Snow and ice are headaches for motorists and pedestrians. To promote public safety, deicing compounds are used to melt ice and snow on sidewalks, driveways, and highways. While deicing materials are necessary for safe winter travel, they can damage automobiles, concrete surfaces, and landscape plants.
Most deicing materials are salts that melt ice by lowering the freezing point of water below 32 F. However, a few other materials can be used on icy and snowy surfaces. Advantages and disadvantages of several commonly used deicing agents are discussed below.
Sodium chloride (NaCl) is commonly known as table salt or rock salt. It is the least expensive and most widely used deicing compound. Sodium chloride is most effective when temperatures are above 15 F. Unfortunately, NaCl is highly corrosive and can damage plants. High levels of sodium are also detrimental to soil structure.
Calcium chloride (CaCl 2 ) dissolves easily and acts quickly. It is also effective in extremely cold temperatures (-20 F) and leaves no visible residue when dry. Calcium chloride is highly corrosive to concrete and metals. It is slightly less damaging to plants than sodium chloride.
Potassium chloride (KCl) is a naturally occurring material used as a fertilizer (0-0-60) and deicing material. KCl is highly corrosive, but is less damaging to plants that sodium chloride.
Calcium magnesium acetate (CMA) is a salt-free deicing compound made from dolomitic limestone and acetic acid. CMA is effective to 20 F. It causes little or no damage to landscape plants and is less corrosive than deicing salts. Unfortunately, CMA is quite expensive (as much as 20 to 30 times more expensive than sodium chloride) and is not widely available.
Abrasive materials, such as sand, cinders, and ash, don t melt ice and snow, but improve traction on slippery surfaces. While these materials are inexpensive and not harmful to plants, they are often tracked indoors and treated surfaces often need to be cleaned in spring.
Effects of Deicing Salts
Deicing salts (NaCl, CaCl 2 , and KCl) 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 turfgrass 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, such as magnesium, by the plant s 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. Damage is most severe on the side of the plant nearest the highway.
The severity of damage depends on many factors. The more salt that is applied to icy and snowy surfaces, the greater the runoff into nearby soils. The degree of salt damage also depends on 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 much more tolerant of salt than poorly growing specimens. White ash, honeylocust, and Norway maple are salt tolerant, while sugar maple, hackberry, littleleaf linden, and white pine are highly sensitive.
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. Mix salt with an abrasive material. 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 plant s root zone. 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.
This article originally appeared in the November 8, 2002 issue, pp. 126-127.