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Minimizing de-icing salt injury to trees

Gary R. Johnson and Ed Sucoff

More than 200,000 tons of de-icing salt are applied to state and municipal roads in Minnesota each winter. Some years, as much as 300,000 tons have been applied. De-icing salts (primarily sodium chloride) are helpful in providing dry, safe pavement for high-speed traffic. They are also used in large quantities within our urban areas to improve safety on streets, driveways, parking lots, and sidewalks. Despite the benefits, the extensive use of salt causes widespread damage. De-icing salt has caused the disfiguration of trees and shrubs along highways, and may have contributed to the decline and death of many city shade trees.

Injury occurs when salt is deposited by spray or drift on 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.

Both spray salt and soil salt can cause stem and foliage disfigurement, reduce growth, and even cause death.

Black and white photo of a salt-truck

Spray-salt damage is most evident along heavily traveled highways where high speed and high volume truck traffic have deposited salt spray on adjacent plants. Damage is most severe within 60 feet of the road, although it can sometimes extend much farther (e.g., spray deposited on elevated highways).

Another source of plant injury occurs gradually, due to the buildup of high salt levels in the soil. This buildup occurs along city streets, driveways, and sidewalks when salt runoff washes into the soil and when salt is plowed and shoveled onto boulevards and lawns.

Toxic quantities of sodium and chloride can damage plants:

  1. by direct absorption into the roots, and
  2. by contributing to the deterioration of soil structure, thereby impeding soil drainage and root growth.

Overmaturity and drought can intensify the problem of high salt levels. For example, prolonged drought interacts with soil salt to increase damage to trees. Also, as trees age they lose their ability to tolerate soil- and salt-related stresses.

Control of infectious diseases is complicated by high salt levels in the soil. For example, the Dutch elm disease epidemic forced the removal of many elms along streets and boulevards. The young replacement trees were subjected to accumulated salt in their planting holes as well as the dangers of additional salt spray.

Salt-related damage to city and highway trees is costly; injury means increased maintenance expenses for pruning, fertilizing, and other extra care, as well as the expense of replacing removed trees.

Symptoms of salt injury

The symptoms of salt injury are similar to injury caused by other stresses. When in doubt, suspected salt injury can be verified with soil and tissue analysis, as well as observation of the planting site location where the damage occurred.

branch with many thin protrusions at ends

Figure 1. "Witches'-brooms" is a common condition along roads because of spray salt injury

Salt spray commonly causes bud death and twig dieback in deciduous plants. Subsequent shoot growth results in the development of "witches'-brooms" (tuft-like growths) from the basal section of branches facing the road (figure 1). The symptoms become evident when growth resumes in the spring. In addition, salt-damaged deciduous trees and shrubs leaf out later in the spring.

On conifers such as pines, spruces, and firs, salt spray causes 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.

drawing of two leaves

Figure 2. Marginal leaf burn or "scorching" is often caused by high soil salt accumulation.

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. However, many years of high soil salt accumulation may pass before injury becomes apparent. The symptoms initially include an abnormal foliage color, needle tipburn, and marginal leaf burn progressing toward the mid-vein of affected leaves (figure 2). Progressive symptoms may include a reduction in leaf, flower, and fruit size; premature fall coloration and defoliation; stunting; and a general decline in health.

Salt injury patterns for metro areas

Common street and landscape trees

Acer negundo (I)
Acer saccharinum (S)
Silver Maple
*Aesculus glabra (T)
Ohio Buckeye
Acer rubrum (I)
Red Maple
*Betulaspp. (I)
Aesculus hippocastanum (T)
Horse Chestnut
Acer saccharum (S)
Sugar Maple
Catalpa speciosa (I)
Northern Catalpa
Amelanchierspp. (S)
Betula nigra (I)
River Birch
Fraxinus pennsylvanica (I)
Green Ash
Elaeagnus angustifolia (T)
Russian Olive
Carpinus caroliniana (S)
Blie Beech
Juniperusspp. (I)
Fraxinus americana (T)
White Ash
Celtis occidentalis (I)
Pinus nigra (T)
Austrian Pine
*Ginkgo biloba (T)
Crataegusspp. (S)
Pinus ponderosa (I)
Ponderosa Pine
Gleditsia triacanthos (T)
Honey Locust
Juglans nigra (T)
Black Walnut
Populus deltoides (I)
Larix decidua (S)
European Larch
Juniperus virginiana (I)
Eastern Redcedar
Pseudotsuga menziesii (S)
Douglas Fir
Picea glauca densata (T)
Black Hills Spruce
Malusspp. (I)
Pyrusspp. (I)
Picea pungens (S)
Colorado Spruce
*Ostrya virginiana (S)
Ulmus americana (I)
American Elm
Pinus banksiana (T)
Jack Pine
Picea abies (S)
Norway Spruce
  Populus alba (T)
White Poplar
Picea glauca (S)
White Spruce
  Robinia pseudoacacia (T)
Black Locust
Pinus resinosa (S)
Norway Pine
  Salix alba tristis (I)
Golden Weeping Willow
Pinus strobus (S)
White Pine
  Sorbusspp. (S)
Mountain Ash
Pinus sylvestris (I)
Scots Pine
  *Syringa reticulata (T)
Japanese Tree Lilac
Prunus serotina (T)
Black Cherry
Quercus alba (T)
White Oak
Quercus macrocarpa (I)
Bur Oak
Quercus palustris (S)
Eastern Pin Oak
Quercus rubra (T)
Northern Red Oak
Taxusspp. (S)
Thuja occidentalis (I)
American Arborvitae
Tilia americana (S)
American Linden
Tilia cordata (S)
Littleleaf Linden
Tsuga canadensis (S)
Canada Hemlock
Populus tremuloides (I)
Quaking Aspen

*Species marked with an asterisk show serious inconsistencies because the evaluations are based on a single parameter and insufficient data.

The symbol after the name of the plant indicates its tolerance to soil salt.
S=sensitive, I=intermediate, T=tolerant

Salt-tolerant species

Although salt-tolerant species are available, there are relatively few of them. If only tolerant species are planted, there are few opportunities to match tree species with soil characteristics, and the risks of a single disease or insect pest destroying a high proportion of the trees are increased. No species is completely tolerant of salt injury; even salt-tolerant trees have limits on the amount of salt they can accept before they weaken and become vulnerable to other problems.

This table lists trees commonly used on streets and landscapes in Minnesota. Plants listed as intermediate or tolerant are recommended for areas where spray salt is common. Note that a species that tolerates spray salt will not necessarily tolerate soil salt.

Minimizing salt injury

drawing of a fence in snow

Figure 3. A simple snow fence can provide effective protection for susceptible plants.

Use the following guidelines to minimize or possibly eliminate salt damage to trees and shrubs in urban areas.

Partial Funding was provided by:
The Minnesota Department of Transportation
University of Minnesota Extension [the Renewable Resources Extension (RREA) program of the University of Minnesota Extension and the U.S. Department of Agriculture--Cooperative States Research, Education and Extension Service (CSREES)]

WW-01413-Go Reviewed 1999

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