Whenever a winter storm is forecast, retail stores stock their shelves with an impressive array of products designed to help their customers do battle with ice and snow. Since we live in a litigious society, ice-melting products are usually prominent in the arsenal on display. Although ice-melt products are effective in reducing injuries to people, plants in areas near ice melter deployment may suffer collateral damage. A little tactical planning before applying ice-melting products can go a long way toward reducing negative effects on plants in the battle zone.
Although there are many products sold for melting ice, most of them work on basically the same principal; freezing point depression. One of the least expensive and most commonly used materials is rock salt, or sodium chloride (NaCl). This is the very same chemical added to an ice cream freezer to allow the melting ice to attain a temperature well below 32 degrees Fahrenheit and remain liquid. Other materials that can be used as sidewalk de-icers include calcium chloride (CaCl), ammonium nitrate (NH4NO3), potassium chloride (KCl), magnesium chloride (MgCl) and urea (CO NH2)2, all of which are ingredients in fertilizer products. Calcium chloride, although more expensive than rock salt, is probably the second most commonly used de-icing salt. This is because it is effective down to –20 F as opposed to +15 F for rock salt and because it is easier on sidewalks and nearby plants than is rock salt.
While de-icing materials may improve traction of shoes and tires in the short term, they may have some longer lasting, unpleasant effects if used carelessly. First, some materials cause pitting and crumbling of concrete sidewalks, making them unattractive or even unsafe. Rock salt (NaCl) is particularly hard on concrete walks, while urea, CaCl, KCl and MgCl cause much less damage.
Indiscriminant application of de-icing materials damages plants in exactly the same way as does excessive application of fertilizer salts. Plants absorb essential plant minerals from the soil as salts in solution in soil water. However, as salts accumulate, they increase the energy with which water is held in the soil. This makes it more difficult for roots to extract soil water, leading to stress and, if the salt concentration gets too high, to "burning" of roots, wilting and scorching of leaves and death. Direct contact of salt with foliage of evergreens, such as junipers, can also cause browning and dieback by drawing water out of plant tissues. Another problem associated with some types of ice-melters is degradation of soil structure. Accumulation of sodium causes clay particles to pack together more densely, leading to poor water infiltration and internal drainage. Thus, de-icing salt applied to sidewalks or roadways can have drastic effects on plants if allowed to wash and infiltrate into nearby soil.
There are several common sense steps that can be taken to reduce the likelihood that sidewalk de-icing materials will damage nearby plants. The most direct approach is to prevent the salt from contacting plants. Consider using dark colored abrasive materials like sand to improve traction. Use de-icers in moderation and channel runoff so that it does not soak into the soil near the sidewalk. However, if salt solution does enter the root zones of plants, damage can be reduced by simply leaching with water, much the same way that excess fertilizer salts are leached from the pots of houseplants by thorough watering. Leaching of clay soil can be difficult, however, since water infiltration is very slow. A simple method to determine whether ice-melting salt has leached into the soil is to take a soil sample and have it tested for soluble salts. Since the electrical conductivity (EC) of water is proportional to the concentration of salts dissolved in it, the salt content can be estimated by measuring the EC of a water extract of the soil with a simple, electronic conductivity meter. An EC value above 5 or 6 millisiemens per centimeter may indicate a problem for salt-sensitive plants such as roses, spireas and some viburnums.
Gypsum (CaSO4) is commonly recommended as a remedy to reduce the impact of salt contamination on plants. While it can help reduce the level of sodium in the soil, gypsum cannot counteract the negative effects of high salts in the soil, no matter how much is applied. Gypsum works by displacing sodium in the soil with calcium, thus allowing sodium to be leached out and helping to improve the structure of high sodium soils. Since it has very low solubility in water, it can be applied at high rates (up to 700 pounds per 1000 ft2) to badly contaminated soil without damaging plants. Unlike lime (CaCO3), gypsum will not raise the pH of soil, even when applied in large quantities. When reclaiming land from the sea, the Dutch plow in gypsum, leach first with seawater and then with river water. If low salt water is used for the first leaching, the soil seals up and cannot be leached further.
Next time you are faced with icy sidewalks, think about nearby plants as you develop your battle plan. Use ice melters in moderation, channel salty runoff away from plant roots and, when in doubt, check the electrical conductivity of possiblyaffected soil. Be prepared to leach, and don't expect gypsum to be a quick fix to salt contamination.
For more information on effects of ice melters on plants see http://www.conservation.state.mo.us/documents/forest/3-124.PDF
REVISED: October 8, 2013