How Salt Damages Trees
Salt damages trees through two pathways: via airborne salt spray, as on a busy highway, and via the soil. Salt spray that lands on a dormant twig can enter the tissue through leaf scar and kill the dormant bud.
When salt in the soil dissolves, it separates into sodium and chloride ions. The ions act differently to damage the tree. In early spring, the chloride ions can be taken up by the roots, enter the sap, concentrate in the shoots, and prevent buds from opening. Later, they can be transported to actively growing leaf margins, causing leaf scorch, curling, or death. Sodium ions use the same “chemical route” as necessary tree nutrients. As George Hudler, professor of plant pathology at Cornell explains, the sodium can “tie up the plant's shuttle system and restrict uptake of magnesium and potassium, two chemicals that are essential for making chlorophyll.” Potassium deficiencies are common in plants suffering from salt injury, says Hudler. Salt in the soil can create a physiological drought. Brine near underground tree roots can be a more concentrated solution than the sap in the roots. The roots therefore can't take in water through osmosis. Water is so unavailable to salt-stressed tree that they are actually dying of thirst.
Diagnose a Salt-Damaged Tree
Salt spray damage can occur in trees that are up to fifty feet from a fast moving, salted highway. Salt spray will damage exposed branches more severely than branches covered by snow. Suspect soil salt damage in trees that are near salted streets and sidewalks. Conifers damaged by salt spray, show the greatest damage in early spring.
On branches facing the road starting from the tips, needles become yellow or broken, and perhaps drop off. Soil salt can cause the needles of conifers to take on a blue-green cast. Deciduous trees affected by salt spray can develop tuft-like “witches' brooms.” Bunches of lateral branches grow to compensate for a terminal bud that was killed by salt. Deciduous trees growing in salty soil might have flower buds that don't open. Leaf scorch, whereby margins of the leaf turn prematurely brown can arise in the spring or during hot, dry weather. Foliage can be sparse, stunted, or yellow and twigs can show dieback.
Spice up your options
People who maintain paved surfaces can choose from a range of products. If you choose low-cost but toxic products, you might have to factor in the replacement cost of future plantings.
Rock salt, consisting of 98.5% sodium chloride, is the cheapest and most widely used of deicing agents, so for highway departments, it is king. The biggest drawback? Corroding bridges, cars and damaging tress. The long-range cost of rock salt damage could be 10-15 times the initial cost.
Calcium chloride, an effective deicer, works best below 15 degrees F. Eight times more expensive than rock salt, it tends to cake, making spreading difficult. It reportedly doesn't damage plants but still contains chloride, which could damage trees.
Potassium chloride is a naturally occurring material that is a fertilizer and a food salt substitute. It is completely eco-friendly, pet safe and easy to use. It is a popular choice among the environmentally friendly inclined.
Calcium magnesium acetate(CMA) is made from dolomitic limestone and acetic acid (the main acid of vinegar). It poses few problems to plants, but unfortunately, is expensive: a ton of CMA costs twenty times more than a ton of rock salt.
Alternatives to salt, such as urea fertilizers, gravel, cinders and ground peanut shells have proven effective in small-scale applications. Homeowners could try a mixture of 3 pounds of urea to 50 to 100 pounds of sand or cinders.
Keep trees out of the brine
Whatever your choice of deicing agent, especially if you've chosen rock salt as the preferred option this winter, but mindful of the following ways to minimize negative effects on your plants.
Reference: Salt injury to roadside Plants. George Hudler, State University at Cornell University, New York