Spreading salt to make treacherous ice magically disappear from road surfaces must have seemed brilliant in the early days of its use. Naturally occurring, abundant, and cheap, road salt allows us to get to work on time and to get home again, even when menacing snow or sleet pelts through the total darkness of an early morning or evening commute.
But there is a rather high price for this peace of mind when everything is counted. Rusted cars, corroded bridges, weakened parking garages, contaminated wells, and a lot of sick roadside vegetation are among the many costs of a safer ride.
Salt works by lowering the freezing point of water. Any substance that will dissolve in water lowers its freezing point, and even sugar water will depress the freezing point slightly below 32˚F. But salt is so much better: a 23.3 percent concentration of sodium chloride freezes at -6˚F. Though considerably more expensive, calcium chloride is even better: a 29.8 percent solution remains liquid to -67˚F.
New Hampshire became the first state to adopt a general policy of using road salt, (sodium chloride), in the winter of 1941-1942. Back then, other northern states used salt for special situations, and that year, a total of 5,000 tons was used nationwide. Now 22.5 million tons are spread annually, amounting to 51 percent of all the salt that is mined in this country.
Sixty or so years of enthusiastic salt use has taught us a lot, and now cars are made of more rust-resistant materials, reinforcing in concrete bridge decks is epoxy coated, new coatings and sealants deter salty water from penetrating concrete, and there are better paints for steel bridges. Some of the most severe damage from salt used to come even before it hit the pavement, a problem that highway crews have corrected by storing salt under cover. Even so, salt continues to be highly corrosive.
Though woody roadside vegetation cannot be modified so simply, it turns out that some species have considerably more resistance to salt than others. Unfortunately, this list does not include many of our most common trees; sugar maple, beech, and white pine are among the worst choices for salty roadsides. And, alas, some of those aggressive non-native species that we love to hate do rather well in brine, including tatarian honeysuckle, Russian olive, common buckthorn, and Norway maple.
Salt enters trees and shrubs either by being splashed onto the aboveground parts or by way of the roots. Curiously, plants that resist salt at one entry point may be highly vulnerable at the other. Northern red oak is a champ at keeping soil-borne salt out, but when grown near the ocean, its leaves turn brown within a few days if they get splashed with salt spray by hurricanes.
It’s probably no coincidence that the most salt-tolerant native conifer is tamarack, for it’s also the only conifer to shed its needles in fall. The large surface area of a white pine in winter collects much more salt than a typical deciduous tree does. Curiously, even a small amount of salt in the buds of some deciduous species can cause them to lose cold hardiness.
In solution, salt separates into sodium and chloride ions, and it’s the chloride ones that cause more damage to plants at the cellular level. Salt tolerance is based more on a plant’s ability to exclude chloride than on any intrinsic ability to tolerate it. Severely damaged plants may have chloride levels in their tissues of 1.25 to 2 percent. Tolerant species growing under the same conditions will have levels of 0.5 to 0.9 percent.
Though sodium ions enter plant tissues more slowly, they are the major culprits in soil compaction – yet another way that salt kills vegetation. Sodium ions change the way soil particles aggregate, leading to a loss of normal spatial distribution. Severely compacted soil restricts access to water and oxygen, and the symptoms on trees and shrubs resemble those caused by drought. Plus, sodium travels along the same physiological shuttle system as potassium and magnesium – both essential to making chlorophyll – and if excess sodium ties up the works, potassium deficiency results.
The highway departments of both New Hampshire and Vermont are committed to reducing the quantity of salt applied to roads. By monitoring road surface temperatures, using accurately calibrated spreaders and proper application procedures, keeping salt stored in sheds, and substituting sand where possible, annual consumption has dropped in recent years. One way to improve the effectiveness of road salt at low temperatures is to pre-mix it with a relatively small amount of calcium chloride.
It’s unlikely that most people will ever tolerate the icy roads of 60 years ago. Though less than ideal, it’s likely to be mostly plain old salt that spews out of the spreaders for quite some time to come.