Saturday, November 10, 2007

Landscape - Winter Deicing Compounds and Salt Injury

De-icing salt injury along route 206 in Sussex County, NJ.
Photo credit: Spence Davis. (bottomthird of pines on right are brown.)

Winter is just around the corner and the first snowfalls cannot be far away. One issue that comes with winter storms is the effect of deicing compounds on plants and potential salt injury. The following is an article on the subject.

Roadside vegetation will be exposed to de-icing compounds in upcoming winter storms. Runoff from treated pavement contains dissolved salts that can injure adjacent vegetation. In plants sensitive to excessive salt, affected foliage may appear “scorched” and drop prematurely. In severe cases, the death of twigs, branches, and sometimes the entire plant, may occur.

Why are De-icing salts used?

Deicing salts make roadways, driveways, and sidewalks safer by melting snow and ice. These compounds are usually applied during snow storms before the snow can accumulate. The salt dissolves in water to form a brine that has a freezing point lower than water. The brine melts ice and helps to prevent the formation of more ice as temperatures drop. The two most commonly applied de-icing salts are sodium chloride (rock salt) and calcium chloride. To improve traction, de-icing salts are usually mixed with abrasives. These abrasives, which include sand, cinders, gravel, and sawdust, can accumulate along roadways and cause problems with drainage.

How does salt affect vegetation?

Plants become injured when salt-laden water contacts roots and foliage. The foliage on roadside vegetation is injured when salted water sprays up from the pavement by passing vehicles. Salt-laden water can also percolate down through the soil profile, coming into contact with soil particles, soil microbes, and plant roots. Salt injures vegetation by:

● Increasing water stress. In the root zone, water molecules are held very tightly by salt ions, making it difficult for roots to absorb sufficient quantities of water. In sensitive species, this “physiological drought” may result in depressed growth and yield.

● Affecting soil quality. The sodium ion component in salt becomes attached to soil particles and displaces soil elements such as potassium and phosphorus. As a result, soil density and compaction increases and drainage and aeration are reduced. Plant growth and vigor are poor under these conditions.

● Affecting mineral nutrition. When the concentration of both the sodium and chloride components of salt in the root zone is excessive, plants preferentially absorb these ions instead of nutrients such as potassium and phosphorus. When this occurs, plants may suffer from potassium and phosphorus deficiency.

● Accumulating to toxic levels within plants. The chloride component of salt is absorbed by roots and foliage and becomes concentrated in actively growing tissue. Plants repeatedly exposed to salt over long periods of time may accumulate chloride ions to toxic levels, resulting in leaf burn and twig die-back.

How do plants respond to excessive salt?

Unlike animals, plants do not have mechanisms to excrete excess salt from tissues and can only “shed” salt in dead leaves and needles. Because conifers do not shed leaves on a yearly basis, they tend to suffer damage from accumulated salt more easily than do deciduous trees.

Plant species vary in their tolerance to salt exposure. Plants that are tolerant of salt grow as well in saline soils as they do under normal conditions. Many herbaceous plants such as grasses adapt fairly readily to high salt levels. Among woody plants, tolerance varies with the species. Plant species with waxy foliage are generally more tolerant of salt spray. In salt-sensitive plants, exposure to salt can result in poor growth, stunted leaves, heavy seed loads, twig and branch die-back, leaf scorch, and premature leaf drop. Plants stressed by excessive salt are also more susceptible to biotic diseases and insect pests. The extent of injury a plant sustains in response to salt depends on:

● The kind and amount of salt applied. Sodium chloride (rock salt) can be very damaging to plants. De-icing compounds without chloride, such as urea, are safer for vegetation.

● The volume of fresh water applied. Although salts are easily leached by water in well-drained soils, they tend to accumulate in poorly-drained soils, so the potential for damage to vegetation in these soils is high. High volumes of water, whether from rainfall or melting snow, will decrease the possibility of injury. Rainfall also washes salt from foliage surfaces.

● The distance plants are situated from treated pavements. Plants within the “spray zone” of moving vehicles are more likely to sustain salt injury. Injury is usually most evident on the side of the plant that faces the highway.

● The direction of surface-water flow. The channeling of drainage water away from susceptible plants will prevent salt from coming into contact with plant roots. If plants are situated up-slope or away from drainage areas, they are less likely to be affected.

● The time of year salt is applied. Salt applied in late winter and early spring is more likely to damage vegetation than salt applied earlier in the winter season. This is because there is less time for winter snow and precipitation to leach salt from the root zone before growth resumes in the spring.

How can we minimize salt injury?

The best solution to the de-icing salt problem is to prevent contamination. Clear snow first, and then use abrasives instead of salt to treat driveways and walkways. If vegetation is located in areas where salt spray occurs, erect barriers or screens to protect plants during the winter months. Anti-desiccants may also help prevent injury when applied to evergreen foliage where de-icing salt will be used. County, state, and municipal officials can help prevent salt injury by carefully training equipment operators and frequently calibrating equipment.

Once soil becomes contaminated with salt, damage can be reduced by leaching the salt with fresh water as soon as possible after exposure. Under certain circumstances, incorporation of gypsum at the rate of 50 lb/1000 sq ft into the top six inches of soil at the drip-line of trees may also be helpful. Furthermore, foliage exposed to salt spray may be washed with salt-free water to remove deposited salt.

When landscaping, place trees and shrubs that are sensitive to salt as far as possible from problem areas consider the following: Select planting sites that are not subject to salt-contaminated waters, and place shallow diversion ditches between roadways and plantings. When vegetation must be placed near roadways, utilize salt-tolerant plants.

Article by Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University in the March 16, 2006 issue of the Plant and Pest Advisory, Landcape, Nursery, and Turf Edition.

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