Assessing elm clones
Below are the factors which seem relevant in selecting elm clones for landscape use in this country. They certainly overlap with the criteria which have been used by the main breeding programmes, but are not identical. With all respect to the nursery industry which has partnered the tree breeders, its main concern has seemed sometimes to be the production of trained, pruned and much-transplanted identical trees in large sizes and quantities for urban landscaping contracts. That has been the main market, but its pressure may have led to the overlooking of clones which could offer more to the rural landscape.
This must come first. An elm which dies of elm disease is an elm which should never have been planted. A new hybrid needs at the very least to demonstrate a level of resistance which suggests that it will rapidly recover from infection in the field, sustaining no lasting damage.
Resistance is assessed by inoculation trials. The hybrid to be tested is propagated by means of cuttings. They are planted out, ideally with control clones whose resistance/susceptibility is well established. Commonly chosen control clones include Commelin (highly susceptible), Lobel (moderately resistant) and Sapporo Autumn Gold (highly resistant).
Around the second or third year, all trees in the trial are inoculated with a mix of selected strains of the pathogen including those most aggressive, in artificially high concentration. The European inoculation method involves making a single cut in the upper third of the plant using a sharp knife on which two drops of the inoculate have been placed.
The timing of this operation has been shown to be crucial. The inoculation is generally performed in mid to late May, which is the period of the elm’s greatest susceptibility to infection. Later inoculation may coincide with slowing growth rate and therefore result in symptoms far less severe than would have been produced earlier in the season.
Independent observers assess the plants for wilting and defoliation a month after inoculation, and for dieback one year later. English elm would typically show defoliation of over 90%, and dieback of more than 85%. In the earlier Italian trials, clones showing dieback of less than 25% were provisionally assessed as “resistant”. At the extreme, the Italian clone FL 493 exhibits defoliation of only 1.44%, and no dieback at all, and at least one other clone developed in Florence is free of all symptoms.
Since the aim is to produce a replacement for a tree which was the largest living thing in most of the landscapes which it dominated, clones need to appear likely to grow big. Only time will tell whether or not they in fact will. Hybrids are frequently vigorous, but vigour does not necessarily translate into ultimate size. However that may be, in any programme of hybridisation there will be some seedlings which are the opposite of vigorous, and it seems reasonable to regard these as holding little interest.
The elm impressed though its combination of size and form. A clone’s mature form only becomes clear once there are mature specimens to be observed, since it is the product of numerous variables. Nevertheless, there are certain key characteristics which need to be present from an early stage if a pleasing final form is to be even a possibility. Most significant of these is that a clone should be monopodial.
A monopodial tree is one in which the crown is borne on a single upright stem (the trunk), which may persist some way within the canopy. Even in mature trees there can be room for debate about whether they are monopodial or not; doubtful examples include London Plane, and Huntingdon Elm itself. In saplings, it is a matter of judging what happens once the stem has forked; will one side of the fork establish apical dominance over the other, or will the plant simply continue to bifurcate and grow in the manner of a bush? The tendency to produce strong but arching shoots (typical of Japanese elm) suggests that the mature tree will be spreading. Stout, upright growth may seem the best promise of an ultimately monopodial habit but in fact it is not reliably so; the result may be competing leaders of equally strong growth later opening up a canopy and depriving it of coherence (this was often the case with Dutch elm). Curiously, a leading shoot which droops to one side may produce the most persistent of stems, as it seems to in FL 462.
This is the windiest part of Europe, and gales during the growing season seem to be becoming more frequent. If a clone makes rapid upward growth, it needs to have a strong enough structure to be able to support itself in all conditions. Root development must be optimal. Beyond that, a plant’s increase in girth must balance its height increment. One promising elm clone with a serious imbalance here is Patriot, which requires careful management in the nursery to equip it to cope in the field. Early lignifying of the current year’s leading shoot is an additional aid to stability, as is early maturing of the trunk, preferably with basal buttressing. The development of side shoots on the current year’s leaders is a positive feature, moderating extension growth and lowering the plant’s centre of gravity.
