'Jefferson' is distinguished by its low, spreading form with arching limbs and broad U-shaped crotches.[2] The parent tree on the National Mall has reached a height of about 68 feet (20.7 m) after 80 years.Ploidy:2n = 42.
Early studies on the parent tree found that the tree hastriploidchromosomes, suggesting that it may be a hybrid between thetetraploid American Elm and an unknowndiploid species.[3] A genetic study that the ARS conducted on the clone at theUnited States National Arboretum in Washington, D. C., during 2004 confirmed the tree asUlmus americana, despite having some atypical features.[4]
A subsequent ARS study also confirmed that the tree is a triploid. The study's investigators concluded that a crossing of two American Elms, one a tetraploid, the other a less common diploid, had created the tree. The investigators found no triploids among the 81 wild trees that they sampled.[5]
The tree proved highly resistant to Dutch elm disease in an ARS trial (as clone N 3487/NA 62001).[6] NA 62001 showed little damage from elm leaf beetle(Xanthogaleruca luteola) feeding during a 2009—2010 survey at an Oklahomaarboretum.[7] 'Jefferson' is susceptible toelm yellowsphytoplasma infection, as are otherU. americana DED-resistant cultivars and native trees of that species.[8]
The Japanese beetle(Popillia japonica) highly prefersU. americana when feeding.[9]Verticillium wilt had a greater effect onU. americana than it had on all other elms studied in an investigation of that fungal disease.[10]
'Jefferson' has not been widely tested beyondWashington D.C.National Elm Trial results were inconclusive and provided no data on ‘Jefferson’ because of an early error in tree identification that occurred in the nursery trade.[11][12] The error may still be causing nurseries to sell 'Princeton' elms that are mislabeled as 'Jefferson', although one can distinguish between the two cultivars as the trees mature.[11][13] The Golden Hill Nursery inKent introduced 'Jefferson' to theUnited Kingdom in 2010, but the clone remains rare in cultivation.
^Sherald, 2009, p. 38. Photographs: (1) Figure 33: "American elm 'Jefferson'. Parent tree on the National Mall in front of the Freer Gallery of Art on Jefferson Drive, flanked on either side by trees vegetatively propagated from it." (2) Figure 34: "Young ‘Jefferson’ elms on the National Mall come into full leaf before native American elms."
^abZetterstrom, Tom (2017). Pinchot, Cornelia C.; Knight, Kathleen S.; Haugen, Linda M.; Flower, Charles E.; Slavicek, James M. (eds.)."Report From the Street"(PDF).Proceedings of the American Elm Restoration Workshop 2016; 2016 October 25–27; Lewis Center, OH. Newtown Square, PA.United States Department of Agriculture,United States Forest Service, Northern Research Station:119–121.doi:10.2737/NRS-GTR-P-174.OCLC1231892730. Gen. Tech. Rep. NRS-P-174. Archived fromthe original(PDF) on May 1, 2019. RetrievedFebruary 7, 2021.Consumers need to also be aware that a nursery trade mix-up a dozen years ago still plays out in the marketplace, and 'Princeton' elms continue to be sold as 'Jefferson' unknowingly by reputable nurseries from New York to Minnesota. .... National Elm Trial (NET) results were inconclusive and provided no data on 'Jefferson' due to the cultivar mix-up..
