It’s the first day of spring… Consider that roadside vegetation has been exposed to de-icing compounds following several recent late-winter storms. Runoff from treated pavement contains dissolved salts that can injure adjacent vegetation. In plants sensitive to excessive salt, affected foliage may scorch and drop prematurely. In severe cases, the death of twigs, branches, and sometimes the entire plant, may occur.
Why is road salt used?
Salts (usually chloride-based) are applied to roadways, driveways, and sidewalks to melt the ice and snow, enhancing safety for motorists and pedestrians. These compounds are usually applied before (anti-icing) or during (de-icing) storms where precipitation is expected to accumulate. The salt dissolves in water to form a brine that has a freezing point lower than water. The brine melts the ice and helps to prevent the formation of more ice as temperatures drop. To improve traction, de-icing salts are often mixed with abrasives such as sand, cinders, gravel, and sawdust.
Anti-icing products are applied before accumulation is expected. Applied most often in liquid form, anti-icers prevent the bonding of ice to the roadway. De-icing products, usually applied as solids, break existing bonds between ice and the pavement and are most often used when plows are needed to clear roadways. In New Jersey, the most commonly applied deicers are rock salt (sodium chloride), liquid calcium, and salt-water brine solutions. To keep the approximately 13,000 lane miles of interstate and state highways clear of precipitation, The New Jersey Department of Transportation stores up to 164,000 tons of rock salt, 720,000 gallons of liquid calcium chloride, and 150,000 gallons of salt brine.
How does road salt affect vegetation?
Plants become injured when roots and foliage are exposed to salt-laden water. The foliage on roadside vegetation is damaged when salted water sprays up from the pavement by passing vehicles. Salt-laden water can also percolate down through the soil profile, coming into contact with soil particles, soil microbes, and plant roots. Salt injures vegetation by:
• Increasing water stress. In the root zone, water molecules are held very tightly by salt ions, making it difficult for roots to absorb sufficient quantities of water. In sensitive species, this “physiological drought” may result in depressed growth and yield.
• Affecting soil quality. The sodium ion component in rock salt becomes attached to soil particles and displaces soil elements such as potassium and phosphorus. As a result, soil density and compaction increases and drainage and aeration are reduced. In addition, chloride and calcium can mobilize heavy metals in affected soils. Plant growth and vigor are poor under these conditions.
• Affecting mineral nutrition. When the concentration of both the sodium and chloride components of salt in the root zone is excessive, plants preferentially absorb these ions instead of nutrients such as potassium and phosphorus. When this occurs, plants may suffer from potassium and phosphorus deficiency.
• Accumulating to toxic levels within plants. The chloride component of salt is absorbed by roots and foliage and becomes concentrated in actively growing tissue. Plants repeatedly exposed to salt over long periods of time may accumulate chloride ions to toxic levels, resulting in leaf burn and twig die-back.
How do plants respond to excessive salt?
Unlike animals, plants do not have mechanisms to excrete excess salt from tissues and can only “shed” salt in dead leaves and needles. Because conifers do not shed leaves on a yearly basis, they tend to suffer damage from accumulated salt more easily than do deciduous trees.
Plant species vary in their tolerance to salt exposure (see plant species listings below). Plants that are tolerant of salt grow as well in saline soils as they do under normal conditions. Many herbaceous plants such as grasses adapt fairly readily to high salt levels. Among woody plants, tolerance varies with the species. Plant species with waxy foliage or scaled, protected buds are generally more tolerant of salt spray.
In salt-sensitive plants, exposure to salt can result in poor growth, stunted leaves, heavy seed loads, twig and branch die-back, leaf scorch, and premature leaf drop. Plants stressed by excessive salt are also more susceptible to biotic diseases and insect pests. The extent of injury a plant sustains in response to salt depends on:
• The kind and amount of salt applied. Sodium chloride (rock salt) can be very damaging to plants. De-icing compounds without chloride, such as urea, are safer for vegetation.
• The volume of fresh water applied. Although salts are easily leached by water in well-drained soils, they tend to accumulate in poorly-drained soils, so the potential for damage to vegetation in these soils is high. High volumes of water, whether from rainfall or melting snow, will decrease the possibility of injury. Rainfall also washes salt from foliage surfaces.
• The distance plants are situated from treated pavements. Plants within the “spray zone” of moving vehicles (about 15 feet, and more if down wind) are more likely to sustain salt injury. Injury is usually most evident on the side of the plant that faces the highway.
• The direction of surface-water flow. The channeling of drainage water away from susceptible plants will prevent salt from coming into contact with plant roots. Plants situated up-slope or away from drainage areas are less likely to be affected.
