Issue: August 28, 2004

Branches breaking in large Siberian elms


I have what I have always called Chinese elm trees. They are 2 huge monsters that are developing a lot of dead limbs. Many limbs that still have leaves on them are breaking off. I also have suckers growing up around them. I really don't want to lose these trees. What should I do? They are so big I can't get my arms around them. I think their roots are so deep that surface water doesn't help.



There are very few true "Chinese" elm trees in New Mexico. Most of the elm trees in the state are Siberian elms. The Chinese elm is a much more desirable tree. It is often smaller and has dark green leaves that are less prone to being damaged by elm leaf beetles. The Siberian elm is the one that produces many seeds in the spring and seedlings in the surrounding landscapes shortly thereafter. The Siberian elm is much more susceptible to branch breakage and elm leaf beetle damage. You probably have the Siberian elm tree. Even though it has the undesirable characteristics I mentioned above, it is well adapted to New Mexico and grows with little care. For that reason, it is a very useful tree for providing shade under difficult growing conditions.

There are several things you can do. The first thing is to water deeply. Moisten the soil to a depth of 3 feet once every 2 weeks in the growing season (when leaves are present). Even though the trees are large, their roots do not grow as deeply as the tree is tall. They spread widely and extract most water and nutrients from the top 1-3 feet of soil. Very little water uptake occurs under the tree's canopy (under the branches). Most water uptake is done by the very small roots beyond the end of the branches. These may be found from the dripline of the tree outward (up to 4 or more times the height of the tree away from the trunk). Apply the deep watering at the dripline and outward up to 10 feet (more if you wish). In the winter, reduce irrigation to once a month but continue to apply enough water to moisten the soil to a depth of 3 feet. Because different soils absorb different quantities of water, you must probe the soil after watering to determine how much water to apply (how long to run the irrigation system) to moisten the soil to the required depth. After watering (perhaps a brief watering), wait a day or two for the water to soak into the soil, and then dig to see how deeply the soil has been moistened. If the soil is moist to a depth of 1 foot, water 3 times as long.

Another important thing to consider is removal of damaged or dead branches. This will reduce risk of injury or proper damage from falling branches. Do not "top" the trees. This results in damage to the tree and increased probability of branch breakage. With such large trees, you should hire a trained and certified tree care professional to do the pruning. An International Society of Arborists (ISA) certified arborist has taken courses in proper tree care, passed certification exams, and will know how to properly care for your tree. You can find ISA certified arborists in telephone directories throughout New Mexico.

A final thought is that trees do not live forever. Some have life spans shorter than a human life. In some situations, it is wise to replace trees as they begin to decline. If your trees seem to be declining, you may want to plant replacement trees near the existing trees and allow them to grow until it becomes necessary to remove the older trees.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.