Adaptability and general resilience
Elm is an inherently unfussy tree with regard to climate and soil type, well known for withstanding urban pollution and maritime exposure. Most of the modern clones are likely to be similarly tolerant. Nevertheless, weaknesses do emerge from time to time. They need to be monitored, or at least taken into account when matching trees to sites. Some examples follow. Columella has poor drought resistance. Accolade is a martyr to attack by snails in early spring. Several Italian clones resent waterlogging in winter. Valley Forge can be torn apart by high winds. Pioneer is at risk from general decline. Lutece is favoured by scale insects. The new growth of the very promising FL 493 needs watching for infestation by blackfly. Most elms perform disastrously if ever allowed to become pot-bound.
Richness of foliage was the essence of elm’s contribution to the summer landscape; a clone which lacks it would have to have other remarkable virtues to be worthy of consideration. Several factors are involved. Perhaps surprisingly, size and colour of leaf are not among the most significant. More important promoters of the desired effect are: persistence of leaves within the canopy and absence of blind buds; shortness (or absence) of leaf stalks; variation of leaf size and orientation within the spray; depth and fineness of leaf serration; absence of undue recurvature, folding or undulation of leaves. The ability of the plant to produce epicormic growth adds to the impression of a full canopy, helping to make up for any death of twigs at its centre. In so far as size of leaf matters at all, the positive characteristics above are more often found in combination with smaller rather than larger leaved clones. Excessive large leaves can also make young growth vulnerable to damage. Preferred colour is fresh green in spring, and considerably darker in summer.
Budburst and leaf-fall
English elm was distinguished by early budburst and late leaf-fall. Of the two, late leaf-fall seems the more important. The spectacle of the U. americana “Princeton” leafless in mid September is a depressing one. Delayed budburst merely postpones enjoyment of the elm’s spring show, though it may also suggest a flowering season too late to be of use to the elm-exclusive White Letter Hairstreak butterfly. Excessively early leafing, found in some Japanese elm cultivars, makes for susceptibility to late frosts.
The numerous elm paintings of Rowland Hilder testify to the impact which the English elm made when leafless in winter; visit Immemorial Elm for some examples. Apart from form (and the cohesion of the crown in particular), the key indicia are the fineness of terminal twigs and the powerful character of the trunk and main branches. English elm develops a robust trunk with semi-mature bark at a very early age; this seems to be linked to its production of patches of corky bark on young branches. Several modern elm hybrids are notably smooth-trunked. Clones with Asian ancestry generally acquire characterful bark early on. Aesthetically, a contrast between the tracery of delicate twigs and the rustication of the trunk is what is wanted.
Trees which sucker from the roots do not endear themselves to some, but this was a famous feature of English elm and most field elms. It was the means by which they managed to maintain themselves unaided and unchanging in hedgerows from generation to generation. An elm which does not sucker is one which will have to be replaced at the end of its life, and probably will not be. To rely on reproduction by seed is unwise for two reasons. One is that most elm produces little if any viable seed in this climate. The other is that even if fertile seed is set, it is likely to result from the spontaneous crossing of the selected clone with wild trees, and resulting seedlings may well have inherited reduced disease resistance. Suckering promotes the appearance of a strong and rustic base in a tree; it is also provides reproduction without genetic assimilation.
Once a selection of apparently suitable clones has been made, they should be tested over a period, ideally of years, in adaptation trials. These involve planting replicates of each clone in a randomised lay-out in grid formation on a range of soil types. Annual height and girth increments are then recorded and compared with a view to identifying elite clones; these will be the ones which show robust growth, adaptability to contrasting conditions, and pleasing appearance.
Remarkably little of this has yet been done in England, despite the fact that this country was arguably more severely affected by elm disease than any other in Europe.