Snow and ice are headaches for motorists and pedestrians. To prevent accidents on slippery surfaces, deicing compounds are used by highway departments, businesses, and homeowners to melt ice and snow on roadways, parking lots, sidewalks, and driveways. While deicing materials improve travel conditions, they can damage automobiles, concrete surfaces, and landscape plants.
Most deicing materials are salts that melt ice and snow by lowering the freezing point of water below 32 degrees Fahrenheit. Commonly used deicing salts include sodium chloride (NaCl), calcium chloride (CaCl2), potassium chloride (KCl), and magnesium chloride (MgCl2).
Abrasive materials, such as sand, do not melt ice or snow. However, they improve traction on snow and ice-covered surfaces.
Effects of Deicing Salts
Deicing salts can damage landscape plants when excessive amounts accumulate in the soil. The most serious damage typically occurs near major streets and highways where salt from run-off accumulates in the nearby soil. Excessive use of salt by homeowners can also create problems. Trees, shrubs, perennials, and turfgrasses are susceptible to salt damage. Additionally, spray from passing vehicles can damage roadside plants, particularly evergreens.
Salts affect plant growth in several ways. When high levels of salt are present in the soil, plants are unable to absorb sufficient water even though soil moisture is plentiful. Plants suffer a salt-induced water shortage termed "physiological drought." High levels of salt restrict the uptake of essential nutrients by plant roots. Excessive amounts of sodium and chloride ions in plant tissue are toxic to many plants. Soil structure is damaged by high levels of sodium. Salt deposited directly on plant foliage can cause dehydration of plant tissue.
The symptoms of salt injury to deciduous trees and shrubs include stunted growth, marginal leaf scorch, early fall coloration, and twig dieback. Accumulation of salt in the soil over several years may result in progressive decline and eventual death.
Salt damage to evergreens results in yellowing or browning of the needles and twig dieback. Evergreens near heavily salted roadways are often damaged by salt spray. Spray damage is most severe on the side of the plant nearest the highway.
The severity of plant damage depends upon the type of salt and other factors. Calcium chloride, potassium chloride, and magnesium chloride are less harmful to plants than sodium chloride. The degree of salt damage also depends upon the amount of salt applied, soil type, amount of rainfall, direction of run-off, and prevailing winds. The condition and type of plant material is also important. Healthy, vigorous plants are more tolerant of salt than poorly growing specimens. Bur oak, northern red oak, honeylocust, northern catalpa, Kentucky coffeetree, horse chestnut, mugo pine, and eastern red-cedar are tolerant of soil-borne salt, while sugar maple, American linden, Canadian hemlock, and eastern white pine are sensitive to soil salt.
Salt Injury Prevention
Homeowners can minimize salt damage by using deicing salts prudently. Before applying salt, wait until the precipitation has ended and remove as much of the ice and snow as possible. Use deicing salts at rates sufficient to loosen ice and snow from driveways and sidewalks, then remove the loosened ice and snow with a shovel. (Deicing salts need to be applied at much higher rates to completely melt ice and snow.) Mix salt with abrasive materials, such as sand or kitty litter. Fifty pounds of sand mixed with one pound of salt works effectively. Avoid piling salt-laden snow and ice around trees and shrubs. While the amount of salt applied to major roadways can not be controlled, steps can be taken to minimize damage. As soon as the ground thaws in early spring, heavily water areas where salt accumulates over winter. A thorough soaking should help flush the salt from the root zone of plants. If possible, alter the drainage pattern so winter run-off drains away from ornamental plants. When planting trees near major streets or highways, select salt tolerant tree species.
Deicing salts are both good and bad. Judicious use of deicing salts helps insure safe travel conditions for pedestrians and motorists and minimizes damage to landscape plants and the environment.