By Rebecca Willison
MWC often gets questioned in the spring about why trees along roadways have brown leaves or needles and how this can be avoided. While tree damage from road salt usage doesn’t become obvious until the spring, now is the time to consider ways to minimize damage over the winter.
Approximately five million tonnes of road salt are released into the environment in Canada each year. The most commonly used road salt is the same salt that is used on food – sodium chloride.
Although road salt assists with keeping pavement dry and safe during the winter and has little negative effect on human health, its widespread use can cause damage to trees, shrubs and other plants along roadways.
How salt damages plants
Road salt can damage plants in two ways. The first is through an airborne spray that kills dormant buds by penetrating leaf scars. The second occurs when salt accumulates in the soil where it breaks down into its two components, sodium and chlorine, both of which act differently to affect the plant.
In extreme cases, chlorine ions in the soil are taken up by the tree in early spring, where they enter the sap and concentrate in shoots, preventing bud openings. Eventually, the chlorine is transported to actively growing leaf margins where it causes leaf scorch, curling and death.
Sodium ions in the soil follow the same route as the tree’s nutrients, blocking magnesium and potassium, both of which are necessary for the production of chlorophyll. Again, in extreme cases, this can result in a potassium deficiency that may inhibit the tree’s resistance to drought and disease.
Salt accumulation in the soil can also cause a physiological drought. Salt solution near tree roots is more concentrated than the tree’s sap, impeding osmosis and preventing the tree from taking up water through its roots.
In general, the primary long-term damage to salt-weakened plants is a result of their increased susceptibility to insects, pathogens, and the environmental stress of drought, wind and ice. Tree death from road salt is less common.
Symptoms of salt damage
Salt damage to conifers is most noticeable in the spring. Branches closest to the road get yellow and brown needles that drop off. This colour change starts at the tips of the needles and progresses to affect the whole needle.
In Muskoka, the browning of trees is often noticeable on the east side of Highway 11 during early spring. By summer, these trees have usually recovered and have new green growth. Damage due to salt spray is usually short-term, with long-term damage occurring where the tree is under other stress.
Deciduous trees damaged from road salt may have many twigs densely clustered together, called witches’ brooms, near the ends of branches as a result of terminal buds killed by salt spray. Other symptoms include unopened flower buds, twig dieback, sparse, stunted or yellow foliage, and leaf scorch.
Young trees are more susceptible to salt damage because they have fewer roots than older ones.
Preventing salt damage to your trees
There are several ways that you can reduce the damaging effects of salt on your trees.
- Avoid the use of de-icing salts. Use coarse sand to help make driveways and sidewalks less slippery. If salt must be used, use as little as possible, lower the throwing distance, and apply before the area freezes.
- Keep your trees and shrubs healthy. A healthy plant is better equipped to survive salt spray and accumulation in the soil.
- Plant salt-tolerant trees near roadways and walkways. Some salt-tolerant native species include Red pine, Red oak, ashes, birches and poplars. Salt-intolerant species, such as White pine, Sugar maple, Eastern hemlock, basswood, and spruce, should not be planted near areas where salt is used.
- Use barriers to protect sensitive species from salt damage. Barriers may include plastic fencing, snow fencing, or burlap.
- Improve the drainage around trees or adjust the grade so salt is easily leached away from the trees.
- Flush the soil with water in the spring when it thaws so salts are sent beyond the trees’ root zones.
Until a cost-effective alternative to the use of conventional road salts is developed to keep our roads safe in the winter, reducing the amount of salt applied and minimizing salt damage to our trees and our watersheds is the best course of action.