In Cleveland, 1930, the first great American elm tree (Ulmus spp., USDA zones 2-9) grew sick and died of an imported disease soon identified as Dutch elm disease. It spread quickly to neighboring trees, then neighboring states, then the entire country. Over the next four decades, the disease decimated the tree-lined streets of America and killed more than 77 million trees, and today the disease still remains a threat. Despite this, the elm tree remains a favorite in America, growing quickly and providing wide, beautiful canopies to green up neighborhoods.
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Why Are Elms So Beloved?
The mighty, hearty elm was considered ideal for urban planning because these trees performed well in compacted soil and thrived even in polluted environments. They even withstood salt, making it roadside-hardy in cities across the northeastern states, rated zone 3 for climates, where temperatures plummet as low as -40 degrees Fahrenheit. The canopies could stretch far, making streets and sidewalks cool in the summer and sheltered during the spring and fall rains.
For bird lovers, the American elm was a preferred roosting tree for the oriole, whose plumage makes it a favorite for spotting. It's the drooping of the tree's branches that made it perfect for placing nests on, where they'd be out of reach and sight of predators.
When leaves of whole limbs, and then whole trees, began turning yellow then brown and dying, it destroyed the aesthetic of street after street. But trees are more than just window dressing, they help prevent floodwaters, clean the air and regulate the ground temperature. So why did the mighty American elm die?
Elm Trees Have Changed
The culprit that killed America's trees was in part caused by monoculture planting. The fast-growing elms wowed city planners in the early part of the century, and streets across America were lined with saplings all grown from monoculture grafted seedlings.
In sharing their rootstock, a chain reaction of disease and subsequent death was able to spread rapidly by fungal pathogens vectored by elm bark beetles, destroying the idyllic tree-lined culture of suburban America.
Today, geneticists have worked hard to clone the American elm (Ulmus americana, zones 2-9) and create a Dutch elm disease-resistant species. The prognosis is good for these beautiful giants returning along the streets of America. New disease-resistant clones of the American elm are beginning to thrive outside the labs they've been created in and include genotypes like the vase-shaped Valley Forge and the wider, taller New Harmony.
The Chinese Elm
In the wake of Dutch elm disease, the Chinese evergreen elm (Ulmus parvifolia, zones 4-9) became highly popular for its hardiness against the disease. Another reason for its popularity is the Chinese elm growth rate, which can be 3 feet per year to a maximum height of 80 feet. Its canopy, though, spreads out quickly, making it a valued tree early in its life.
Also called the Drake elm and the lacebark elm, the Chinese elm is a beautiful tree that can be among the first to green up in the spring and the last to turn colors in the fall, long after most other trees have submitted to the season. To stay healthy, pruning is needed regularly on younger trees but is more of a cosmetic task on older trees. Beware of wide, shallow root structures that can be pesky for sidewalks and pipes.
Be on the lookout for lesser imitators, though, because the Siberian elm (Ulmus pumila, zones 4-9) is similar in appearance while far less hardy or desirable in urban planning. Chinese elms have leaf buds that are pointed, small and reddish-brown and the bark has cinnamon-colored patches, whereas the Siberian leaf buds are round and black.
The Evergreen Elm
Also called the "Chinese evergreen elm," owners of these fast-growing semi-evergreens can see saplings spring as high as 30 feet in five years. Top heights are around 40 to 60 feet, but canopies can spread from 50 to 70 feet. With that quick growth and wide canopy comes extensive root systems, making it less ideal for smaller residential areas.
For properties that can take the evergreen elm's roots, owners can expect to enjoy a broad, drooping, dense canopy of green with great summer shade, perfect for resting under on a hot summer day.
Ideally grown in the southwest, this is the only elm that can tolerate Arizona's low elevation areas, but root rot can be a concern in desert-like regions. They like to be in full sun and deep soil with good drainage and they need infrequent but deep irrigation. Younger evergreen elms need maintenance like staking and pruning to prevent wind damage, but they stand up well against pests and diseases.
The “Princeton” American Elm
Of the Ulmus americana cultivars, the Princeton elm (Ulmus americana 'Princeton,' zones 3-9) has been among the most resistant to the Dutch elm disease that killed so many American elms in the 20th century. The Princeton was first cultivated in 1922 and its survival rate has been impressive among the early trees planted in its first decade; few new elm species meet its track record.
The Princeton is a fast grower with an average of 4 to 6 feet of growth per year, reaching peak heights of 60 to 80 feet. It's a vase-like shape with a wide canopy at the top and wide shallow roots that sometimes cause infrastructure damage, but some success in avoiding that can be had with root barriers.
The Princeton's fast growth means this tree benefits from twice-a-year pruning, which will help it bounce back from storm or other damage. This is an ideal tree for streets and large yards, and homeowners will enjoy its canopy in just a few short years. In a 10-year trial by the USDA, the Princeton had survival rates over 80 percent, a near tie with the New Harmony cultivar.
Other American Elms
The Princeton's track record is now long and strong, but two newer cultivars are considered very hardy against the Dutch elm disease — the Valley Forge (Ulmus americana 'Valley Forge') and the New Harmony (Ulmus americana 'New Harmony'), which have a 96 percent and 86 percent rate of resistance against Dutch elm disease, according to the USDA.
New Harmony is hardy in zones 4 through 7 and grows to be a maximum of 60 feet high with a 60-foot wide canopy; in fact, the canopy can exceed the height of the tree. It does need pruning in March or after Labor Day to rid the tree of dead branches and to prevent beetles (but pruning from April through August can attract beetles).
Valley Forge is hardy in climate zones 5 through 7 and enjoys full sun and partial shade with good drainage. It can reach 80 feet high while boasting a 60-foot canopy. Some exceptional trees can hit heights of 110 feet. On average, they grow 3 feet per year once established and can achieve a crown of 30 feet within 12 years.
Avoiding Dutch Elm Disease
To protect even the hardiest of American elms, they need pruning. Professional pruning by a certified arborist will remove dead branches and spot the early stages of the disease. Pruning should be done at least annually, but twice a year is even better for these fast-growing, large-canopied trees.
If Dutch elm disease is suspected, it's critical that the diseased sections of the tree be destroyed at an approved facility or burned immediately, where allowed. Why? Because the disease can live in the sap long after the tree has died, which is part of why it succeeded in spreading so widely and so quickly.
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- Pacific Northwest ISA: Princeton Elm, Ulmus americana ‘Princeton’
- Invasive Species Initiative: Dutch Elm Disease
- University of Florida IFAS: Ulmus americana - New Harmony', American Elm