Friday, 10 February 2012
Disease-resistant cultivars could help the elm to launch a comeback, says Miranda Kimberley.
Since the virulent outbreak of Dutch elm disease (DED) in the 1960s that killed more than 20 million elm trees in Britain, gardeners and landscapers have shied away from planting elms, believing them too great a risk. But in recent years, several disease-resistant cultivars have been put forward as a way of reintroducing elm. So are these efforts proving to be successful or not?
DED is caused by two related fungi that are spread by various elm bark beetles. All our main native elms - English elm (Ulmus procera), smooth-leaved elm (U. carpinifolia or U. minor) and wych elm (U. glabra) - are susceptible to the more aggressive DED fungus Ophiostoma novo-ulmi.
Some mature trees escaped the outbreak, such as those in Brighton and Hove, but the council's tree officers must still be vigilant. Native elms also survive in hedgerows as long as they do not grow too tall and attract beetles. There are also pockets of U. glabra in Scotland that remain unaffected, but again the disease is still present.
The majestic elm was an iconic tree in the British landscape, so people are still investigating how we can reinstate it. The East Asian species have built up resistance to the disease so have been crossed with European species to produce resistant hybrids, and cultivars have also been bred from the American elm, U. americana.
Arguably, the most exciting new introduction is U. americana 'Princeton Riveredge', which has been rated by US Department of Agriculture National Arboretum scientists as the most resistant to DED in the USA. Prince Charles has planted an avenue of the trees at Highgrove. Barcham Trees currently has the UK rights to the variety and is bulking up stock with more than 1,500 trees in production.
Another cultivar of note is U. 'Morfeo', which is being championed by Butterfly Conservation. Elm trees are the hosts for the endangered white-letter hairstreak butterfly and the charity has been carrying out trials on all the recently-introduced resistant elm varieties. It has found 'Morfeo' to be the most suitable because its leafing up time suits the butterfly and because it is best suited to the British climate.
Unfortunately, this Italian-bred cultivar is not yet available in the UK. Butterfly Conservation also recommends US-bred 'Valley Forge' and Dutch-bred 'Lutece' and 'Vada'. The charity has concluded that many of the disease-resistant cultivars bred from Asian species are unable to thrive during the wet British winter and on our clayey, waterlogged soils.
Meanwhile, the Conservation Foundation is carrying out the Great British Elm Experiment, which aims to reintroduce native elm across the country. Cuttings have been taken from healthy, mature native elms growing in the British countryside and micropropagated. The resulting saplings are being distributed to hundreds of schools, community groups, local authorities and private landowners.
Participants are asked to log their tree's progress for up to 15 years. To get involved, email email@example.com.
What the specialists say
- Neil Lucas, owner, Knoll Gardens, Dorset:
"One of my first jobs as a young head gardener was to remove more than 400 elms that had succumbed to DED. The resulting flat landscape left a very firm impression on me, with the realisation of just how vital trees are. Like our other great trees - oak, ash and beech - elm is iconic and associated with British history and places of great beauty.
"Although I am very keen on cultivated varieties such as the 'Princeton', especially in urban areas and parks, I always love to see plants that fit into their natural surroundings, so any of the species that can be found in the UK are great for me. Unfortunately, we had to stop selling 'Princeton' several years ago because we were no longer able to get a regular supply.
"Elms are actually very tough and durable but the better we can prepare the planting areas the sooner they will establish. The biggest issue with elms now is disease, which is why truly tolerant ones are so valuable, at least in the interim, in our built landscapes. I am hopeful that many of the planting initiatives currently underway will be successful and of course the English elm survives in hedgerow form almost everywhere. But it is unlikely that the English elm as we knew it will be seen in tree form for some decades to come."
- Simon Toomer, director, Westonbirt Arboretum, Gloucestershire:
"On the one hand, there is a search for disease-resistant elms for arboricultural planting in urban and garden situations. On the other, a strong nostalgic desire to replace the iconic trees of the countryside.
