Ulmus americana 'Jefferson'

Ulmus americana
'Jefferson' elm in The Mall, Washington D. C.
Cultivar	'Jefferson'
Origin	National Park Service, USA

The American Elm cultivar Ulmus americana 'Jefferson' was cloned from a tree that grows along the National Mall in Washington D. C. [2]. Planted in the 1930s, it remains (2013) unscathed by Dutch elm disease, and was cloned (NPS 3487) by the U. S. National Park Service, which released it as 'Jefferson' in 2004. Early studies on this clone indicated 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 confirmed the tree as Ulmus americana, despite its atypical features.[3] A later study by the USDA's Agricultural Research Service confirmed the tree as a triploid, but derived from a crossing of two American Elms, one a tetraploid, the other a rarer diploid.

'Jefferson' is distinguished by its low, spreading form with arching limbs and broad U-shaped crotches; the tree in the National Mall has attained a height of about 21 m (68 ft) after 80 years.Ploidy: 2n = 42.

The tree proved highly resistant to Dutch elm disease in USDA trials (as clone N 3487/NA 62001), and very resistant to the Elm Leaf Beetle Xanthogaleruca luteola. No specific information is available, but the species generally is highly susceptible to Elm Yellows and highly preferred for feeding by the Japanese Beetle Popillia japonica [4] in the United States. U. americana is also the most susceptible of all the elms to verticillium wilt.