Winter Salt Injury and Salt-tolerant Landscape Plants

Overview

Winter in Wisconsin often means snow and ice. To allow safe travel for pedestrians and motorists, walkways and roads must be kept as ice-free as possible. While snow and ice removal is best done with shovels, snow blowers, and plows, this may not remove all of the snow, and ice can quickly form, leaving slick, hazardous surfaces. Deicing salts are used extensively to melt this ice and snow.

Sodium chloride (NaCl), or rock salt, is the most commonly used deicer due to its effectiveness, availability, and comparatively low cost. However, it also has significant drawbacks. It’s highly corrosive, causing significant environmental damage and corroding vehicles and concrete. The national cost of damage to vehicles and infrastructure alone is estimated at $3.5 to $7.0 billion annually.

This article focuses on recognizing and preventing plant damage caused by deicing salts, evaluates the pros and cons of alternatives to rock salt, and provides an extensive list of salt-tolerant plants.

Types of Salt Damage

Soil Damage

When salt accumulates in the soil, excessive sodium (Na) from salt destroys soil structure, raises soil pH, and reduces water infiltration and soil aeration, leading to soil compaction, increased erosion, and water runoff. The soil becomes unsuitable for proper root growth and plant development. Deicing salts are also detrimental to beneficial mycorrhizal fungi in the soil, which form a symbiotic relationship with roots assisting with nutrient uptake.

Plant Damage

Salt is moved off the pavement and into the environment by one of two mechanisms: it may accumulate in the soil along streets or sidewalks from melted, salt-laden snow or it may become dispersed in an aerosol spray by fast-moving traffic and high winds along wet, salted roads. This dispersed salt causes significant damage to turfgrass as well as landscape and wetland plants growing near or along roadways, highways, driveways, and sidewalks. While salt runoff is typically limited to areas bordering salted roadways, salt spray can travel hundreds and even thousands of feet from the road.

Plant roots can take up salt, leading to accumulation in the plant. Plants may also have salt deposits on surfaces, such as twigs, buds, or leaves, causing tissue dehydration. Build-up of deicing salts in the plant can interfere with photosynthesis and other plant processes, like respiration and transpiration. Chloride (Cl) ions in salt can injure plant tissue, reduce water uptake, and cause nutrient imbalances such as magnesium (Mg) and potassium (K) within a plant.

Salt applications made in late winter or early spring cause the most damage compared to applications made earlier due to active uptake of water by the root systems as plants are beginning to break their dormancy. In areas where salt concentrations are high, native vegetation is being replaced by salt-tolerant, invasive species such as reed canarygrass (Phragmites australis) and/or narrow-leaved cattail (Typha angustifolia) leading to landscapes dominated by a single species.

People

Well water contaminated with excessive chloride contributes to hypertension and other heart-related conditions in humans. The intake of high chloride levels from salt may also be associated with elevated mortality from cancer. Maximum safe level of chloride in drinking water is 250 mg/L (250 ppm) but salt runoff often exceeds these levels.

Water Quality

Water quality can be adversely affected by road salt runoff into lakes, ponds, rivers, and wetlands.

Wildlife

Local wildlife lose food and shelter as plants die due to excess salt.

Diagnosing Salt Injury

Diagnosing salt injury can be difficult because similar symptoms may result from a wide variety of causes. The following problems all cause similar symptoms: drought stress, root damage from construction, girdling roots, winter burn of foliage, air pollution, compacted soil, grade changes, nutrient deficiencies, insects and diseases, natural gas leaks, water-logged soils, and improper planting depth. There are, however, some details to look for that may help:

Indications of Salt Spray Injury

• The damage is more severe on side of plant facing the road or on outer portions of dense plants.
• Severity of damage increases with volume and speed of traffic and amount of salt used.
• Plants downwind from road show more damage than upwind.
• Most damage occurs within 60 feet of road and decreases with distance from road.
• Sensitive plants show symptoms of salt damage at distances as far as 1,000 feet or more from major highways.
• Branches covered by snow or sheltered from road show no damage.
• Branches growing above spray drift zone show no damage.
• Plants that are less cold hardy show more injury.

Indications of Soil Salt Injury

• Most damage occurs within 30 feet of road and decreases with distance from road.
• Plants are severely injured in poorly drained soils or where runoff from salt-laden snow collects.
• Damage increases with amount of salt used.
• Plants growing near areas that receive frequent salt applications (walkways, parking lots, entrances) show most damage.
• Plants damaged over several years lack vigor and are in decline.

