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Scientific classification
Order: Urticales
Family: Ulmaceae
Genus: Ulmus
  Ulmus alata - winged elm
  Ulmus americana - American elm
  Ulmus crassifolia - cedar elm
  Ulmus glabra - Scotch elm, Wych elm
  Ulmus parvifolia - Chinese elm
  Ulmus procera - English elm
  Ulmus pumila - Siberian elm
  Ulmus rubra - slippery elm (red elm)
  Ulmus thomasii - rock elm
Elms are trees of the Genus Ulmus, Family Ulmaceae. They have alternate, simple, doubly-serrate leaves, usually with uneven bases, often with rouch fine bristles. The fruit is a round samara.

Species of elm are:

Another important genus in the Ulmaceae is the genus Celtis, or hackberry.

Table of contents
1 Landscape Use
2 Dutch Elm Disease
3 Resistant Trees

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:

From ca 1850 to 1920 the most prized small specimen elm was the Camperdown Elm, a contorted weeping mutation of the Wych Elm, grafted on a standard Wych elm trunk to give a wide, spreading and weeping fountain shape in large garden spaces.

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. 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 Harmoney" 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.