Salt And Pollution: A Toxic Mix For Trees

Trees and plants are often affected by exposure to salt and pollution. De-icing salt, which is often used to improve safety on roads, streets, driveways, and parking lots, can cause significant damage to plants and trees. This damage can lead to reduced growth, disfigurement, and even death. The salt accumulates in the soil, affecting the nutrient balance and water absorption, and can also directly damage the foliage through spray from passing cars. Additionally, pollution from vehicles and other sources can further harm trees and plants, contributing to a decline in their health. Understanding the impact of salt and pollution on trees is crucial for developing strategies to protect and preserve our natural environment.

Salt spray on foliage

Salt spray can also cause salt burn on buds, leaves, and small twigs. The unprotected developing leaves and flower buds dry out and are often killed by the cold winter wind. Salt spray can further desiccate the bud scales, exposing the tender tissues of the developing leaves and flowers.

The worst damage occurs to sensitive tree species planted near heavily salted roads with high traffic, especially when they lie downhill, downwind, or have poor drainage. In such cases, the salt spray from passing vehicles and ploughs can directly contact trees and plants, causing salt accumulation on their surfaces.

To mitigate salt spray damage, physical barriers such as burlap, plastic, or wood can be used to protect plants. Additionally, salt-tolerant plant species can be chosen for areas near roads, driveways, and sidewalks. Examples of salt-tolerant deciduous trees include ash, birch, honeylocust, poplar, red oak, tamarack (larch), weeping willow, and white oak.

Salt modification of soil

Salt modification of the soil can have detrimental effects on trees and other plants. The most commonly used salt for de-icing roads is sodium chloride (rock salt) due to its low cost, effectiveness, and accessibility. While it improves safety on roads, sidewalks, and parking lots, it can cause significant damage to plants.

Salt affects plants in two main ways. Firstly, direct contact with foliage (spray zone damage) is observed, particularly on evergreen plants along roadsides. The salt spray causes salt burn on buds, leaves, and twigs, and can desiccate bud scales, exposing developing leaves and flowers to damage. Secondly, salt modifies the soil chemically, leading to toxic levels of salt ions in the root system, which then scorches the foliage. This uptake of salt ions also interferes with nutrient availability, slowing tree growth and increasing stress.

In addition, sodium and chloride ions from dissolved salts in runoff water can displace essential mineral nutrients in the soil. Plants then absorb these ions instead of necessary nutrients like potassium and phosphorus, leading to deficiencies. Chloride ions can reach the leaves, disrupting photosynthesis and chlorophyll production. This accumulation of chloride can result in leaf burn and dieback.

The impact of salt on soil can also lead to physiological drought. Salts in the soil absorb water, reducing the amount available for plant uptake, causing water stress and root dehydration. This, in turn, leads to decreased plant growth. Furthermore, the displacement of mineral nutrients by sodium ions affects soil quality, increasing compaction, and reducing drainage and aeration.

The extent of damage caused by salt modification of the soil depends on various factors, including plant type, type of salt, freshwater availability, and the timing of salt applications. Late-season applications of salt are particularly harmful to trees, as there is less time and runoff to remove salt from the soil near root zones.

To mitigate the effects of salt modification of the soil, it is important to reduce salt use, carefully apply salt in targeted areas, and use alternative de-icing materials when possible. Improving drainage and leaching soils with fresh water can also help remove salts.

Salt-tolerant trees

While salt can be detrimental to many trees, some species are more salt-tolerant than others. These salt-tolerant trees can be planted in areas where other trees would suffer from salt exposure.

Salt damage to trees occurs in two main ways. Firstly, direct contact with foliage, known as spray zone damage, which is often seen on evergreen trees along roadsides. Secondly, through the chemical modification of the soil, where trees absorb salt ions through their root systems, which can reach toxic levels and damage the foliage. Salt uptake also interferes with nutrient availability, slowing tree growth and increasing stress.

Some trees that are more tolerant of high soil salt levels include the common horse chestnut, Canadian serviceberry, and the honeylocust. Trees that can tolerate moderate soil salt levels include the arborvitae, white cedar, and the river birch.

It is important to note that even salt-tolerant trees can still be affected by high salt levels. The degree of tolerance varies among species and can also depend on factors such as the method of salt exposure and the soil type.

To reduce salt damage, it is recommended to use salt-tolerant species, reduce salt application rates, and avoid piling snow containing salt around plants. Additionally, improving drainage and leaching salts from the soil with repeated applications of fresh water can help mitigate salt damage.

Salt damage symptoms

Evergreen Trees:

• Needles turning pale green or yellow
• Yellow, brown, or fallen needles, especially on the side facing the road
• Witches' brooms (clusters of twigs growing out of branch ends)
• Marginal leaf scorch
• Needle tip burn

Deciduous Trees:

• Marginal leaf scorch
• Reduced or distorted leaf or stem growth
• Witches' brooms (cluster of twigs growing out of branch ends)
• Wilting during hot, dry conditions
• Flower and fruit development delayed and/or smaller than normal
• Fewer and/or smaller leaves than normal
• Premature fall color
• Early leaf drop

The signs of salt damage in evergreen trees are usually visible in late winter, while for deciduous trees, the signs may not appear until early spring.

Preventing salt damage

Choose salt-tolerant plants

When planting trees and other plants near roadways, it is important to select salt-tolerant species. Salt-tolerant trees include:

• Junipers
• Eastern redcedar
• Mugo pine
• Southern magnolia
• Longleaf pine
• Yellow birch
• Thornless honeylocust
• Kentucky coffeetree
• Black walnut
• Sweet gum
• White spruce
• Colorado blue spruce
• Gingko
• Paper-Bark Maple
• Horsechesnut
• Red buckeye
• River birch
• Paper birch
• Catalpa
• Hackberry
• Thornless cockspur hawthorn
• Honeylocust
• London plane
• Tolerant maples (Norway, hedge, or sycamore)
• English and red oak
• Tolerant pines (Austrian, pitch, or Japanese black)
• Sargent cherry
• Scholar Tree

Avoid piling snow near plants

When shovelling, plowing, or snow blowing, avoid piling snow against shrubs and trees, and keep it off your lawn. Snow will often contain a high concentration of salt, which can leach into the soil and harm your plants when it melts.

Use salt alternatives

Road salt isn't the only product that can be used to prevent ice from forming. Alternatives include sawdust, sand, kitty litter, or chicken grit, which provide traction without the harmful effects of salt.

You can also use products that use salts other than sodium chloride, such as calcium magnesium acetate (CMA), which is less damaging to plants.

Protect plants with physical barriers

Using physical barriers such as burlap, landscape fabric, wood, or plastic can help keep salt off your plants. Remember to remove the coverings once the temperatures warm.

Water plants to reduce salinity

Regularly watering plants in late winter or early spring can help dilute salt in the soil. Rinsing salt-covered leaves and needles when temperatures are above freezing is another way to get salt off plants that are exposed to salt spray.

Add a layer of mulch

Adding a layer of organic mulch around trees and shrubs can help dilute salt in the soil while also improving the soil’s structure and drainage.

Frequently asked questions