"Our native elms - U. procera and U. glabra in particular - are still abundant in our hedgerows and woods, though they rarely get very large before falling victim to DED.
"There has been an enormous amount of research and effort put into hybridising and cloning elms and even attempts to genetically modify native elms with genes from resistant Asian species.
"The problem is that although resistant trees can and have been propagated, they are not good replacements for our native species since they are quite different in appearance. The question also has to be asked that even if they were, would they replace them ecologically, supporting the same flora and fauna?
"Much of the effort has been focused on using Asian species. The University of Wisconsin has produced a number of hybrids of Siberian elm (U. pumila) and Japanese elm (U. japonica).
"One of the earliest of these was 'Sapporo Autumn Gold', a fast-growing, medium-sized hybrid. Another is 'New Horizon', one of the so-called Resista elms. Other hybrids have been raised from the Chinese species U. chenmoui. These include the Italian cultivar 'Morfeo'.
"Other cultivars arise from more complex threeor four-parent origins. One of the most recent is the Dutch U. 'Nanguen'. Like other newer forms, it will be many years before we know how successful, or even how large, these will be.
"From time to time there seem to be individual trees that survive despite elms all around dying of DED. One such tree is a magnificent Huntingdon elm U. x hollandica 'Vegeta' at Westonbirt. This variety is not usually known for its resistance but has thrived while all other large trees in the arboretum have succumbed. We have repeatedly propagated it and planted its offspring elsewhere at Westonbirt."
Species and varieties
- U. americana 'Princeton Riveredge' currently has the highest rating for resistance to DED in the USA and trees planted there 90 years ago are still healthy. It is a broad-leaf tree, with large, leathery foliage, an upright habit when young and a uniform arching vase shape when mature. Height: 20m.
- U. carpinifolia 'Wredi Aurea' is a slow-growing elm with luminescent yellow foliage. It becomes a tree of small to medium size and oval habit. Its small size probably protects it against DED. Height: 5-10m.
- U. 'Clusius' is a fast-growing Dutch hybrid that becomes a large broadly oval tree. It has large leaves and a broad crown. It has some resistance to DED but is more susceptible than 'Princeton'. Height: 20m.
- U. 'Dodoens' is a very fast-growing tree released in 1973 in Holland. It was derived from a seedling of the U. glabra 'Exionensis' (the Exeter elm) and U. wallichiana.
- U. glabra 'Camperdownii' is a form of wych elm. It is a small, weeping tree with a dome-shaped head and looks good growing in a lawn and parks. It remains neat and compact and is generally considered to be resistant to DED.
- U. x hollandica 'Jacqueline Hillier' is known as the dwarf elm. It is a slow-growing, rounded, bushy shrub with green leaves. Height: 2m. Spread: 2m.
- U. 'Lobel' is a Dutch hybrid cultivar. It is a large and fast-growing narrow columnar tree that is notably late to flush with leaves rarely developed before mid May. It has proven resistant to sea winds.
- U. Lutece = 'Nanguen' is a disease-resistant hybrid that has very similar leaves to the field elm, U. minor, bright-green in colour with a rough upper surface and serrated margins.
- U. 'New Horizon' is a vigorous, large tree of upright habit with a straight central leader and a dense conical crown. It is a cross between U. pumila and U. japonica, both of which are tolerant of DED.
- U. procera is the native English elm that has suffered so terribly from DED. An imposing tree with a dense broad-headed crown, it has dark-green leaves that have toothed edges and are slightly hairy underneath.
- U. 'Sapporo Autumn Gold' is a fast-growing, medium sized tree of spreading habit with good resistance to DED. It is a hybrid of the Japanese elm and the dwarf Siberian elm, U. pumilia. The leaves are glossy and green, turning a warm yellow in autumn. Height: 15m.
Thank you to Floramedia, which supplied images for this article from its photo library www.floramedia-picture-library.com
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