If salt injury is suspected, either foliar analysis or soil salt tests are needed to confirm the diagnosis. These tests are available at a soil and plant analysis lab for a nominal fee. Collect soil samples in early spring before rain leaches salts out of the sampling area. Leaf and stem tissue samples can be submitted during the growing season for chloride content.

Foliar chloride concentrations that exceed 0.5% dry weight for conifers and 1.0% for deciduous plants are considered harmful. Use clean pruners and wear gloves when handling the samples to avoid contamination of sample, or prune the tissue directly into plastic bags. Contact Extension for specific instructions, location, and costs for this service.

Symptoms of Salt Injury

Salt Spray

Salt spray enters plants through buds, bud and leaf scars, young shoots, as well as the leaves of evergreens species. Salt deposited on plants draws moisture out of the tissues causing desiccation (drying) and burn.

Recent research indicates that salt spray injury contributes to winter injury, although it does not normally affect cold hardiness of dormant buds on woody onamentals. The morphology and physiology of a plant bud appear to work together determining tissue sensitivity to deicing salts and freezing temperatures. For example, species that have naked buds (no scales on buds) appear more susceptible to freezing and to deicing salt than species with multi-scaled (covered) buds.

Buds are most susceptible in late winter and early spring as they break dormancy. The degree of bud injury can be influenced by the plant’s genetic differences, type of salt exposure (aerial spray or soil salt), intensity of the salt exposure, biotic factors, climate, and seasonal differences.

Symptoms on Deciduous Plants

• Injury appears in early spring
• Delayed budbreak
• Reduced leaf size and stem growth
• Off-colored foliage
• Outer edges of leaves turn brown
• Plants may fail to flower
• Death of buds and twigs leading to misshapen, unsightly looking plants
• When the tip of a branch dies back, many new shoots sprout just below the dead area, creating a ”witches’ broom”
• Crown of the tree begins to thin
• Large branches may die if severe injury occurs over several years

Symptoms on Evergreens

• Injury evident in late winter to early spring
• Tips of mature needles turn brown or yellow; discoloration moves down the needle, eventually killing entire needle; damaged needles eventually fall off
• Discoloration of needles is often hidden by new growth in spring
• Twigs die back
• Symptoms occur primarily on the side of the plant facing the road

Soil Salt

Unlike salt spray injury, soil salt injury is slow to develop and injury progresses over several years. Deicing salts create a wide range of problems for plants. Salt is highly absorbent and binds tightly with water, preventing plant roots from taking up water. Thus, even when soil moisture is plentiful, high salt levels create an artificial, drought-like environment for plants.

Compounding the problem, high concentrations of soil salt can actually draw water out of the roots. When roots do absorb salt, high concentrations will eventually kill roots, causing plants to decline over several years. At high concentrations in the soil, sodium (Na) will compete with essential nutrients for uptake by plants. Roots will also absorb toxic levels of chloride (Cl) from deicing salts, which accumulates in buds, leaves and twigs, causing desiccation (drying).

Since an affected plant’s health is compromised, it become more susceptible to insects, diseases, and drought stress. With regard to soil texture, plants tend to be more salt tolerant in sandy, well-drained soils than in heavy clay soils as the salt is more easily leached through the soil by rainwater.

Symptoms on Deciduous Plants

• Browning along edges of leaves
• Wilting during hot, dry weather when soil moisture is limited
• Off-colored or discolored foliage
• Nutrient deficiencies, in particular magnesium and potassium
• Stunting and reduced plant vigor
• Plant produces fewer, smaller leaves that are often chlorotic (yellow)
• Premature fall coloration and leaf drop
• Flowers and fruit smaller than normal
• Twig dieback
• Turfgrass and herbaceous perennials may die due to excessive soil salt

Symptoms on Evergreens

• Similar to salt spray injury, but both old and new needles are affected
• Eventual plant death

Prevention of Salt Injury

There are many ways to prevent or alleviate salt injury. Deicing salt applications should be used primarily in high-risk areas such as highways, intersections, hills, steps, and major walkways; limit applications in non-critical areas.

If possible, avoid using pure sodium chloride (NaCl), a common deicing salt. Instead, mix less than 5% of sodium chloride (NaCl) with abrasive materials such as sand, crushed rock, kitty litter, ash or cinders, or use an alternate deicing product (see Table 2. Products for Ice Control). These mixtures are particularly useful on roads with low traffic volume and when temperatures fall below 20°F (sodium chloride is ineffective at colder temperatures).

For example, 1 pound of deicing salt mixed with 50 pounds of sand makes an effective abrasive compound, particularly on walkways where good foot traction is required. This combination stays loose and unfrozen and is easy to spread; however, there is usually not enough salt to do any appreciable melting. Do not use commercial fertilizers as a deicing salt as these products will burn plant roots.

