. If the new trees are able to thrive here, as seems likely, and pose no threat to existing ecosystems, there is no reason to shun them. In fact there are native species of lepidoptera and lichens which are severely at risk in the near-absence of mature elm, so a concern for native species positively favours the planting of replacements.

Since most of them are comparatively recent developments and have yet to reach maturity, it is impossible to be precise. For the same reason the question of their ultimate size is largely a matter of educated guesswork. Dutch breeding tended to favour the production of upright trees for urban planting or formal landscape designs such as avenues. The Italian programme also valued upright growth. However, the range of materials in use ensured a variety of outcomes. American breeding has always prized the iconic “vase shape” of Ulmus americana, but resulted in some trees which were rather closer to European types. It is unlikely that any of the existing hybrids is going to look exactly the same as an English elm, or grow so large. However, just about any elm is going to look more “elm-like” than any other tree, and there is a wide range of specific characteristics from which to choose.

Chiefly because of lack of knowledge within the domestic trade and the presumed lack of a market. The later Dutch products were taken over by a French body which did very little to promote them. The American programmes were insular, and in addition the difficulties in importing plant material from the States are daunting. The Italian programme has done much more to maintain links with experts and enthusiasts outside Italy, but was not primarily interested in commercial sales abroad. Add to this the fact that elms were not being sold because none had been sourced, and it was easy to conclude that there was no demand.

All the clones which seem fit for landscape planting are covered here. There are many more which have been excluded because in my assessment they have major defects – eg. insufficient disease resistance – and I could not counsel anybody to plant them except on a purely experimental basis.

Please see the “Availability” section of this website. In most cases, I can either obtain trees from specialist nurseries (if they are subject to plant breeders’ rights) or propagate them myself to order if they are patent free.

No. Its primary purpose is to make the new trees better known, and to try to dispel some of the misunderstandings around the subject of elm disease. I source or propagate the modern elm hybrids because I consider that someone in this country ought to do so. At present it is a very low-key hobby mainly serving the needs of like-minded enthusiasts, but I am also involved with larger landscaping projects.