With winter comes the potential for deicing salt damage to garden plants. Passing cars and snow plows spray salt-laden slush onto garden beds, winter buds of deciduous plants, and the needles of conifers. Salts slowly spread from driveways, sidewalks, and roadsides into nearby garden soil. Decisions made last May about where to plant that new tree or shrub come into question as the white crust accumulates.
Salt Damage to the Soil
In areas where sodium chloride (NaCl, rock salt) is used for deicing, excessive sodium ions (Na+) from the salt destroys soil structure, raises the pH of the soil, and reduces water infiltration and soil aeration. These damaging factors lead to soil compaction, increased erosion, and increased water runoff. Affected soils become unsuitable for proper root growth and plant development.
Deicing salts are also damaging to beneficial mycorrhizal fungi in the soil. Under healthy soil conditions, these fungi form a symbiotic relationship with plant roots, assisting with nutrient uptake. When the mycorrhizal fungi are damaged by salt build up in the soil, garden plants in that soil grow poorly or die.
Symptoms of Salt Damage to Plants
Symptoms of damage from airborne salts deposited on plants during the winter become evident the following growing season: complete branch die-back, browning of needles or leaves (particularly on the side of the plant facing a road), lack of flowering, and “tufting” of twigs at the ends of branches where the terminal bud was killed (also know as “witch’s broom”).
Symptoms associated with accumulation of salts in the soil include stunted growth, early defoliation, yellowing of leaves and, in severe cases, branch die-back and plant death. All of these symptoms are similar to those associated with drought stress. Indeed, high salt levels in the soil severely impede water uptake by the plant.
Avoiding Salt Damage
The distance from the road that salts are carried depends on the traffic speed and prevailing wind direction. Along high-speed highways, plants should be placed at least 60 feet from the road; along busy city streets, a distance of at least 30 feet is recommended. Also, avoid planting where runoff accumulates.
Sensitive plants placed too close to the road can be protected from aerial deposition of salts with physical barriers made from burlap, snow fencing, or plywood. As unattractive as these barriers are, they are effective. Remember that the height of air-borne salt damage increases with distance from the road.A dense screen of evergreen salt tolerant plants will provide a permanent and more attractive physical barrier to wind-borne salts. Recommended salt tolerant native conifers that grow tall enough to form an effective screen include arborvitae, Thuja occidentalis, and white spruce, Picea glauca.
Risk of damage from deicing salts can be reduced by using alternatives to sodium chloride (table salt), the least expensive and thus most frequently used salt. Effective alternatives that are more vegetation (and pet) friendly include potassium chloride, magnesium chloride, and calcium magnesium acetate (CMA). A February 2014 Consumer Reports analysis showed both potassium chloride and magnesium chloride to be safer for pets and garden plants, with magnesium chloride more effective at colder temperatures. The report stressed that both of these alternatives can damage grass and other plants when over-applied. One way to avoid this problem is mixing the salt with sand or sawdust.
Late winter and spring rains help to wash salts from the above-ground portions of plants while leaching salts from the surrounding soil. Later in the spring, after these early rains, apply several inches of water to salt-exposed plants and surrounding soil to ensure that the accumulated salts are washed below the root zone.
Salt-tolerant Trees and Shrubs
Although often seen planted along roadsides and in highway medians, the white pine, Pinus strobus, is very salt sensitive and exhibits the classic symptoms of salt injury. Needles on the side facing the road are often brown, except for lower portions of the tree covered by snow. Also, trees planted farthest from the road suffer the least amount of damage. Other species of pines, however, are considered more tolerant of deicing salts and should be considered as alternatives to white pine in areas of extensive salt deposition. These include jack pine (P. banksiana), mountain pine (P. mugho), Austrian pine (P. nigra), Japanese white pine (P. parviflora), and Japanese black pine (P. thunbergii).
Among deciduous trees, those more tolerant of salt than others include common horsechestnut (Aesculus hippocastanum), serviceberry (Amelanchier laevis), birches (Betula sp.), maidenhair tree (Ginkgo biloba), honey locust (Gleditsia triacanthos), Kentucky coffee tree (Gymnocladus dioica), blackgum (Nyssa sylvatica), poplars (Populus sp.), white oak (Quercus alba), red oak (Q. rubra), Japanese pagoda tree (Sophora japonica), mountain ash (Sorbus sp.), northern catalpa (Catalpa speciosa), cockspur hawthorne (Crataeus crus-galli), black walnut (Juglans nigra), tamarack (Larix laricina), sweet gum (Liquidambar styraciflua).Salt tolerant deciduous shrubs include chokeberry (Aronia melanocarpa), witch hazel (Hamamelis virginiana), winterberry (Ilex verticillata) – as indicated by the abundance of this shrub along New England roadsides, northern bayberry (Myrica pennsylvanica), staghorn sumac (Rhus typhina), fragrant sumac (R. aromatica), smooth sumac (R. glabra), Virginia rose (Rosa virginiana), snowberry (Symphoricarpos albus), summersweet clethra (Clethra alnifolia), sweet fern (Comptonia peregrina), beach plum (Prunus maritima), nannyberry viburnum (Viburnum lentago), and blackhaw viburnum (V. prunifolium).
Salt-tolerant GroundcoversI get more requests for a list of salt-tolerant groundcovers than for any other type of garden plant. Unfortunately, the list is short, consisting of low-growing species and cultivars that are very popular among coastal gardeners. It includes bearberry (Arctostaphylos uva-ursi), heaths (Erica sp.), blueberries and cranberries (Vaccinium sp.), creeping juniper (Juniperus horizontalis), Sargent juniper (J. sargentii), eastern sandcherry (Prunus pumila var. depressa), and Gro-low sumac (Rhus aromatica ‘Gro-low’).