If using a deicer, wait to apply until after all the snow has been plowed or shoveled. Early applications of small amounts of salt can be very effective in keeping ice from bonding to the pavement, which improves removal of snow and ice after a storm. Remove slush before it has a chance to refreeze. Once ice has bonded to the pavement, it takes more salt to remove the ice than if a lighter application been made earlier. If possible, avoid application of salt in late winter and early spring as the plants are coming out of dormancy.

Prevention Tips

• One of the best preventive techniques is to plant salt-tolerant species in areas that are subject to salt spray or runoff (Table 1. Salt-tolerant Landscape Plants). Be aware that a plant’s degree of tolerance to soil salts may be different from its aerial salt tolerance.
• In areas where soil salt is a problem, planting trees and shrubs on berms (raised, mounded beds) will prevent salty runoff water from moving into root zones.
• Protect plants from salt spray by placing physical barriers such as plastic, burlap, or snow fencing around or near plants in late fall.
• Avoid shoveling salt-laden snow over the root zones of sensitive plants.
• During a warm spell in winter, rinse off plants to eliminate residual salt before budbreak.
• In early spring, water soil heavily (at least 6 inches) to flush salt out of the plant’s root zone.
• Direct salt runoff away from plants using barriers such as gutters or alter drainage patterns away from planting areas. (This method works best on well-drained soils and is not very effective on clay soils and compacted soils where water does not drain readily.)
• Application of gypsum (CaSO4) to heavy clay soils that are high in salt will displace the sodium ion with calcium, improving both aeration and drainage.
• If salt-sensitive plants are to be used, plant them at least 60–100 feet from highways and 30–40 feet from city streets where salt runoff into soil is common.

Use of anti-transpirants and dormant oil sprays are ineffective in prevention of salt spray injury and are not recommended.

Products for Ice Control and Alternatives

There are several chemicals commonly used as deicers. They are often blended together or combined with other materials such as sand, cinders, ash, agricultural byproducts, or other proprietary material to improve performance, limit environmental and concrete damage, and reduce cost. There are many commercial blends available in local stores with different percentages of deicing chemicals. Some deicing compounds are sold with anti-corrosive additives. While additives help, they will not eliminate corrosion entirely and their effects on plants and the environment are often unknown.

Performance of a deicing product is influenced by many factors including chemical concentration, air and pavement temperatures, traffic and weather conditions, type of road surface, topography, traffic volume, width of application, duration of the deicing salt melting action, shape of the deicer particles, and the time it takes the deicing salt to form a brine. Consider a balance between safety, cost, and practical storage and application issues when selecting a final product to use. Always follow label directions when using a deicing product. Never over-apply deicing products as this may cause serious damage to plants and the environment.

Why some plants are salt tolerant

A variety of physiological characteristics are involved in making plants tolerant or susceptible to salt.

Protection against salt spray. Some plants have features that physically prevent salt spray from penetrating. Such features include hidden or submerged buds; thick surface wax; numerous, tightly arranged bud scales; fuzzy buds; or sticky resin-coated buds. (None of these structures protect the plant against soil-borne salt, which is taken up through the roots.) In contrast, plants that produce naked buds (no bud scales) are very susceptible to salt spray injury in winter, more so than plants with scaled buds. Examples of plants that have naked buds include wayfaringtree viburnum (Viburnumlantana), Koreanspice viburnum (Viburnumcarlesii), Judd viburnum (Viburnumx juddii), pawpaw (Asimina triloba), witchhazels (Hamamelis spp.), fothergillas (Fothergilla spp.), and the highly invasive tallhedge buckthorn (Rhamnus frangula ‘Columnaris’).

Protection against soil salt. Some plants, such as green ash (Fraxinus pennsylvanica), are able to exclude soil salt from entering their cells or they may be able to withstand higher concentrations within the cells.

Varying degrees of tolerance. An individual plant’s tolerance to salt will vary depending on its state of dormancy, on temperature fluctuations, and on morphological changes. Plants exhibit the greatest resistance to salt damage in early winter (December through early January). In March and April as spring approaches, buds become much more susceptible to salt injury. Newly planted trees and shrubs, which have small root systems and little stored water, are more susceptible to salt injury in winter than established plants. For these plants, extra attention should be given to preventive measures during the first few years after planting to help protect plants from salt damage.

Salt-Tolerant Landscape Plants

Deciduous Trees

Deciduous Shrubs

Evergreen Trees & Shrubs

Groundcovers

Vines

Ornamental Grasses

Herbaceous Plants

Products Available for Ice Control

(Not including products used only in airports)