Jefferson Elm (Ulmus americana 'Jefferson')

'Jefferson' is a pure American elm cultivar selected from a 1932 specimen planted at the Tidal Basin in Washington, D.C. and released by the USDA National Arboretum in the mid-2000s. It is genetically unusual among American elms in being hexaploid rather than the more common tetraploid, and it shows high DED resistance.

Origin

'Jefferson' was selected from a single tree planted in 1932 at the Tidal Basin in Washington, D.C., one of a group of American elms that had survived multiple DED waves through subsequent decades despite being grown in close proximity to numerous infected trees. The tree was identified by USDA researchers Susan Bentz and Alden Townsend during a survey of long-lived survivor elms in the region.

Cytological analysis revealed that 'Jefferson' is hexaploid (six sets of chromosomes), whereas most American elm cultivars are tetraploid. The cultivar was released for commercial propagation after extensive inoculation testing confirmed its DED resistance.

Identification

Form: Upright vase shape, somewhat narrower than 'Valley Forge'
Mature size: 50–70 feet tall, 40–50 foot crown spread
Leaves: Standard Ulmus americana foliage, 3–5 inch oval, double-serrated, asymmetric base
Bark: Light gray, deeply furrowed at maturity
Growth rate: Fast

Hardiness and adaptability

USDA hardiness zones: 4–9
Site preferences: Adaptable; tolerates urban conditions
Stress tolerance: Drought-tolerant once established

Dutch Elm Disease resistance

'Jefferson' demonstrated high resistance in USDA inoculation trials, comparable to 'Valley Forge' and 'New Harmony'. The hexaploid genome is hypothesized to contribute to the cultivar's resistance through multiple resistance-allele copies, though the exact mechanism remains under investigation.

The cultivar has performed strongly in the National Elm Trial and in National Mall replantings.

Landscape uses

Street trees in urban environments
Restoration plantings (the cultivar is among those used at the National Mall and around Washington, D.C. monuments)
Mixed plantings with other resistant American elm cultivars

Limitations

Hexaploid genetics make breeding crosses with tetraploid cultivars complicated
Like all American elms, susceptible to elm yellows (a phytoplasma disease distinct from DED)
Slightly less broad canopy than 'Valley Forge', which some plantings will prefer

Similar cultivars

'Valley Forge' — broader form, similarly high resistance
'New Harmony' — narrower form, similarly high resistance
'Princeton' — older selection with moderate resistance

References

Whittemore, A. T., & Olsen, R. T. (2011). "Ulmus americana (Ulmaceae) is a polyploid complex." American Journal of Botany, 98(4), 754–760.
Townsend, A. M., Bentz, S. E., & Douglass, L. W. (2005). "Evaluation of 19 American elm clones for tolerance to Dutch elm disease." Journal of Environmental Horticulture, 23(1), 21–24.
Sherald, J. L., et al. (1994). "Evaluation of American elm for resistance to Dutch elm disease." Journal of Arboriculture, 20(3), 162–170.