As the snow begins to fall and the temperatures hover around freezing, the great Minnesota season of “deicing” has begun. The rattle of snow plows portends large piles of gray dirty snow along streets and at the ends of parking lots and driveways. For safeties’ sake, tons of chloride salts will be spread on sidewalks and streets throughout the Twin Cities metropolitan area to melt snow and ice and provide better traction for pedestrians and motorists.
While chloride salts can be helpful for decreasing accidents due to snow and ice, they can cause serious problems to trees, shrubs, turf grass and perennial plants. Plants are exposed to salts either by direct misting with road spray or by saturating the ground where their roots are growing.
Direct misting can result in direct foliar damage to evergreens such as arborvitae, spruce and pine lined adjacent to roadways. This damage becomes obvious as winter progresses. In the spring, if the misting was severe, dead limbs and foliage will be obvious on the road side portions of the plants. In some cases, the stem tissue may just be damaged and not dead. In these cases, latent buds may pop out in the spring and begin re-foliation in the damaged areas of the plants (Figure 1). While the trees aesthetic character may be diminished by the salt mist injury, the trees will survive (Figure 2). If the tissues are dead, recovery will not occur. The only way to determine if the tissues are dead or alive is to wait and see what happens in the spring.
Unfortunately, the only way to protect trees from this damage is to create barriers that keep the mists from contacting the living tree tissues. The use of anti-desiccant/anti-transpirant applications will not usually provide adequate protection from the salt damage to living tissues. Spraying the plants with water during warm periods that are above freezing in the winter can wash some of the salts off the foliage.
The other form of salt injury that occurs to plants is through the chloride salts soaking into the soils. Salts can raise the pH of soils, can prevent the uptake of moisture through plant roots and can be directly toxic to the fine roots that acquire nutrients for the trees.
In one situation in Saint Louis Park, Minnesota we found sidewalk plantings of trees growing in clay soils with a pH of 9.4 (Figure 3). Not even sagebrush can grow in soils with that high a pH. Of course, the soils also had an abundance of sodium and magnesium since they were using both magnesium chloride and sodium chloride as deicing salts on the property. The property owners were replacing the trees every other year. We were only able to make an impact on the trees when we built planting boxes with curbs (Figure 4). This allowed the salt used on the sidewalks to drain around the trees into the street drains. We also had to replace the soils when we replanted trees.
Building salt barriers to prevent deicing salts from getting into the soils where plants are growing is one option. It is also a good idea to create a snow piling area that is on a hard surface. When melting does occur the runoff will run into the storm drain system rather than onto the plants in the landscape.
If barriers and piles on hard surfaces are not an option, then it is a good idea to heavily irrigate salt impacted soils and plants early in the spring season to wash the surface salts away from the plants before they have an opportunity to penetrate the soil. Once the salts are in the soil, water can be applied to wash it down through the soil profile away from where the roots are growing. This flushing is more effective in soils with significant sand content and less so with clay and loam soils that aren’t as porous and hold onto water.
Invariably there will be some damage on the edges of lawns and to trees and shrubs where salt accumulates (Figure 5). In the spring, application of organic amendments to impacted soils can help to mitigate the damage. Organic amendments help to bind salts, increase the porosity of soils for flushing and stimulate microbes that utilize and render the salts into less damaging compounds. This effect is limited to mitigating minor salt issues with soils. As with the Saint Louis Park example, nothing short of replacing soil can help when the soils have become toxic.
The arborists at Ostvig Tree Care have been protecting trees from salt injury for decades. They are also on the forefront of using new organic amendments opportunities to ameliorate soils to better help plant recovery in impacted landscapes.
Dr. John Lloyd, Ph.D. contributes to the Ostvig Tree Care website blog. He is available through Ostvig Tree Care to answer questions on tree care and plant health issues. Feel free to send any questions you may have about this blog or any other tree issues to firstname.lastname@example.org.