Most de-icing materials are salts that melt ice by lowering the freezing point of water below 32 degrees fahrenheit. However, a few other materials can be used on icy and snowy surfaces. Advantages and disadvantages of several commonly used de-icing agents are listed below.
- Sodium chloride (NaCl) is commonly known as table salt or rock salt. It is the least expensive and most widely used de-icing compound. Sodium chloride is most effective when temperatures are above 15F. Unfortunately, NaCl is highly corrosive and can damage plants. High levels of sodium are also detrimental to soil structure.
- Calcium chloride (CaCl2) dissolves easily and acts quickly. It is also effective in extremely cold temperatures (-20F) 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 de-icing material. KCl is highly corrosive, but is less damaging to plants than sodium chloride.
- Calcium magnesium acetate (CMA) is a salt-free de-icing compound made from dolomitic limestone and acetic acid. CMA is effective to 20F. 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.
So what kind of damage is caused by de-icing salts?
Hardscape: Salts are corrosive and accelerate rusting of metal railings, grates, drains, and door frames, and underground utility lines if they are not properly protected. Salts may also cause scaling, or flaking of surface layers from concrete. Salt solutions enter void spaces in concrete and expand by 10 to 20 percent in volume when they freeze. The pressure exerted by this expansion fractures the surface of concrete. Porous brick, masonry, and natural stone are especially vulnerable to damage and should be avoided in areas where de-icing salts are used. Concrete which is properly formulated for environments where freeze-thaw cycles are common resists scaling caused by de-icing salts. Corrosion resistant paints and sealers will also minimize the impact of de-icing compounds on the hardscape, and are recommended for areas where de-icing salts are used.
Indoor Surfaces (floors and carpets). Residues from de-icing chemicals are frequently tracked into buildings and deposited on floors and carpets. Salts degrade wax and other finishes, leaving a dull appearance on floors and requiring more frequent cleaning and maintenance of indoor surfaces. Sodium and potassium chloride salts are relatively easy to remove from floors and carpets. Calcium and magnesium chloride salts, however, leave a greasy film and require wet cleaning with detergents to remove the residue. Abrasive materials such as sand also mar the finish on floors and can be difficult to remove from carpets.
The Environment. Salts move rapidly with water off surfaces and into the surrounding soil. If sufficient water is moving through the soil, components of the salt may leach to ground water. Salt may also run off and enter surface waters, potentially degrading quality and killing fish and other organisms. Certain salts have greater potential for environmental damage than others. For example, nitrogen salts have a high risk for surface and ground water pollution while organic salts (calcium magnesium acetate, CMA) have a high risk for surface water pollution.
Vegetation. De-icing salts (NaCl, CaCl2, 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, lawns, and other landscape plantings are damaged by de-icing salts accumulating in the soil. The salt accumulation usually occurs where snow is deposited directly by plowing or through runoff as snow melts in spring.
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. Salt deposited directly on plant foliage can cause dehydration of plant tissue. Soil structure is also damaged by high levels of sodium.
Diagnosing salt damage sometimes is difficult. The symptoms of salt injury to deciduous trees and shrubs include stunted growth, marginal leaf scorch, early fall coloration, and twig dieback. 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. Accumulation of salt in the soil over several years may result in progressive decline and eventual death.
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.
Homeowners can minimize salt damage by using de-icing 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. Exposure to de-icing salts may be unavoidable if your property is adjacent to a street or road subject to frequent salting and plowing. In these cases, it may be possible to select specific plant material that is more resistant to salt accumulation in the soil and salt exposure to the foliage.
Year-round practices of proper pruning, watering, and fertilization, that promote plant health, will also make plants more tolerant to salt exposure. However, for the overall health of the landscape, the goal should be to reduce or eliminate the use of de-icing chemicals if possible.
I hope that you have a safe and happy holiday season.