| Ulmus americana | |
|---|---|
| Details | |
| Cultivar | 'Jefferson' |
| Origin | National Park Service, USA |
The American Elm Ulmus americana cultivar 'Jefferson' was cloned from a tree that grows along the National Mall in Washington D. C. [1]. Planted in the 1930s, it remains unscathed by Dutch elm disease, and was cloned (NPS 3487) by the U. S. National Park Service, which released it as 'Jefferson' in 2004. Although early reports of studies on this clone had indicated that it had triploid chromosome levels (suggesting it may be a hybrid between the tetraploid American Elm and a diploid species), a genetic study performed by the United States National Arboretum in 2004 [1][2] confirmed the tree as an American Elm, despite its atypical features.
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'Jefferson' is distinguished by its low, spreading form with arching limbs and broad U-shaped crotches; the tree ultimately reaching a height of about 12 m (40 ft).
The tree proved highly resistant to Dutch elm disease in USDA trials (as clone N 3487/NA 62001) [2], and very resistant to the Elm Leaf Beetle Xanthogaleruca luteola [3]. No specific is information available, but the species generally is highly susceptible to Elm Yellows and highly preferred for feeding by the Japanese Beetle Popillia japonica [3] [4] [5] in the USA. U. americana is also the most susceptible of all the elms to verticillium wilt [4].
Neither 'Jefferson' nor the cultivar 'Washington' has been widely tested beyond Washington D.C., although it is now being evaluated in the National Elm Trial coordinated by Colorado State University. The tree was introduced to the UK in 2010.