One of the many problems plants encounter in the landscape is salt damage. The two major problems are saline soils and salt spray. Saline soils are a general problem with less than 20 inches of rainfall per year, but Delaware gets 43 inches of rain per year, so why are we concerned? While we do not have saline soils throughout the state, we can have specific site problems such as coastlines with seawater over wash or salt accumulation in the soil from salt spray. Flooding from brackish tidal rivers and estuaries happens in wooded wetlands. Sidewalks and roads where deicing salts are used may have soil affected within 30 to 50 feet of the roadway. Over application of fertilizer (especially animal manures), irrigation with high salt water and high ground water tables can also cause specific site problems.
High salt soils cause plant injury because plant membranes allow water to pass through but prevent salt from entering the plant. Lots of salt makes it hard for water to pass through the membrane. Especially high salt may even draw water out of root cells. Salt also changes soil structure. It binds with clay soils and causes compaction resulting in less room for water and air in the soil. Seedlings and young transplants are more sensitive. The amount, duration and concentration of exposure to salt matter and damage is more severe during dry weather.
Saline soil injury to plants will show up as stunted growth, dead leaves, dead areas on leaf edges, early fall color or leaf drop and buds that fail to open.
How do you know how much salt is present in the soil? One strategy is to look for injury in plants. You can also take a soil test and anything greater than 2000 ppm is a problem.
Plants injury can also be due to salt spray. Waves break and throw tiny droplets of salt water into the air. Salt laden water droplets land on leaves. When droplets evaporate, sodium and chlorine ions penetrate stems, buds and leaves, causing injury.
Symptoms of salt spray injury include bud death and twig dieback producing a witch’s broom appearance to the branches. Leaf burn or scorch on foliage can also occur. Sometimes there is even a white residue on leaves once water evaporates. With salt spray damage, the plant injury is usually on one side of the plant—the side facing the roadway or ocean.
Why are we talking about salt injury in August? This seems more like a winter problem, right? The reason to think about salt injury now is that selecting tolerant plants for landscape situations prone to high salts is the best method of preventing injury and we are coming up to the best planting time of the year—fall. Fall is good for planting because we normally get adequate rainfall (certainly not an issue this year, but…); plants tend to grow roots in the fall, so they become established more quickly; and the weather gets cooler throughout the fall providing great conditions for growth. Evergreens are the exception because they do not grow roots in the fall, but most other plants establish quickly when planted in the fall, giving them a great start for the spring growing season.
So, what makes a plant salt tolerant? Salt tolerance is the ability of plants to grow and complete their life cycle in substrates with a high concentration of soluble salts. These plants are called “halophytes.” There are several different mechanisms of tolerance. Plants use avoidance to keep salt ions away from the parts of the plant they might harm. Salt exclusion means filtering salt at the root membrane, allowing water in, but keeping salt out. Red mangroves use this method of salt avoidance. Salt excretion involves removing salt into glands, bladders or though leaf cuticles. Tamarix secretes salt through its leaf cuticle. Succulent plants use salt dilution. By having plenty of water in the leaf, they dilute the salt content. High salts are compartmentalized in plants. Groundsel bush, a common beach and roadside plant in Delaware, has vacuoles (large open structures between leaf cells) filled with salt. Some plants have biochemical mechanisms that allow them to tolerate salt.
To reduce salt injury from salt saturated soils, improve soil structure by adding organic matter to soils exposed to salt. Leach the soil with irrigation, applying two inches of water over a two to three hour period. If runoff occurs, repeat in three days. Always irrigate thoroughly, rather than lightly. Mulch to reduce evaporation. Fertilize only as needed and keep plants healthy to reduce stress. If you have control over de-icing, use cinders, fly ash or sand, rather than salt.
For salt injury caused by salt spray, design sites with windbreaks to prevent salt spray from hitting plants or use salt tolerant plants as windbreaks. Rinse salt spray off plants after a storm that might have caused excessive spray.
If salty soil or salt spray is a problem for your landscape, select salt tolerant plants for planting this fall. Here are two good references that list many salt tolerant plants.
Highly tolerant large trees include southern magnolia and willow oak. For smaller trees, use Japanese black pine. Highly tolerant shrubs include groundsel bush, wax myrtle and yucca. Lyme grass and muhly grass are both highly salt tolerant. Blanket flower and daylily are flowering perennials with high salt tolerance. There are many more moderately tolerant trees, shrubs and perennials on the referenced lists.
Groundsel bush (Baccharis halmifolia) is a highly salt tolerant shrub that grows along the Delaware coast and sequesters salt in large vacuoles within the leaves to prevent injury. (Photo credit: Rick Darke