During the latter part of February and early March, the harmful effect of excessive use of de-icing salts used along sidewalks and roadways becomes crystal clear. Salts deposited on beds and branches can cause damage ranging from complete branch dieback, browning of the needles or foliage, to lack of flowering, and tufting, which is also known as "witches broom". Grass along sidewalks can be completely dead due to overuse of salt.

"Witches Broom" on Tilla (Linden).

Closeup view of "Witches Broom" on Tilla (Linden).

Years ago, I can vividly remember the lawsuit conducted by Schenck Farms and Greenhouses of St. Catharines with the Province of Ontario. Because of the proximity of the Queen Elizabeth Highway and the resultant drift of salt spray onto their nearby peach orchards, this condition resulted in a significant loss of fruit production. It took years for Schenck's to successfully win their case.

Salt damage on turfgrass.

Road salt can also cause damage to the plants when it is absorbed by the roots and accumulated in the plant to toxic levels. Root physiology is drastically affected. Photosynthesis is dramatically reduced as is the production of chlorophyll. Root dehydration is increased due to the fact that the solute concentration is higher outside of the cell than it is inside the cell. In a nutshell, when salt goes into solution it interferes with water uptake by the plant. Water leaves the cell faster than it may be absorbed due to the fact that the sodium ions in the salt tend to replace both the phosphorus and potassium ions in the soil, making them unavailable to the plant. With a high accumulation of salt around the roots, plant growth is adversely affected and the roots die of drought.

Pinus strobus (White Pine) showing damage on windward side.

Closeup view of damage to Pinus strobus (White Pine).

On a small scale, roadside plants can be limitedly protected by wrapping plants with burlap, or constructing burlap or durable plastic screens along fence lines to shield them from traffic splash. At the homeowner level, avoid sodium and calcium chloride salts near salt sensitive plants. Use other alternative products that are not harmful to plants. When salt is used, avoid throwing salt laden snow onto adjacent shrubs or groundcovers.

Oakes Garden Theatre in Niagara Falls, Ontario. Protection of boxwood hedge from salt damage.

Salt Spray Symptoms

In 1974, G.P. Lumis, Department of Horticulture Science, and G. Hofstra and R. Hall, Department of Environmental Biology, University of Guelph, published results of their research on Salt Damage to Roadside Plants. Their description of injury symptoms for trees and shrubs is as follows:

Symptoms Specific to Evergreens

• Needle browning moderate to extreme, beginning at the tip.
• Needle browning and twig dieback on the side facing the road but none or very little on the back side.
• No needle browning or dieback in branches near the ground under continuous snow cover.
• Needle browning and twig dieback less severe further from the road.
• Browning usually first evident in late February or early March and becoming more extensive through spring and summer.

Symptoms Specific to Deciduous Plants

• Leaf buds on the terminal part of branches facing the road very slow to open or do not open.
• New growth arises from the basal section of branches facing the road, resulting in a tufted appearance.
• Flower buds on the side facing the road do not open but flowering normal on back side.

General Injury Patterns

• Injury more severe on side facing the road, plants one-sided due to branch dieback.
• Damage more pronounced on downwind side of highway.
• Plants further from road injured less.
• Branches covered by snow not injured.
• Injury to evergreens apparent in late winter, injury to deciduous plants not evident until spring.
• Branches above the spray-drift zone not injured or injured less.
• Damage increased with the volume and speed of traffic and the amount of salt applied to highway.
• Plants damaged over several years lack vigor and soon begin to die.
• Less winter-hardy plants injured more severely.
• Salt spray penetrates only a short distance into dense plants.
• Plants in sheltered locations lack injury symptoms.

Which Salt Tolerant Plants Should You Use?

