Elm
Elms are deciduous trees of the genus Ulmus, family Ulmaceae. They have alternate, simple, single- or doubly-serrate leaves, usually with asymmetric bases, often rough with fine bristles. The fruit is a round samara.
There are between 20 to 45 species of elm; the ambiguity in the number is a result of difficult species delimitations in elms, due to the ease of hybridisation between them and the development of local seed-sterile vegetatively-propagated microspecies in some areas, mainly in the field elm group.
Classification
- "Mountain elms": spring flowering; flowers subsessile; leaves very rough above
- Ulmus bergmanniana. Eastern Asia.
- Ulmus glabra - Wych Elm. Europe, western Asia.
- Ulmus laciniata. Eastern Asia.
- Ulmus macrocarpa. Northeast Asia.
- Ulmus rubra - Slippery Elm or Red Elm. Eastern North America.
- Ulmus wallichiana - endemic, often treated as a variety of U. carpinifolia.
- Ulmus canescens. Southeast Europe.
- Ulmus carpinifolia - Smooth-leaved Elm or European Field Elm. Europe.
- Ulmus chumlia. Himalaya.
- Ulmus davidiana. China.
- Ulmus japonica - Japanese Elm. Eastern Asia.
- Ulmus plottii - Lock Elm. England; local endemic, often treated as a variety of U. carpinifolia.
- Ulmus procera - English Elm. England; local endemic.
- Ulmus pumila - Siberian Elm. Northern Asia.
- Ulmus villosa. Himalaya.
- Ulmus wilsoniana - Winged Elm. Eastern North America.
- Ulmus americana - American Elm. Eastern North America.
- Ulmus androssowii - Narwan Elm. Southwest Asia.
- Ulmas laevis - European White Elm or Fluttering Elm. Eastern Europe, western Asia.
- Ulmas thomasii - Cedar Elm. Eastern North America.
- Ulmus parvifolia - Chinese Elm or Lacebark Elm. China.
- Ulmus serotina - September Elm. Eastern North America.
- Hybrids and hybrid origin cultivars
- Ulmus x brandisiana. U. chumlia x U. wallichiana.
- Ulmus x hollandica - Dutch Elm. U. glabra x U. carpinifolia.
- Ulmus 'Accolade'. U. japonica x U. wilsoniana.
- Ulmus 'Coolshade'. U. rubra x U. pumila.
- Ulmus 'Frontier'. U. carpinifolia x U. parvifolia.
- Ulmus 'New Horizon'. U. japonica x U. pumila.
- Ulmus 'Regal'. U. carpinifolia x U. pumila.
- Ulmus 'Sapporo Autumn Gold'. U. japonica x U. pumila.
- Ulmus 'Sarniensis' - Jersey Elm. Backcross (U. x hollandica) x U. carpinifolia.
- Ulmus 'Vegeta' - Huntingdon Elm. Cultivated origin; parentage unknown.
- and many more without formal hybrid names
Two other important genera in the Ulmaceae are Zelkova (Zelkova) and Celtis (hackberries).
Landscape Use
From the Civil War period to the early 20th century, the American Elm was the most widely planted ornamental tree in the United States and Canada. It was particularly popular for boulevard plantings in towns and cities, creating high tunneled effects that formerly characterized old towns in the U.S. Northeast. The American Elm has unique properties that made it ideal for such use: rapid growth, wide adaptation to a broad range of climates and soils, strong wood, resistant to wind damage, and vase-like growth habit requiring minimal pruning.
From ca 1850 to 1920 the most prized small specimen elm was the Camperdown Elm, a contorted weeping cultivar of the Wych Elm Ulmus glabra 'Camperdown', grafted on a standard Wych Elm trunk to give a wide, spreading and weeping fountain shape in large garden spaces.
Large numbers of English Elms were planted in Australia in the early 20th century. Because of its geographical isolation, Australia has so far been unaffected by Dutch Elm Disease (see below), and as such retains some of the world's best stands of elms; the long avenues of Royal Parade and St Kilda Road in Melbourne are perhaps the most beautiful examples.
Dutch Elm Disease
Dutch elm disease has been devastating to elms throughout the Northern Hemisphere. This is a fungal disease that is borne by a vector, the elm-bark beetle. It affects all species of elm native to North America and Europe to some degree, while at least some Asian elms are much more resistant, having evolved alongside the disease. Woodland trees in North America are not quite as susceptible to the disease because they usually lack the root-grafting of the urban elms and are somewhat more isolated from each other.
The disease was first introduced to North America in 1928 and has since become endemic.
Resistant Trees
Well-funded efforts to develop resistant trees have been underway since the 1960s. Research has followed two paths.
Hybridization of the American Elm with the Chinese Elm has produced trees with the greatest disease resistance. A number of named hybrids are commercially available. However, these trees have a smaller mature size and lack the vaselike form for which the American Elm was prized.
Separately, efforts have been made to develop resistant cultivars of Ulmus americana. The 'Liberty Elm', available commercially, represents the results of one such effort, and though marketed as a single product, consists of five cultivars chosen at random. These cultivars were the result of field selection of trees that survived in a region where the disease was endemic, followed by 2-3 generations of selection. Some of the cultivars are patented.
The 'Valley Forge' and 'New Harmony' elms are competing cultivars, produced using selection techniques similar to those used for the 'Liberty Elm'.
Since elm trees take decades to grow to maturity, and these introductions are recent, the performance of these trees in actual landscape conditions is not known with certainty.
A related effort is the commercial reintroduction of the 'Princeton Elm', which is a cultivar selected in 1920 for its landscape qualities. Large plantings have survived the disease, and testing in laboratory conditions revealed that this cultivar has considerable resistance.
