American Elm Restoration Awareness Month Recap

American Elms on the National Mall.

Thanks for tuning in to our American Elm Restoration Awareness Month blog coverage! We hope you learned a lot about these fantastic trees. We had a successful month planting 100 'Valley Forge' American Elms -- today the last six trees went into the ground in the Capitol Hill area. We are really excited about planting 20 more American Elms, particularly because they are the 'Jefferson' cultivar elms that Carol Herwig wrote about last week. They will be planted in the median of Washington Avenue SW near Independence Avenue SW, directly south from the U.S. Botanic Gardens. The site is a newly created ARRA-funded planting location, where cobblestones and fill soil will be replaced with structural soils and 'Jefferson' American Elms.

If you would like to learn more about what Casey Trees is doing to promote American Elms in DC, visit the American Elm Restoration page on our website. We hope you'll join us to plant elms and other trees this spring at one of our Community Tree Plantings. We will be announcing the schedule very soon, but in the mean time you can prepare for the planting season and sign up for a Tree Planting class or take a look at our other free course offerings for February and March.

Threats to American Elms

During the past month, we have discussed introducing cultivars of the American Elm to resist Dutch elm disease. It has been a devastating threat to elms in DC and around the country, but there are other pests and diseases that can harm elms as well. Many American Elm cultivars are selected and propagated for their DED resistance, but not yet for their resistance to other threats. Here is a comparison of some of the major threats to the American Elm:

Dutch Elm Disease

• Fungal infection - Ophiostoma novo-ulmi and O. ulmi
• First described in 1921 in Holland and spread to the United States by 1930. Reached Washington, DC mid-20th century.
• Transmitted primarily by the native and European species of the elm bark beetle that feed on elms. Can also spread from tree to tree when roots cross and become grafted together.
• American Elm street trees can have rates of death higher than 5% annually. DC lost 70% of its street elms between 1959 to 2002 when Casey Trees did its first city-wide tree inventory.

A withered DED-infected branch alongside healthy branches.

Leaves on affected individual branches shrivel and brown. Symptoms commonly are observed in early summer, but  can be seen at other times of the year. Other elms are susceptible to DED, but the American Elm is especially vulnerable, unless it is a cultivar that has significant resistance (but no immunity). The disease is spread by beetles that carry fungal spores from the diseased elms to healthy elms. The beetles movement and the threat of spreading from a DED-infected tree to neighboring trees means that an entire street of elms can be at risk if there is even one diseased tree in the area. 

The virulence of DED necessitates an intensive management strategy that requires infected trees to be identified and removed as quickly as possible. Insecticide or fungicide methods are also used, but may be expensive. Planting resistant cultivars is a good strategy, but even those trees may be susceptible to elm yellows. Often only one branch is initially infected. In such cases, an arborist can quickly undertake a sanitation pruning to remove a diseased limb. Without intensive management, DED is fatal for an American Elm. 

Phloem Necrosis (Elm Yellows)

• Viral infection - Morsus ulmi
• Trees usually die within a year of symptoms.
• Transmitted by the whitebanded elm leafhopper, and also through root grafts.

Healthy elm (left), elm yellows (right).
Photo credit: Wayne A. Sinclair, Cornell University, bugwood.org

The entire crown of leaves turns yellow (not brown and shriveled as with DED) and drop prematurely, often at the end of summer, far ahead of the normal fall cycle. One of the distinguishing characteristics of elm yellows is a wintergreen odor emanating from the inner bark. 

Bacterial Leaf Scorch

• Bacterial infection - Xylella fastidiosa
• The disease clogs the xylem, impeding water transportation through the tree which prevents water from reaching the leaves.
• Causes annual summer/fall symptoms of browning (scorched) edges of leaves, creating a "halo" effect.
• Can be transmitted by a xylem-feeding leafhopper.
• Trees decline over many years rather than dying immediately.

Verticillium Wilt

• Fungal infection - Verticillium albo-atrum
• Carried through the roots of elms, transmitted through the soil.
• Dieback symptoms are similar to DED - leaves begin to wilt on individual infected branches.

Verticillium wilt can affect the growth of twigs and branches, and may result in discolored leaves. Branches may die over the winter. An infection can be managed by pruning wilted branches.

Friday Photo Feature - January 28, 2011

Snow falling on leaves, Northwest DC.

Students Care for District's American Elms

At this point in our inaugural American Elm Restoration Awareness Month I am certain you have fallen back in love with the American elm. Maybe you never fell out of love with them. This hearty tree is ideal for urban environments and by planting disease-tolerant American elm cultivars in combination with a number of other tree types we are advancing species diversity, an urban forestry best practice. 

So what happens once the American elms are in the ground? We do just what we ask you to do when you plant a new tree - properly water and care for it.

In addition to our tree planting crew who provides regular tree care to the trees we plant, we hire high school students to serve as Summer Crew members for eight weeks each summer. These dedicated teenagers travel in teams by truck and bike to tree sites all across the District to water, weed and mulch the trees. In 2010, Summer Crew members cared for 4,500 trees, far surpassing their goal of 2,500 trees. That is a lot of tree love!