• The time of year salt is applied. Salt applied in late winter and early spring is more likely to damage vegetation than salt applied earlier in the winter season. This is because there is less time for winter snow and precipitation to leach salt from the root zone before growth resumes in the spring.
How can we minimize salt injury?
The best solution to the de-icing salt problem is to prevent contamination. On sidewalks and driveways, clear the snow first, and then use minimal de-icing product to treat the pavement. If vegetation is located in areas where salt spray occurs, erect barriers or screens to protect plants during the winter months. Anti-desiccants may also help prevent injury when applied to evergreen foliage where de-icing salt will be used. County, state, and municipal officials can help prevent salt injury by carefully training equipment operators and frequently calibrating equipment.
Once soil becomes contaminated with salt, the damage can be reduced by leaching the salt with fresh water as soon as possible after exposure. Under certain circumstances, incorporation of gypsum at the rate of 50 lb/1000 sq ft into the top six inches of soil at the drip-line of trees may also be helpful. Furthermore, foliage exposed to salt spray may be washed with salt-free water to remove deposited salt.
When landscaping, place trees and shrubs that are salt-sensitive as far as possible from problem areas. Select planting sites that are not subject to salt-contaminated waters, and place shallow diversion ditches between roadways and plantings. When vegetation must be placed near roadways, utilize salt-tolerant plants. Keep in mind that stress due to de-icing compounds may predispose plants to diseases and insects and may enhance their sensitivity to other environmental stresses.
Salinity tolerance of selected trees and shrubs.
Note: These listings were compiled from several sources (below). Sensitivity to salt depends on plant species, stage of growth, and environmental conditions. Note that in many cases, tolerance or sensitivity of host plants to salts is based on anecdotal information or studies where responses to soil salinity or salt spray were assessed. Note also that different sources may report different tolerances for the same species.
Moderately tolerant to tolerant trees and shrubs
- Acer platanoides (Norway maple)
- Aesculus hippocastanum (Horse-chestnut)
- Ailanthus altissima (Tree of heaven)
- Baccharis halimifolia (Eastern baccharis)
- Betula sp. (, Gray birch, Paper birch, Yellow birch, Sweet birch)
- Cryptomeria japonica (Japanese cryptomeria)
- x Cupressocyparis leylandii (Leyland cypress)
- Elaeagnus angustifolia (Russian olive)
- Euonymus japonicus (Japanese spindle)
- Fraxinus americana (White ash)
- Gleditsia triacanthos (Honeylocust)
- Ilex sp. (American holly, Inkberry, Yaupon holly)
- Juniperus sp. (Chinese juniper, Common juniper)
- Lonicera tatarica (Tatarian honeysuckle)
- Picea pungens (Colorado blue spruce)
- Pinus sp. (Austrian pine, Japanese black pine, Mugo pine)
- Populus sp. (Eastern cottonwood, White poplar)
- Pyrus calleryana (Callery pear)
- Quercus sp. (English oak, Red oak, White oak, Willow oak)
- Robinia pseudoacacia (Black locust)
- Salix sp. (Weeping willow, White willow)
- Spiraea x vanhouttei (Vanhoutte spirea)
- Syringa reticulata (Japanese tree lilac)
- Syringa vulgaris (Common lilac)
- Taxodium distichum (Bald cypress)
- Thuja occidentalis (Arborvitae)
- Ulmus sp. (Chinese elm, Siberian elm)
- Zelkova serrata (Japanese zelkova)
Sensitive trees and shrubs
- Abies sp. (Balsam fir, Concolor fir)
- Acer negundo (Boxelder)*
- Acer sp. (Black maple, Red maple, Silver maple)
- Aesculus glabra (Ohio buckeye)
- Albizia julibrissin (Silktree)
- Alnus sp. (European alder, Gray alder, Hazel alder)
- Amelanchier sp. (Serviceberry)
- Asimina triloba (Pawpaw)
- Berberis thunbergii (Japanese barberry)
- Betula sp. (Bog Birch, European white birch, River birch)
- Buxus sempervirens (Common boxwood)
- Carpinus sp. (American hornbeam, European hornbeam)
- Carya sp. (Bitternut hickory, Pignut hickory, Shagbark hickory)
