Q. An upswing in traffic on the road running next to our property, combined with a few consecutive years of heavy snowfall, has resulted in increasing amounts of plowed snow piling up near a row of eastern hemlocks each winter. The trees also catch spray from passing vehicles, and now the branches that face the road are turning brown at the tips. These trees help to screen our house from the road, and I would hate to lose them. Is there any way to help them recover from what appears to be damage caused by salt applied to the road?
Derry, New Hampshire
A. Two types of damage may occur to plants growing near surfaces treated with deicing salts. Discolored needles on evergreens, stunted or dying buds and branch tips, and delayed bud break and flowering, all on the side nearest the road, can result when salt from road surfaces splashes onto the plants' branches and stems. (This type of injury also happens to sensitive species planted in seaside gardens.) Salt that washes into the soil typically causes a more general lack of vigor that may include shortened shoots, early fall color and leaf drop, smaller leaves, and fewer flowers and fruits. These symptoms tend to develop gradually over years of exposure and can vary widely depending on the sensitivity of the plants, the type of deicing agent used, and the extent of the contact with the deicing salt. The effect of deicing salts on plants may also resemble damage resulting from drought, air pollution, or injury to the roots, making it difficult to diagnose.
Where public thoroughfares are involved, homeowners have little say in how much or what kind of deicing salt is applied. While deicing compounds such as magnesium chloride, potassium chloride, and calcium chloride cause less damage to plants, all are significantly more costly than the inexpensive sodium chloride favored by cash-strapped municipalities.
Ideally, plants growing in proximity to salty sources like roads or major bodies of water would be chosen for their ability to tolerate expsure to salt. Among evergreens, for example, eastern hemlock (Tsuga canadensis) and white pine (Pinus strobus) are prone to injury from both salt spray and salt in the soil, while eastern red cedar (Juniperus virginiana) and Austrian pine (Pinus nigra) have high tolerance to salt exposure. When changing traffic patterns or changing winter road-management practices create conflict between salty conditions and mature plants, gardeners are left to choose between relacing the plants with more suitable selections or trying to mitigate the effects of the salt.
Start by ensuring that plants in the "danger zone" are in good condition and adequately watered before winter arrives, advises extensiion horticulturist Richard Jauron of Iowa State University. Healthy, vigorous plants are more tlerant of salt exposure, he explains, noting that salt in the soil reduces plants' ability to take up moisture, even when it is plentiful When salty surface treatments begin, consider barriers of burlap or snow-fencing to limit contact with salt spray. As much as possible, avoid piling salt-laden snow and ice around trees and shrubs.
As soon as the ground thaws in early spring, heavily water areas where snow and salt accumulates during the winter, Jauron recommends. A thorough soaking will help to flush salt away from the root zone and minimize any damage. If possible, alter the drainage pattern to divert the salty runoff from roadways away from your landscape plants, he adds.