Lebanon Reporter

March 11, 2014

Landscape plants and road salt

By Curt Emanuel Boone County Extension
Lebanon Reporter

---- — The practice of salting the earth to destroy, at least symbolically, the product­ivity of a defeated enemy’s lands, has been in existence since ancient times. During a winter such as we have “enjoyed” this year, large amounts of salt have been used to clear streets and highways of snow and ice. Additional salt may have been used as a melting aid for driveways and sidewalks. While salt today does not have the kind of symbolism it did back then, it can still result in damage to landscape plants.

Salt causes damage to plants in two ways. Salt spray from roads may cause contact injury to leaves, buds and small twigs. Evergreens are particularly susceptible. With this type of injury, damage is usually more severe on the side of the plant closest to the source of the spray. Unless damage is very severe, plants usually recover from this type of injury.A more serious cause of damage is when melting salt alters soil chemistry. The most commonly used form of salt is sodium chloride. When this salt dissolves in water, it breaks down into sodium and chloride ions, both of which are toxic to plants if present in high concentrations in soil. Sodium blocks necessary nutrients such as potassium, calcium and magnesium from being available to plants. Chloride ions are absorbed by plants and accumulate in the leaves where they interfere with the production of chlorophyll and photosynthesis. If the salt does not break down, the water in soil becomes salt water resulting in damage to roots. Nobody recommends using seawater to irrigate plants.

In a normal Indiana winter, snowfall is often interspersed with periods of warmer weather and, frequently, rain. This helps prevent salt buildup in the soil and, when it does accumulate, can help to flush the salt out. This has not been the pattern this year. Much of the salt that has been sprayed out from the roads has remained in place. While spring may arrive with a great deal of rain, it may not.

Reducing or preventing plant damage from road salt is simple. Once the ground has begun to thaw, irrigate the soil around plants to leach out the salt and the sodium and chloride ions. You should be careful not to cause damage to plants by over-watering so water thoroughly but not excessively and repeat this several times.

If you are intending to establish new plantings in areas that may receive a lot of salt, plants vary in their salt tolerance. Salt tolerant plants are known as halophytic. You can find lists of these types of plants from various sources, including Purdue Extension Publication ID-412-W, “Salt Damage in Landscape Plants,” which you can either find on the Internet or you can pick up at the extension office. Use caution in selecting plants. Those that are salt hardy are often hardy in other ways, sometimes to the point of being invasive.

Scale insect controlOne of the more difficult insect groups to control are the scales. I personally consider them sneaky as they are hard to spot until their numbers become quite large. If you have had a problem with scale in the past, late winter can be a great time to control them. Many species of scale overwinter as adults or immature crawlers and are susceptible to applications of dormant horticultural oil.

These products are called dormant because they should be used before plants begin growing in the spring. These products act by smothering pests. Unfortunately, they can also smother growing plants, so be sure to apply them before plants begin to break dormancy. While they are ineffective against scale insects that spend the winter as eggs, they are effective against some of the most common scale pests, including euonymus, pine needle and cottony maple scales.

Scale insects usually have a body covering — either armored or waxy — which is difficult for conventional pesticides to penetrate, making dormant sprays a preferred treatment. Also, by applying a pesticide in late winter, negative impacts to most desirable insects are avoided as they are not yet active.

Be sure to always read and follow all label directions for any pesticide. For additional information on controlling scale, including available products and which species are susceptible to dormant oils, see Purdue Extension Publication E-29-W, “Scale Insects on Shade Trees and Shrubs.” You can find this publication online or at the extension office.

Curt Emanuel is a Boone County Extension Educator for Agriculture and Natural Resources