With winter comes questions from readers about avoiding salt damage to garden plants. Passing cars and snow plows spray salt-laden slush onto garden beds, winter buds of deciduous plants, and the needles of conifers. De-icing salts slowly spread from driveways and sidewalks into nearby garden soil. Decisions made last May about where to plant that new tree or shrub come into question as the white crust accumulates.

Symptoms of salt damage

Symptoms of damage from salts deposited on plants during the winter become evident the following growing season: complete branch die-back, browning of needles or leaves (particularly on the side of the plant facing the road), lack of flowering, and “tufting” of twigs at the ends of branches where the terminal bud was killed (also know as “witch’s broom”). All of these symptoms are caused by the deposition of airborne salts.

Although often seen planted along roadsides and in highway medians, the white pine, Pinus strobus, is very sensitive to salt and exhibits the classic symptoms of salt injury. Needles on the side facing the road are often brown, except for lower portions of the tree covered by snow. Also, trees planted farthest from the road suffer the least amount of damage.

Symptoms associated with soil accumulation of salts include stunted growth, early defoliation, yellowing of leaves and, in severe cases, branch die-back and plant death.

All of these symptoms are similar to those associated with drought stress. Indeed, high salt levels in the soil severely impede water uptake by the plant.

Avoiding salt damage

The distance from the road that salts are carried depends on the traffic speed and prevailing wind direction. Along high-speed highways, plants should be placed at least 60 feet from the road; along busy city streets, a distance of at least 30 feet is recommended. Also, avoid planting where runoff accumulates.

Sensitive plants placed too close to the road can be protected from aerial deposition of salts with physical barriers made from burlap, snow fencing, or plywood. As unattractive as these barriers are, they are effective. Remember that the height of airborne salts increases with distance from the road.

A dense screen of salt-tolerant plants will provide a permanent and more attractive physical barrier to windborne salts. Recommended native conifers include arborvitae, Thuja occidentalis, and white spruce, Picea glauca. A more complete list of salt-tolerant plants can be found at http://www.rittenhouse.ca/hortmag/glynis/salty.asp.

Risk of damage from de-icing salts can be reduced by using alternatives to sodium chloride (table salt), the least expensive and thus most frequently used salt. Effective alternatives that are more vegetation (and pet) friendly include calcium chloride and calcium magnesium acetate (CMA). Also, mix the salt with sand or sawdust to reduce the threat to garden plants.

Late winter and spring rains help to wash salts from the above-ground portions of plants while leaching salts from the surrounding soil. Later in the spring, after these early rains, apply several inches of water to plants and surrounding soil to ensure that the accumulated salts are washed below the root zone.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.