Summer Crew members in action.

Not only are these students providing much-needed tree care, they are gaining professional development skills and serving as tree ambassadors. While they are watering and weeding, they are answering questions from passerbys. Just the sight of them watering trees prompts drivers and pedestrians to remember to water trees on and surrounding their property. We couldn't be more proud of the work they do.

If you know of a student who might be interested in joining the Summer Crew team, encourage them to apply. Applications are due May 1, 2011.

Our commitment to our American elms doesn't stop there. We also do structural pruning in years three and four as needed. If you think your trees need to be trimmed, consult Good Tree Care to find a certified arborist.

Additionally, we keep residents updated on the work we are doing with American elms near their homes. We have shared our planting schedule with the Councilmembers and ANCs representing areas where American elms are being planted, posted updates on listservs and distributed door hangers in advance of our work. Let us know if there are ways we can improve!

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.

Friday Photo Feature - January 21, 2011

Young American Elms in the snow.

Notable American Elms

Contributing Writer - Carol Herwig, Volunteer Coordinator

Although Dutch elm disease (DED) has diminished many of Washington D.C.'s elm-lined streets and parks, there are a few grand survivors that can be appreciated. Some, such as the grand Lafayette Park elm facing the White House at Madison Street NW and those on the National Mall, receive special attention from the National Park Service. Elm lovers in Mount Pleasant raise money to inoculate their old trees against DED.

A massive American Elm at the southeast
corner of Lafayette Park.

The American elm towering over the Metro bus stop at Sherman Circle and Crittenden Street NW has a diameter at breast height (DBH) of 5 feet (15.5 feet in circumference) and a crown spread of 90 to 100 feet. Its future is threatened – a lower limb was torn off in last winter's windstorm, leaving a large wound. Rock Creek Cemetery boasts an elm with a DBH of 45 inches and a hundred-foot spread that shades New Hampshire Avenue NW.

The biggest elm in Ward 1, on Ontario Place NW, is nearly five feet in diameter. In Capitol Hill, south of the Capitol off 3rd Street, are some great elms. North Carolina Avenue SE has an mix of survivors and the selections arborists have turned to as replacements: Japanese zelkovas, Chinese elms and more recently, disease-resistant 'Princeton' cultivar American Elms planted by Casey Trees.

The long-lived John Quincy Adams Elm in 1965.

Sadly there are many great American Elms that have not survived. An elm on the southeast grounds of the White House, known as the John Quincy Adams Elm, lived from 1826 to 1991 when it was taken down because of structural concerns. A sapling was derived from a root cutting from the original tree and was planted in 1991 by First Lady Barbara Bush, so in a way the ancient tree lives on.

There are also many notable American Elms outside of the District. The national champion elm, according to American Forests' Big Tree survey, is in Ross County, Ohio. It has a circumference of 273 inches (DBH of just over 7 feet), stands 118 feet tall and has a crown width of 100 feet. American Forests, in selecting Big Trees, follows the following formula to determine the Big Tree champion for each species: Trunk Circumference (DBHx3.1416 or pi) + Height + ¼ Average Crown Spread = Total Points. Using those standards, the Sherman Circle tree's formula is 186 inches + 55 (feet) + 25 (feet) = 266. The Ohio tree received 416 points.

The national champion elm in Ross County, Ohio.

You can view notable American Elms on the Casey Trees Map. You can even nominate a favorite elm in your neighborhood if it has not yet been selected or look at historic American Elm corridors throughout the District.

Friday Photo Feature - January 14, 2011

American Elm leaves on the ground, Daingerfield Island.

How to Identify an American Elm

How do you know an American Elm when you see one? Here are some characteristics of American Elms that will help you identify these trees on the streets of DC.

Note the serrated edge and ovate shape of these emerging elm leaves.

Features of the American Elm:

• Size: Can reach 100 feet tall or more, but generally mature city trees are 60 to 80 feet.
• Trunk and branches: Vase-shaped with a canopy almost as wide as its height. Thick columnar trunk divides into large branches and then smaller branches that arch outwards. On classic elm-lined streets, mature elms often branch across the roadway.
• Leaves: Alternating simple leaves that are ovate in shape. Doubly serrated with straight veins. Various sizes, but leaves usually are 3 to 6 inches long. Dark green in the summer. Notably, the base is asymmetrical (meaning one side of the leaf is broader than the other). Twigs and buds appear in striking zig-zag formation.
• Fall appearance: Yellow or brown leaves.
• Bark: When mature, dark gray-brown with deep, crossing ridges. Bark can be scaly when young.
• Flowers: Tiny petal-less reddish flowers on pendulous stalks. Not showy. One of the first trees to flower, often in late winter before the leaves emerge.
• Fruit: Small round, flat and papery-looking, with a notch at the apex. They are samaras, containing a single seed, and can be green or yellow.

