American elms on the National Mall

American Elm Restoration

The American elm was once the street tree of choice for America’s main streets, particularly in the District. This beautiful native tree is hearty and adaptable to a wide range of conditions found in cities, such as compact soil and increased pollution.

The Challenge of Dutch Elm Disease

Dutch elm disease (DED) devastated many strands of American elms across the country including here in the District. As many as 100 million American elms have succumbed to DED since it was introduced from Europe almost 85 years ago.

Though DED has taken its toll, and American elms remain susceptible to DED today, it did not wipe them out.  There are still many types of elms across the landscape, including those that are wild and naturally occurring, along river banks, alleys, fence-lines and abandoned areas.  These include several natives such as: winged elm, slippery elm and rock elm, as well as several non-native introduced elms such as the Wych elm, Siberian elm and Chinese elm.

Elms in general comprise a significant percent of the canopy in Washington D.C., and depending on land use type, typically range from 5% to 8% of the total canopy.

The most visible and well-known stand of American elms in the District are growing on the National Mall between the U.S. Capitol Building and the Washington Monument.

How to Recover Our Elms

As of 2013, our organization has planted 2,250 disease-tolerant American elm cultivars and hybrids across D.C. Planted cultivars include Jefferson, New Harmony, Princeton and Valley Forge.

Few elms are absolutely immune to Dutch elm disease, and the various planted and naturally occurring elms throughout the landscape provide many vectors for DED to continue to spread.

The typical annual infection rate in Washington D.C. is approximately 7%. Elms are also susceptible to drought stress, and other diseases and pests, such as elm yellows, and Asian Long Horn Beetle. A coordinated program of monitoring and management is required across jurisdictions. Currently the National Park Service, Urban Forestry Administration and the U.S. Forest Service collaborate and coordinate annual inspections, sanitation, treatment and removals.  Private homeowners should be encouraged to keep an eye on the elms in their landscapes, and seek the advice of a professional arborist in identifying symptoms and diagnosing problems.

Elm Pruning

Elms are alternate branching and decurrent in growth form, with several co-dominant trunks.  This is what yields the classic graceful arching and spreading canopies that many elm lovers are nostalgic for.  Yet some of the disease resistant elm cultivars also tend to have narrow crowns and tight branching habits, which along with their rapid “weed-like” growth, require an aggressive regime of pruning, especially in their earlier years, to adequately “train” their trunks and influence their mature spreading form. This can be a challenge of resources and time.

Annually, we partner with the National Park Service to host a pruning workshop at Daingerfield Island, instructing staff, volunteers and interested citizens. We gain experience pruning these special trees, before they get transplanted out into the National Mall.

See more photos from one of our pruning workshops on Flickr.