Whenever salt is used, it should be used as limitedly as possible. Better than this, alternative products should be considered for use to avoid ground water contamination as well as damage to landscape and roadside plants. Some species of trees and shrubs are much more tolerant of salt than others. A selective list of salt tolerant plants is as follows:

Deciduous Shrubs

• Amelanchier spp. (Serviceberry)
• Aronia spp. (Chokeberry)
• Caragana arborescens (Siberian Pea Shrub)
• Cytisus spp. (Broom)
• Hamamelis spp. (Witch Hazel)
• Hibiscus syriacus and cvs. (Rose of Sharon)
• Hippophae rhamnoides (Sea Buckthorn)
• Hydrangea spp.and cvs. (Hydrangea)
• Hypericum (most) (St. John's Wort)
• Ilex verticillata (Winterberry)
• Lonicera tatarica 'Zabelli' (Zabel's Honeysuckle)
• Lonicera xylosteum (European Fly Honeysuckle)
• Myrica pennsylvanica (Northern Bayberry)
• Philadelphus spp. and cvs. (Mockorange)
• Potentilla (Cinquefoil)
• Rhamnus frangula (Glossy Buckthorn)
• Rhodotypos scandens (Black Jetbead)
• Rhus (Sumac)
• Ribes alpinum (Alpine Currant)
• Rosa (shrub types) (Shrubby Roses, e.g.. Rosa rugosa)
• Shepherdia argentea (Silver Buffalo Berry)
• Symphoricarpos albus (Snowberry)
• Tamarix ramosissima (Fivestamen Tamarisk)
• Viburnum dentatum (Arrowwood)

Deciduous Trees

• Acer campestre (Hedge Maple)
• Acer pseudoplatanus (Sycamore Maple)
• Aesculus hippocastanum (Common Horsechestnut)
• Ailanthus altissima (Tree-of-Heaven)
• Betula (Birch)
• Elaeagnus angustifolia (Russian Olive)
• Fraxinus americana (White Ash and cvs.)
• Fraxinus excelsior (European Ash)
• Fraxinus pennsylvanica (Green Ash)
• Ginkgo biloba (Maidenhair Tree)
• Gleditsia triacanthos inermis and cvs. (Honey Locust)
• Gymnocladus dioica (Kentucky Coffee Tree)
• Magnolia (most)
• Nyssa sylvatica (Blackgum)
• Populus spp. (Poplar)
• Prunus x cistena (Sand Cherry)
• Quercus alba (White Oak)
• Quercus rubra (Red Oak)
• Robinia pseudoacacia (Black Locust)
• Salix and cvs. (Willow)
• Sophora japonica (Japanese Pagoda Tree)
• Sorbus (Mountain Ash)

Broadleaf Evergreens

• Cotoneaster divaricata (Spreading Cotoneaster)
• Yucca filamentosa (Adam's Needle)

Conifers

• Larix decidua (European Larch)
• Larix kaempferi (Japanese Larch)
• Picea glauca (White Spruce)
• Picea pungens glauca (Colorado Blue Spruce)
• Pinus banksiana (Jack Pine)
• Pinus mugho mughus (Mountain Pine)
• Pinus nigra (Austrian Pine)
• Pinus parviflora (Japanese White Pine)
• Pinus ponderosa (Ponderosa Pine)
• Pinus thunbergii (Japanese Black Pine)
• Platycladus orientalis (Oriental Arborvitae)
• Taxodium distichum (Bald Cypress)

Groundcovers

• Arctostaphylos uva-ursi (Bearberry)
• Erica spp. (Spring Heath)
• Iberis sempervirens (Edging Candytuft)
• Potentilla (Shrubby Cinquefoil)
• Symphoricarpos albus (Snowberry)
• Vaccinum and cvs. (Blueberry/Cranberry)

It should be clearly understood that salt tolerance for plants depends upon a great number of factors as previously discussed. The above list is representative of plants that will grow in an environment that is periodically subjected to air-borne road salt spray.

Salt damage on Taxus (Japanese Ewe)

Acer platanoides, the Norway Maple, has been purposely left off this list. While this tree is incredibly hardy and very resistant to salt spray damage, there are other good reasons why I feel that it should not be used as a roadside tree or used nearly as extensively as it is in the landscape. This will be another article appearing in Hort-Pro at some meaningful time in the future.

References

Lumis, G.P, Hofstra, G., and Hall, R. "Salt Damage to Roadside Plants" in Ontario Shade Tree Newsletter, Vol.5, No.1. Toronto, 1973.

Lumis, G.P., Hofstra, G., and Hall, R. "Plants Resist Salt" in Landmark 2(4), 1990. Pp.76-79.