Careful landscaping helps protect plants from salt toxicity
To a traveler caught in a snowstorm, a salt truck is a happy sight. Yet the salt not only pits the bodies of cars, it scars the landscape as well. Landscape plants suffer from de-icing salts in two ways:
Salts wash into the soil during rains and snow melt-off, creating saline soils around plant roots.
They may also be sprayed onto plants as vehicles splash by.
Roots may take up excess sodium and chloride or they may be deprived of moisture because of the increased osmotic ``pull'' of the soil water. Both salt uptake and water deficits can decrease growth and even kill sensitive plants.
Initial symptoms of injury -- browned leaf margins, leaf tips, and twig tips -- are most often noticed in the spring when the buds begin to open.
Plants injured by salt sprays can develop toxicity symptoms similar to those indicated above, but often injury occurs only on the windward side of the plant.
Sometimes ``witches' brooms'' are seen on trees along highways. These are branches whose sensitive tips die back each winter or spring; then side shoots proliferate later in the growing season. Repeated cycles of dieback and proliferation result in a dense mass of branches, resembling a broom, near the shoot tip.
Salt toxicity can be reduced or even avoided if you take some preventive steps to secure your landscape.
First, if a road or a landscape is just being installed, be sure that plants are not set in low spots. Also, grade all roads, driveways, and walks so that drainage water enters a sewer and not the soil.
Second, use de-icing salts (sodium chloride) around the home only when necessary. If the ice has not accumulated too much, substitute cat litter, sand, or calcium chloride. Although calcium chloride is a salt, it is much less toxic to plants than sodium chloride.
Physical barriers can reduce the amount of salt sprayed on the landscape. A permanent wall, temporary snow fence, or a planting of salt-tolerant species can protect more-sensitive species.
Some authorities recommend wrapping plants with burlap, but tight wrappings can reduce air circulation and light needed by plants. Use a loose wrap instead, tacking burlap to four stakes in the ground around the plant. Leave the top open and leave an air space between the bottom of the burlap and the ground.
On warm winter days, wash salt sprays off of roadside plants with a hose or sprayer. Keep a mulch around plants to reduce evaporation and subsequent accumulation of salt in the surface soil. An organic mulch should be at least two inches deep to benefit plants.
In the spring, prune any damaged plant parts. Purdue University horticulturists recommend that, ``where watering is feasible, thoroughly wet down the aboveground portions of the plant in early spring and, if soils are well drained, apply several inches of water to the root zone.''
Attempts to leach salts can backfire, however, if irrigation water is saline (soft water) or if soil is poorly drained.
Before you get carried away with curing a salinity problem, first be sure that you actually have one. Symptoms similar to those caused by excess salt can also be caused by drought or the lack of hardiness of a species. Soil-testing laboratories can determine salinity easily by measuring the electrical conductivity of a soil solution.
When you add plants to a roadside landscape, try to select salt-tolerant species or cultivars. Some conifers, for example, have more wax on their needles than others, so they shed salt sprays more easily. Tolerant deciduous trees may have very resinous buds (horse chestnut and cottonwood) or buds submerged in twigs (black locust and honey locust). A particular species, however, may resist salt sprays and not saline soils, and vice versa.
Purdue lists salt-tolerant woody ornamentals in its publication, No. HO-142, ``Roadside De-Icing Salts and Ornamental Plants,'' by Ruth Kvaalen (available from the Cooperative Extension Service, West Lafayette, Ind. 47907). Similar lists are found in ``Hillier's Manual of Trees and Shrubs'' and in ``Wyman's Gardening Encyclopedia.'' Books about seaside gardening are also valuable, since plants along the ocean are subject to stresses that are similar to those along the road.
If you want to grow salt-sensitive species, set them at least 60 feet from high-speed highways and 30 feet from all other roads. Plant them in the spring, since established, healthy plants tolerate salt stresses more easily than newly set or weak plants.
Gardening near salted roads presents the same challenges as keeping a car intact in these areas. Just as you would protect a car by rustproofing, protect a garden with mulches and barriers. Just as you would wash excess salt off of a car on a warm winter day, shower your plants as well.