The 'Jefferson' American Elm

Contributing Writer - Carol Herwig, Volunteer Coordinator

The first elm to earn the name 'Jefferson' is on the National Mall near the Freer and Sackler Galleries. It stands out for its shape — it has the traditional vase shape, but with a more rounded crown than the typical American Elm — and its color — it is a darker green. It also leafs out earlier and holds its leaves longer. Those are desirable traits, but more significantly, it has shown resistance to Dutch elm disease (DED).

The 'Jefferson' elm was previously thought to be a hybrid of the Ulmus americana and another species. However, scientists from the National Arboretum have determined the 'Jefferson' elm on the National Mall is a true varietal of the species instead of a hybrid. Planted in the 1930s, the original specimen had survived DED.

However, if you were to purchase a 'Jefferson' — and they are just becoming available commercially — your elm would be a clone of that elm on the Mall. That is the only way to assure that you would be getting the same genetic makeup — the disease-resistance, color and shape characteristics so prized in the original. The first clones were made in 1993 and are just now becoming available commercially. Small 'Jefferson' elms in one quart or one gallon containers are selling for $25 and $40 at the Botany Shop Garden Center in Joplin, Missouri. By contrast, a three foot tall 'Princeton' elm, a disease-resistant cultivar, can be purchased for $12.

I had the privilege of taking a tour of the National Mall's elm trees with Jim Sherald, a plant pathologist with the National Park Service's Center for Urban Ecology who helped test the 'Jefferson' for DED resistance. He showed how the 'Jefferson,' clearly a favorite of his, stands out among the many elms of the Mall. Take the test yourself this spring.

Cultivars of the American Elm

'Princeton' elms near the White House.

Why do we plant different cultivars of the American Elm? Surely there's nothing quite like the original Ulmus americana. The classic American Elm is hardy, fast-growing and can grow into a healthy and aesthetically beautiful specimen with ease. However, the prevalence of Dutch elm disease (DED) since the middle of the 20th century has threatened the future of the  elms that once lined streets and parks throughout the District and populated America's main streets. Certain cultivars have been developed to be resistant to DED, or have been discovered to be resistant to the disease.

Certain varieties, such as Washington and Jefferson are classic American elms that were planted by the National Park Service in Washington, and can be found on the National Mall and the monumental core of the city. These were naturally occurring varieties but are now reproduced as clones to ensure the identical genetic makeup.

The 'Princeton' cultivar is a variety developed in a nursery; it must be reproduced as a clone or it will revert to one of its parents. 'Princeton,' which can be seen planted in rows in front of the White House along Pennsylvania Avenue, was developed in the 1920s before the threat of Dutch elm disease. It has come into wider use because it has shown good resistance to DED. 'Valley Forge' is a more recent U.S. National Arboretum cultivar and is probably the most resistant to DED. ‘Princeton’ and ‘Valley Forge’ are the primary cultivars Casey Trees has planted during the past seven years. Casey Trees is planting 100 'Valley Forge' elms this winter; in past years, we planted mostly 'Princetons.'

Cultivars vary not just in their disease and pest susceptibility, but also in their form. A ‘Princeton,’ for example, has a dense, symmetrical form with highly acute angles, making for an upright profile. It is notably vase-shaped, a good way you can identify it. The ‘Valley Forge’ also has a vase-shape, but the branching is looser.

There is a good reason why American Elms are cultivated carefully from single specimens rather than hybridized, as is done with many trees to select for certain traits. Hybridization within the genus Ulmus (crossing with other elm species) has been aimed primarily at breeding for DED and phloem necrosis resistance. However, most of the breeding and selection work does not include American elm because it is a tetraploid — that is, it has a chromosome number twice that of all other elms (56 versus 28). Therefore, most hybrid breeding and selection work does not include American elms. Thousands of attempts to cross the American with the Siberian elm have failed. Reports of successful artificial hybridization and verification of hybridizing American elm with other elms are rare.

Facts about American Elm cultivars:

• Cultivars are crosses between clones. They are derived from root cuttings (rather than planted from seeds) to ensure their DED-resistance.
• Although cultivars may be selected for DED-resistance, they may still be at risk for other elm diseases, such as elm yellows and verticillium wilt.
• DED-resistance does not mean a cultivar is immune, but certain ones have significantly improved survival rates compared to other elms.
• There are dozens of named cultivars of Ulmus americana.

History of the American Elm in the District

Contributing Writer - Carol Herwig, Volunteer Coordinator

Lafayette Square in 1919. Photo by Martin Gruber.

Since Washington, DC’s origin as the nation’s capital, trees were a key element in the plan. George Washington (a noted plantsman) and planner Pierre L’Enfant envisioned tree-lined streets and parks. Thomas Jefferson bemoaned the loss of native groves, and followed up with a sketch and a planting of Lombardy poplars that, unfortunately, were spectacularly unsuited for Washington’s climate.

But the American elm, Ulmus americana, was already here, along with white pine and white oak and other native species. A mature elm can grow 60 to 80 feet tall with strong limbs creating a vase-shaped spread nearly equal to its height. The elm has an interlocking grain that is resistant to splitting, which made it valuable for wagon wheel hubs. Its wood is also resistant to rot in wet conditions.

So many of those big native elms were lost to the building of the city, cut down for timber. The Civil War further decimated Washington’s tree canopy, as most trees were felled to build the circle of forts that defended the capital.

The focus on Washington as a tree-lined city returned in the mid-1870s, post-Civil War, when Governor Alexander “Boss” Shepherd laid out his grand plan for the city: planting 60,000 trees, building sidewalks and installing curbs. Shepherd created a “parking commission,” a precursor to the District Department of Transportation, that was responsible for planting and maintaining the city’s trees. Shepherd’s term barely lasted two years, but the modern DDOT agency continues to be responsible for city trees though its Urban Forestry Administration.

Shepherd’s plan was the beginning of what we now know as the historic elm corridors. Streets were planted each with a single species, ignoring the risk of losing whole blocks to disease. This monoculture approach to tree planting only made the spread of Dutch elm disease easier when it arrived in the early 20th century and many elm-lined DC streets were decimated. Remnants can be found throughout the city: along South Carolina Avenue in the Southeast, to Branch Avenue east of the river,  to the state streets, New Hampshire, Arkansas, Kansas and Illinois, in the Northwest. Often diseased trees were replaced by Japanese zelkovas or Chinese elms and more recently by disease-resistant cultivars of the American Elm. But there are some grand survivors — at the southeast corner of Lafayette Park; along the western edge of Rock Creek Cemetery; on the Capitol Grounds; along the West Potomac Park waterfront and of course, on the National Mall.

Washington’s world-famous elms — the double allees on the Mall — are part of the McMillan Commission plan, influenced by Western Europe’s great avenues. Elms were chosen by the McMillan Commission because of the “architectural character of its columnar trunk and the delicate traceries formed by its wide-spreading branches.” Installation of the plan began in 1902 and was largely complete by 1922, after a pause for World War I.

Elms and the Lincoln Memorial in the snow.

The American Elm is a classic native tree of our city although it certainly has seen better days. At Casey Trees we are committed to maintaining and restoring the legacy of the American Elm as a street tree, which is why we have been planting over 1,750 elms in the city since 2003. Learn more about our American Elm Restoration program on our website.