University of Vermont Extension 
Department of Plant and Soil Science

Fall News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
Most people are only too aware of the damage and corrosive effects of salt on automobiles, but may not be aware of damage that winter salt on roads and walks can cause on plants. On heavily traveled highways from 40 to 80 tons of salt per lane mile per year may be applied. Landowners along these roads also are aware of the damage to plants that such salt can cause.

Deicing salt is usually refined rock salt consisting of about 98.5 percent sodium chloride, 1.2 percent calcium sulfate, 0.1 percent magnesium chloride, and 0.2 percent rock. Calcium chloride is reported to be less toxic to plants but is seldom used on roads because it is much more expensive than rock salt and more difficult to handle in bulk.

When sprayed onto plants from passing cars and plows, salt may enter plant cells or the spaces between the cells directly. One result of this "salt application" is that buds and small twigs of some plant species lose cold hardiness and are more likely to be killed by freezing.

Salt accumulation in the soil also may cause plant injury. This frequently occurs when salt-laden snow is plowed off streets and sidewalks onto adjacent lawns.

Anyone who has tried to get table salt out of a wet shaker knows how readily salt absorbs water. Rock salt exhibits the same property in the soil and absorbs much of the water that would normally be available to roots. Thus, even though soil moisture is plentiful, high amounts of salt can result in a drought-like environment for plants.
When salt dissolves in water, sodium and chloride ions separate and may then harm the plants. Chloride ions are readily absorbed by the roots, transported to the leaves, and accumulate there to toxic levels. It is these toxic levels that cause the characteristic marginal leaf scorch.

The symptoms of excessive salt resemble those caused by drought or root injury. They include stunted, yellow foliage; premature autumn leaf coloration; death of leaf margins (scorch); and twig dieback.  When conifers are injured by salt spray, the affected foliage turns yellow or brown in early spring. If spray is the primary cause of the salt deposit, discolored needles are soon masked by the new season's growth. However, if salt is excessive in the soil, the new needles may die as chloride ions accumulate in them. This could be lethal to the entire plant if it occurs for several consecutive years.

One characteristic of salt injury that aids in diagnosis is that it is often confined to branches facing the road. Trees closer to the road suffer more damage than those set farther back. You can often observe this on pines and other evergreens near interstates where they’ve been affected by the salt spray thrown from passing plows at high speeds and with wind.

Here are 10 measures you can use to prevent or lessen plant injury from salt.
-- Rather than sodium chloride, use calcium or magnesium or potassium chlorides.  These are more expensive but you need to use much less.   Or use sand, cinders, or one of the relatively new and safer deicing products (these are often liquids and consist of chemicals more akin to fertilizers). Liquid products wont track about outside or indoors as do granular products.
--In the coldest areas, use a calcium chloride product as it works down to about 5 degrees (F).  In contrast, rock salt only is effective down to about 15 degrees (F).  Other products fall in between.  If below 5 degrees (F), don’t use chemicals but rather sand for some traction if really icy.
-- Pre-apply any chemicals prior to icing or snow, as they are more effective then and you can use less.  Don’t apply on top of snow, rather shovel it away and then apply.
-- Generally use less salt and deicing chemicals, as many landowners tend to apply too much—small applications more often are more effective.
-- Late season applications (after March 1) are most detrimental and should be avoided if possible since this is the time plants are coming out of dormancy and are most susceptible to injury.
-- Screens of fencing or burlap may be erected to ward off salt spray from roads.
-- Salt and snow should not be piled around plants or in places where the resulting salt water will drain into plants when the snow melts.
-- Construct a shallow trench between walks or drives and plants or lawns.  This will help channel salt runoff away from desirable garden areas.
-- If weather permits, it's a good idea to flush the area around roots exposed to salt with fresh water as soon as the snow melts.
-- Where new trees and shrubs are to be planted and where exposure to salt is likely, select species or cultivars resistant to salt spray injury. Examples of salt-tolerant evergreens include white spruce and Austrian pine.
Salt-tolerant deciduous trees include ash, birch, honey locust, poplar, tamarack (larch), white oak, red oak, mountain ash, and weeping willow. Salt-tolerant shrubs include serviceberry, broom, pea shrub, hydrangea, winterberry, northern bayberry, mockorange, potentilla, most shrub roses, tamarisk, and spreading cotoneaster.

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