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HOW to... Recognize and Minimize De-Icing Salt Injury to Trees

From the February 2003 issue of The Forestry Source

De-icing salt spread by trucks like these often causes the disfiguration of trees and shrubs and may contribute to the decline and death of trees in urban areas.
Although de-icing salts are beneficial winter tools that help keep pavement dry and safe, the extensive use of such salts causes widespread damage to many trees and shrubs along roadways and often contributes to the decline and death of many city shade trees.

"Extensive use of salt causes widespread damage, causing disfiguration of trees and shrubs," says Gary Johnson, an urban forester with the University of Minnesota's Extension Service.

Salt injury occurs when it is deposited by spray or drift on the dormant stems and buds of deciduous woody plants and on the stems, buds, and needles of evergreens. Injury may also occur when excessive amounts of salt accumulate in the root zone of these plants.

Recognizing Salt Injury

The symptoms of salt injury are similar to injury caused by other kinds of tree stress and therefore may be hard to identify. When in doubt, says Johnson, suspected salt injury could be verified with soil and tissue analysis, as well as observation of the site where the damage occurred.

Nevertheless, Johnson says the effects of salt injury often appear in the following ways.

In deciduous plants, the symptoms of salt injury become evident when tree growth resumes in the spring. Salt spray can causes bud death and twig dieback, as well as the development of "witches' brooms," tuftlike growths that sprout from the basal section of branches facing the road.

On conifers, Johnson says the symptoms of salt spray damage are often moderate to extreme needle browning, beginning at the tips of needles and twigs facing the road. Browning usually is first evident in late February or early March and becomes more extensive through spring and summer.

Soil salt damage to deciduous species often becomes evident late in the summer following the growing season in which the salt damage occurred or during periods of hot, dry weather.

Minimizing Salt Injury

To minimize salt injury to trees, Johnson recommends taking the following steps to protect trees and shrubs:

  • Avoid de-icing salt completely or reduce quantities applied by pre-wetting the salt with a liquid such as salt brine or by mixing the salt with abrasives such as sand, cinders, and ash. Johnson also recommends using alternative de-icing salts such as calcium chloride and calcium magnesium acetate.

  • Limit treeplanting in areas at high-risk for salt injury, such as high-speed roads, intersections, hills, steps, and walkways.

  • Improve the structure and drainage of poor draining soils.

  • Reduce sodium content in soils by adding organic matter, activated charcoal, or gypsum and then thoroughly leaching the soil.

  • Protect susceptible plants by constructing a physical barrier made of plastic, burlap, or snow fencing and then placing it between the pavement and the plants.

  • Avoid sites at high risk from salt injury by planting trees and shrubs away from salt spray drift zones and areas where salt-laden snow are likely to be deposited.

  • Keep plants healthy throughout the year by provide adequate irrigation and mulching to reduce water loss, pruning, adding fertilizers to correct nutrient deficiencies, and control damaging diseases and pest infestations.

  • Use plants that are sufficiently tolerant of the expected exposure to salt. This, says Johnson, is the only successful technique when planting in high-salt locations. Salt-sensitive tree species include little leaf linden, sugar maple, red maple, crabapple, white spruce, and white pine. Instead, says Johnson, plant salt-tolerant tree species such as Ohio buckeye, Black Hills spruce, ginkgo, and white ash.

  • When planting trees near roadways, Johnson recommends planting them at least 60 feet away. Plants that are closer to the road stand a higher chance of being affected.

Adapted from the University of Minnesota Extension Service publication Minimizing De-Icing Salt Injury to Trees by Gary R. Johnson and Ed Sucoff. Copyright 2000; www.extension.umn.edu.

For information, contact Gary R. Johnson, Urban and Community Forestry, University of Minnesota Extension Service, 115 Green Hall, 1530 Cleveland Avenue North, St. Paul, MN 55108; (612) 625-3765; fax (612) 625-5212; grjonson@umn.edu.

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