If you're looking for American elms in the city, the obvious place to look is the National Mall where there are hundreds lining the walkways and surrounding the monuments. Additionally, at Lafayette Park and other sites near the White House there are many elms to see including mature specimens and the "Princeton" cultivar planted by Casey Trees along Pennsylvania Avenue. When Casey Trees inventoried the trees on the grounds of the Smithsonian Institution last year, we documented 165 American Elms, making it the Smithsonian’s most common tree species. Other locations include: New Hampshire Avenue NW (including Dupont Circle), North Carolina Avenue SE/NE and East Capitol Street in Capitol Hill, and Lamont and Park Streets NW in Mount Pleasant.

The vase-like spread of American Elms on the National Mall.

Learn more about our American Elm Restoration program on our website, including new planting locations for elms this winter.

Elm Pruning Workshop Recap

Contributing Writer - Carol Herwig, Volunteer Coordinator

Twenty Casey Trees Citizen Foresters, staff members and other volunteers joined National Park Service horticulturist Barry Stahl on Saturday, January 8, for a tree pruning workshop at Daingerfield Island. This wooded land just north of Alexandria doubles as a boat marina and tree nursery for the Mall. Stahl propagates elms from cuttings: at present, he’s growing the original American Elm species Ulmus americana, the disease-resistant cultivars Valley Forge, Princeton and New Harmony, plus the Jefferson and Washington elms.

Citizen Foresters work together to prune elms.

Our job was to prune up the young trees, generally 10-12 feet tall, nipping out dead and broken branches, removing too-low branches, establishing strong central leaders (that is, leading branches) and helping young trees get ready for their starring role on the Mall. This takes years and more than one pruning session. Casey Trees’ visit to Daingerfield Island has become a regular event on our winter calendar.

Stahl began the morning workshop with background on the various elms cultivars. The Jefferson and Washington cultivars have more rounded shapes than the others; the original Jefferson stands near the Freer and Sackler galleries and is distinctive for its rounded crown and because it greens up early and holds its leaves early; the newest elm in front of Natural History is a Jefferson, and the White House is getting a Washington. Stahl believes that there is no street tree quite like the American Elm and his passion and commitment to these trees comes through clearly.

Elms on Daingerfield island, nearly ready for planting in the District.

He explained there are no secrets to the Park Service’s Dutch elm disease maintenance plan. It is labor-intensive diligence with annual spring inspection and quick removal of diseased branches. Then we went to work, learning whys and hows of pruning. We prune in winter because the plant is dormant and so are the insects and diseases that can enter a tree through a pruning wound. Also, the tree’s structure is revealed. Conversely, late fall, when the sap is still running, is the worst time to prune.

Stahl teaches the three-part cut and the six steps of pruning. These include the five basics, plus one special one:

1. Remove broken, diseased and dying branches.
2. Select or create a central leader and cut back or remove competing branches.
3. Select and establish the lowest permanent branch (depends on the tree’s location).
4. Select scaffold branches and remove competing branches and plan for proper spacing
5. Select temporary branches below the lowest permanent branch.
6. Before you make a single cut, feel the ground, address the tree, walk around it and touch it.

With that, we went to work, bundling up with mufflers and handwarmers and armed with pruners, loppers and handsaws. We looked, we asked questions and we cut – carefully, thoughtfully.

Climbing a ladder to make a careful cut.

Warming up in the greenhouse afterward, we saw one- to three-year-old elms that are cared for indoors, to be planted outside when their time arrives. These young elms are closely cultivated, growing quickly, perhaps one day to stand among the magnificent trees on the National Mall.

Very young elms in the greenhouse.

View more pictures from the Tree Pruning Workshop on Facebook. We will be repeating the program on January 22 - please contact Carol Herwig if you are a Citizen Forester who would like to attend.

Friday Photo Feature - January 7, 2011

American Elms on the National Mall

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.

Celebrating the American Elm

American Elms on the National Mall.

This January, we are celebrating the American Elm. This native tree is one of the oldest continuous residents of Washington, DC, and is a hardy street tree that provides shade and beauty for many neighborhoods in our city. As any District resident knows, or any visitor to the National Mall can see, elms are a defining part of DC’s status as the city of trees.

Winter 2011 should be a great season for elms – Casey Trees is planting 100 “Valley Forge” American Elms in Wards 1, 3, 5 and 6, in locations identified by the Urban Forestry Administration. Areas in which we are planting include Capitol Hill, Mount Pleasant, the Bladensburg Road NE corridor near the National Arboretum, and Nebraska Avenue NW near Tenleytown. You can view maps of the proposed planting locations on our website.

Throughout January we will share articles featuring the American Elm both past and present, and we will discuss our current planting efforts. You can keep up with us here by subscribing to our RSS feed, or by following us on Twitter or liking us on Facebook. We invite you to learn more about our American Elm Restoration program on our website, where you can find out where we are planting American Elms this year. You can also use the Casey Trees Map to explore the historic American Elm corridors, elms planted by Casey Trees, and learn about some of the elms featured as Trees of Note in our city.