De-icing salt is the major culprit.
Salt readily absorbs moisture, tying it up in the soil and extracting it from plant roots.
Sodium and chloride ions also travel throughout the plant, accumulating in various plant parts such as leaves and stem at a toxic level. Affected evergreen foliage turn yellow or brown in the early spring. Damage usually occurs only on the side of the plant facing a road, driveway or sidewalk. Avoid injury by leaching the soil thoroughly with water. Alternatives to salt such as sand, cinders or light gravel can be used.