Planting Salt Resistant Vegetation
A new resident of New Hampshire will soon recognize what road salt does to vehicles, but it is harder to see what it does to certain plants. Planting salt resistant vegetation along the roadside is essential since vigor is necessary to survive in our sometimes intolerable weather conditions. Hardy vegetation will save you from having to replant the area if it dies because it is incompatible with its surroundings. Vegetative cover is necessary to control erosion and protect water quality (by filtering runoff). It is an important safety element since sediment from runoff can cause blockage of culverts resulting in flooding and creating driving hazards.
How Road Salt Affects Vegetation
Since salt in soil absorbs water it can injury plants if it accumulates to excessive amounts. Even if moisture is plentiful, high amounts of salt can result in a drought-like condition because sodium results in compacted soil. Compacted soil doesn’t allow the water and oxygen to get to roots of the plant.
Salt affects plants as it is absorbed through roots and leaves or needles. As salt dissolves in water, sodium and chloride ions separate, and chloride ions are absorbed by the plant roots. The ions are carried by the plant into growing sections and can accumulate to toxic levels. Toxic buildup results in marginal scorch (death of leaf margins). Plowing and vehicle traffic can spray salt on the roadside. Where salt may enter plant cells directly through the leaves or needles affected plants lose their cold hardiness. Salt also reduces plant vigor, therefore they become more susceptible to attack by insects and diseases.
Symptoms of salt injury are similar to those caused by drought or root injury: stunted yellow foliage, premature autumn leaf coloration, scorch, and twig die back. The conifer foliage often turns yellow or brown in the early spring. If spray causes salt damage, discolored needles are soon masked by new year’s growth. If the salt damage is caused by excesses in the soil, new needles may die as chloride ions accumulate in them. Either type of damage could be lethal to a plant if it occurs for several consecutive years.
Common Sense Solutions
One obvious solution to curb salt damage is to sow salt tolerant plants. To protect intolerant plants use salt spray barriers. Snowfencing or plastic shields also prevents spray to trees. Intolerant species shouldn’t be planted within 30 feet of roads or on slopes below roadbeds.
After March 1, salt applications are most detrimental because plants are breaking from their dormancy and are beginning to actively absorb nutrients and water from the soil. Toxic ions are more likely absorbed during this period. Crews should avoid piling salt and snow around plants or in places where melting water will drain into them.
Proper planting procedure is important. Newly planted trees are under stress and less able to cope with external factors (such as salt). The depression created by settling of new transplants is an excellent place for salty water from melting snow to accumulate. Depressions should be filled or leveled as soon as new transplants become established.
As mentioned earlier, soils containing large amounts of sodium frequently develop poor drainage. Treating roadside soil with gypsum appears to be a successful method of correcting the soils poor drainage. Also, one can create adequate drainage systems for highways, curbs and gutters divert street runoff into storm sewers and clean ditches and culverts carry water away from roadsides. Adequate drainage this will substantially reduce salt’s effects on vegetation.
What to Plant
The University of New Hampshire’s Cooperation Extension program suggests planting salt tolerate trees such as horsechestnut, black locust, honey locust, red oak, white oak. Moderately salt tolerant trees are ash, poplar, birches, cherry, and red cedar.
Permanent seeding of grasses and legumes should occur before August 15. If this does not fit into your schedule, annual rye grass is a quick temporary cover. Also, annual rye grass is a good choice in ditches that are cleared out yearly.
Permanent seeding should occur during April and May or the first two weeks in August. The Maine Department of Transportation (MEDOT) suggests planting crown vetch and roseacaciait is non-invasive in forest canopied areas, wetlands, and ditches. The MEDOT warns that these species will invade croplands, pastures, lawns, and shrub beds.
Warm-season grasses are more useful for erosion control than are cool-season grasses. Reliable warm-season grasses are switchgrass, little blue stem, big blue stem, indian grass, and prairie grass, Although in droughty sand and gravel, the seedbed is usually too dry most of the time to allow the plants to germinate. Most warm-season grass will remain viable in the soil for several years and will not germinate until prolonged moisture is available. The most dependable method for seeding is to track using a bulldozer, the groser tracks provide an important function of reducing erosion during the seedling establishment period. Reliable warm-season grasses are switchgrass, little blue stem, big blue stem, Indian grass, and prairie grass.
Sources:
“Roadside Vegetation and Salt,” Deicing Salt and Our Environment, Salt Institute, 700 North Fairfax St, Fairfax Plaza, Suite 600; Alexandria VA 22314-2040 (703) 549-4648
Quick Guides Vegetative Erosion & Sediment Control No. 3
Hudler, George W. “Salt injury to Roadside Plants,” New York State College of Agriculture and Life Sciences, Cornell University, Ithaca NY
Kelsey, Theodore “Gravel Pit and Other Sandy and Drought Site Renovation Trials and Experiences in New Hampshire,” Technical notes PM-NH-26 May 1991 Soil Conservation Service, Durham NH
Walton, Clyde, “Using Sustainable vegetation management on Maine Roads,” Better Roads, Vol. 64 n 7., July 1994, p. 21-22.