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An elm tree is a majestic tree that once lined the streets of many American and European cities. It suffered from a debilitating disease that killed most of the trees on these continents. Resistant varieties are now being bred, and hopes are that these stately trees will once again grace streets as they did before.
There are about 30 to 40 species of elm trees, belonging to the genus Ulmus. From the 1700s to the early 1900s, elm trees were very popular along streets in Europe and North America. They could tolerate air pollution, and their leaves decomposed quickly. The trees were also useful to farmers as shade for livestock and protection from winds and storms. The branches were used to feed animals.
In the 1900s, the trees came under attack by a fungal elm tree disease known as Dutch elm disease that is carried by bark beetles. There were two waves of this disease. The first, lasting into the 1940s was not very serious. In the late 1960s, however, a strain appeared that was three times more lethal. It wiped out the majority of elms in most of the areas in which they were grown, except for China and Australia.
Resistance breeding programs have been underway for decades. In the 1970s, trees that were supposed to be disease-resistant turned out not to be. Subsequent breeding efforts have produced a new crop of resistant lines, but the previous, failure has soured some people on planting these current resistant trees. So far, however, these new lines appear to be holding up.
The most common elm tree in North America was the American elm, or U. americana. It grows rapidly, is resistant to wind damage, requires little pruning, and is adapted to a wide range of soils and climates. The most common street name in the United States is Elm Street, due to its one-time prevalence. Trees that are isolated, such as on a farm, are more likely to have survived, without contracting Dutch elm disease.
This deciduous tree can grow to 100 ft (30.5 m) with a spread that can be even greater. Trunks typically reach up to seven ft (2.1 m) wide. Before the leaves appear, the trees bloom with inconspicuous small flowers, followed by seedpods that are eaten by wildlife and birds. The elm tree leaves have a rough surface, and are 3 to 6 in (7.6 to 15.2 cm) long, with serrated edges. The bark of the tree is grayish-brown, turning to ash gray as the tree matures.
In the United States, the American elm tree is hardy in planting zones 2 through 9. This tree is still planted by people willing to be vigilant about monitoring for Dutch elm disease. Cultivars with the most resistance to this disease are Valley Forge and New Harmony varieties. Some people have abandoned trying to grow the American elm and instead grow the Chinese or lacebark elm, U. parvifolia. The Chinese elm, while resistant to Dutch elm disease, is not a good shade tree.
Control of Dutch elm disease relies heavily on sanitation measures. Weakened or dead trees and logs should be removed and destroyed. Sometimes the disease can be eliminated by pruning infected branches and twigs. Healthy trees can be sprayed with insecticide in the winter and spring, to kill the beetles that spread the fungus. These measures have been only partially effective, however.
The roots of nearby trees can sometimes join together. Killing these grafts with chemicals can limit the fungus to a particular tree. A promising new treatment is to inject the trunk or roots with a systemic fungicide to attack the fungus directly. This technique has been used to cure diseased trees, but is not always dependable. There is currently research on using viruses that attack the fungus to control Dutch elm disease.
Elm trees should be planted 40 to 50 ft (12.2 to 15.3 m) away from other trees, and at least 15 to 20 ft (4.6 to 6.1 m) away from houses. They prefer full sun and regular watering in soil with good drainage. When planting the trees, one should plant bare roots in holes slightly more shallow than the roots. To allow the roots to spread out, one should dig more deeply around the edges of the holes. The young trees can be transplanted in early spring or fall, when they are between 4 and 7 ft (1.2 and 2.1 m) tall.
There are some downsides to planting elm trees. The roots grow near the surface, and can interfere with mowing or lift sidewalks. The trees require pruning and produce quite a bit of litter. Many people feel the beauty and shade produced by these trees, however, more than makes up for any negative attributes. Hopefully, elm trees will once again be part of the urban landscape in Europe and North America.