- Ceanothus americanus (New Jersey tea)
- Cedrus atlantica (Atlas cedar, Blue atlas cedar)
- Celtis occidentalis (Common hackberry)
- Cercidiphyllum japonicum (Katsura tree)
- Cercis canadensis (Eastern redbud)*
- Chaenomeles japonica (Japanese flowering quince)
- Chamaecyparis sp. (Altantic white cedar, Sawara)
- Cornus sp. (Cornelian cherry, Flowering dogwood, Redosier, Tatarian dogwood)
- Corylus sp. (American filbert, European filbert)
- Cotoneaster sp. (Cotoneaster)
- Crataegus crus-galli (Cockspur hawthorn)*
- Cytisus scoparius (Scotch broom)
- Elaeagnus umbellata (Autumn elaeagnus)*
- Euonymus alatus (Winged euonymus)
- Fagus sp. (American beech, European beech)
- Forsythia suspensa (Weeping forsythia)
- Fraxinus nigra (Black ash)
- Fraxinus pennsylvanica (Green ash)**
- Gaultheria sp. (Creeping snowberry, Eastern teaberry)
- Gaylussacia sp. (Black huckleberry, Blue huckleberry, Dwarf huckleberry)
- Gingko biloba (Gingko)*
- Hamamelis virginiana (American witchhazel)
- Hypericum sp. (St. Johnswort)
- Ilex cornuta (Chinese holly)*
- Ilex sp. (Burford holly, Common winterberry, Japanese holly)
- Juglans sp. (Butternut, Eastern black walnut)
- Juniperus virginiana (Red cedar)*
- Kalmia latifolia (Mountain laurel)
- Lagerstroemia indica (Crape myrtle)
- Larix decidua (European larch)*
- Larix laricina (Tamarack)
- Ligustrum amurense (Amur privet)*
- Liquidambar styraciflua (American sweetgum)*
- Liriodendron tulipifera (Tuliptree)
- Lonicera maackii (Amur honeysuckle)
- Maclura pomifera (Osage orange)
- Magnolia sp. (Cucumber-tree, Sweetbay, Umbrella-tree)
- Mahonia aquifolium (Oregon grapeholly)
- Malus sp. (Crabapple)*
- Metasequoia sp. (Redwood)
- Morus alba (Common mulberry)*
- Morus rubra (Red mulberry)
- Nyssa biflora (Swamp tupelo)
- Nyssa sylvatica (Black tupelo, black gum)*
- Ostrya virginiana (American hophornbeam)
- Paulownia tomentosa (Princess tree)
- Physocarpus opulifolius (Common ninebark)
- Picea sp. (Black spruce, Norway spruce, Red spruce, White spruce)
- Pinus sp. (Eastern white pine, Loblolly pine, Red pine, Scots pine, Shortleaf pine, Virginia pine)
- Platanus occidentalis (Planetree)
- Prunus sp. (American plum, Black cherry, Cherry plum, Chickasaw plum, Kwanzan cherry, Sand cherry)
- Pseudotsuga menziesii (Douglas fir)
- Pyrus communis (Common pear)
- Quercus sp. (Black oak, Chestnut oak, Chinkapin oak, Pin oak, Post oak, Scarlet oak, Shingle oak, Southern red oak, Swamp chestnut oak, Swamp white oak, Water oak, Willow oak)
- Quercus macrocarpa (Bur oak)**
- Rhododendron sp. (Azalea, Rhododendron)
- Rhus copalinum (Winged sumac)
- Rosa sp. (Rose)
- Salix sp. (Coastal plain willow, Golden weeping willow, Laurel willow, Pussy willow, Sageleaf willow, Sandbar willow)
- Sambucus canadensis (American elder)
- Styphnolobium japonicum (syn. Sophora japonica) (Japanese pagodatree, Scholar-tree)*
- Sassafras albidum (Sassafras)
- Sorbus americana (American mountain ash)
- Spiraea sp. (Steeplebush, White meadowsweet, White spirea)
- Taxus sp. (Yew)
- Taxus canadensis (Canadian yew)
- Tilia sp. (American basswood, Basswood, Littleleaf linden)
- Tsuga canadensis (Canadian hemlock, Eastern hemlock)
- Ulmus sp. (American elm, Chinese elm, Rock elm, Slippery elm)
- Vaccinium sp. (Blueberry, Deerberry)
- Viburnum sp. (Blackhaw, Cranberrybush, Mapleleaf viburnum, Nannyberry)
*Sensitive to moderate tolerance depending on source.
**Sensitive to high tolerance depending on source.
- Sinclair, W., and Lyon, H. 2005. Diseases of Trees and Shrubs, 2nd. Comstock/Cornell University Press.
- Urban Horticulture Institute. 2009. Recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance. Department of Horticulture, Cornell University.
- Miyamoto, S., Martinez, I., Padilla, M., Portillo, A., and Ornelas, D. 2004. Landscape Plant Lists for Salt Tolerance Assessment. Texas A&M University Agricultural Research and Extension Center at El Paso, Texas Agricultural Experiment Station.
- Woody plants database, Cornell University: http://woodyplants.cals.cornell.edu/
- USDA PLANTS database: http://plants.usda.gov/java/