Salt Damage to Woody Plants
By Nicholas J . Brazee , UMass Extension Plant Pathologist
Damage to woody plants from exposure to de-icing salts can be a major concern during the winter season in New England . The injury to trees and shrubs occurs two possible ways , either as salt spray to the canopy or through root uptake of salts in the soil . Symptoms of salt damage vary considerably depending on the plant , distance from a treated surface , type of exposure ( spray or root uptake ), amount of salt applied , and soil type , among other factors . These symptoms can include : reduced vigor , stunted and / or deformed growth , marginal leaf / needle browning , shoot dieback , witches ’ brooming and even outright death of the plant .
There is considerable variation in the susceptibility of both deciduous and coniferous plants to damage from de-icing salts , even within the same genus . For example , it ’ s well known that eastern white pine ( Pinus strobus ) is very sensitive to salt damage , whereas pitch pine ( P . rigida ) is very tolerant of salts . Meanwhile , sugar maple ( Acer saccharum ) is also very sensitive to salt damage in urban environments , yet honeylocust tolerates salt exposure . Unfortunately , sensitivity to salt damage is often a major determining factor for municipalities when selecting tree species for urban planting .
The damage that woody plants experience from de-icing salts is caused by chloride and sodium as these ions accumulate in stems and leaves . High levels of chloride negatively affect a variety of important processes and structures at the cellular level , such as inhibiting nitrate and phosphate uptake , damaging cell membranes and disrupting enzyme function . Plants respond to excess salts by translocating chloride and sodium ions to shoot tips and leaf margins , where they accumulate and symptoms manifest . In addition to causing damage directly to plant tissues , excess sodium in the soil raises soil pH and can make essential nutrients , such as calcium , magnesium , and iron , unavailable for root uptake . Chloride and sodium can leach from soils in certain settings , but they can also build up over time with repeated , annual applications .
A variety of de-icing salts are available for commercial and residential use , and it can be challenging to determine which is best when it comes to plant health . Two common forms , both used by the Massachusetts Department of Transportation , include rock salt ( sodium chloride ) and magnesium chloride . While highly effective , both are harmful to plants when taken up either as salt spray or through root uptake in the soil . Liquid brine , which is used to prevent ice from forming over treated surfaces , is created by mixing water with rock salt or a combination of salts ( i . e . rock salt and magnesium chloride ). Alternatives , such as calcium chloride and calcium magnesium acetate ( CMA ), are also effective and widely available . While both calcium chloride and CMA are believed to be less harmful to plants in comparison to rock salt and magnesium chloride , the results of various research trials do not always support this assertion . ( continued on page 18 )
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