National Park Service (USA)
Jefferson was cloned from a tree of approximately 80 years old growing in the National Mall, Washington D.C. The identity of the parent tree has been the subject of several genetic studies which have produced inconsistent findings; the earlier indicated that it was likely to be a hybrid between U. americana and another species, while it seems subsequently to have been shown to be true U. americana. The tree was released in 2005, and introduced into Britain in 2010.
26 specimens of Jefferson were assessed in inoculation trials carried out by James L. Sherald and Alden M. Townsend in 2002, alongside various other American elm selections. The clone is reported to have shown 0% foliar symptoms at 4 weeks, 1% dieback after one year, and 0% dieback after two years. However, there are now doubts about the accuracy of some of the American resistance testing. It should be noted that U. americana “Princeton” performed nearly as well as Jefferson in the same trials, but has died of elm disease in Britain.
The parent tree reached approximately 70 foot in as many years, with a spread of 50 foot. As the photographs indicate, the trunk is impressive, but the main branches arch in a way not seen in any European elm, and the silhouette is parasol-like. Branch unions are reputedly broad and strong (narrow junctions with associated bark inclusion and risk of splitting are a weakness of many elm clones). In saplings, the bark has a russet colour more reminiscent of cherry than of elm. The mature bark is presumably typical of U. americana.
Here, Jefferson possesses some notable advantages. It comes into leaf early, and holds its leaves considerably later than most U. americana selections. The leaf is smaller than usual in this species, which may give it some protection against the wind damage suffered by clones such as Valley Forge. It is also darker, and therefore closer to typical European elm. Most importantly the leafing is abundant; Jefferson seems to avoid the tendency to blind buds which detracts from the overall appearance of many hybrids and some American elm. The summer canopy of the tree on the National Mall is dense and reminiscent of English elm even though its form is so different.
If European inoculation testing confirms its disease resistance, Jefferson will probably prove to be the best of the U. americana cultivars. Its form is very different from European elm, and that would seem to limit it to ornamental planting, but the massing of its foliage strongly recommends it.