Saturday, December 22, 2018

Minimizing Salt Injury to Trees and Shrubs

Every winter brings its annual a-salt on roads and walkways. In icy conditions, salt may be necessary for safety, but too much of it is worse than a bad pun. Cars, equipment, and concrete suffer in obvious ways, but damage to trees and other woody plants is less visible. Salt injures trees and shrubs by several means.

When road-salt spray hits twigs, buds and, in the case of evergreens, foliage, such direct contact causes yellowing of needles, and subsequent death of evergreen twigs and limbs. It also leads to stunted or deformed growth, such as witches’ brooms, on hardwoods. Severe or repeated direct exposure, especially for sensitive species like white pine or cedar, can kill the whole tree.

Less noticeable is the effect salt has on roots when it is directly deposited onto a tree’s root zone by plowing or through runoff. For an established tree, its root zone is two to three times its branch length or drip line. High concentrations of salt in the soil may kill a tree over the course of just a few years. But even at lower concentrations it makes water less available to tree roots, producing drought stress even in the presence of moisture.

This latter injury is chronic, and can show up as browned leaf edges, a condition known as marginal leaf scorch, in the hottest part of the summer. In July and August, deicing salt is the last thing on people’s minds in terms of a diagnosis. It can also manifest as subtle, cumulative damage that weakens a tree year after year until eventually it succumbs to opportunistic agents such as insects or diseases. Many large old sugar maples planted on roadsides have fallen victim to cumulative stress, with salt at or near the top of the list of factors.

Salt actually damages soil structure, causing what is known as sodium compaction. Roots need to get oxygen through soil pores, and healthy soil forms tiny clumps which form natural channels for air to pass. The chemical bonds holding the clumps together are broken by salt, and as a result the pore spaces collapse, restricting roots’ access to air and further stressing trees.

While it was formerly assumed that rain or heavy irrigation could wash road salt from the soil each summer, it now appears this is rarely the case, and that salt levels often slowly build over time. An approximation of soil-salt levels can be made using a conductivity meter to check the electrical conduction in a soil slurry. Readings over about 3,000 micro-mhos per square centimeter (the standard units) may indicate a problem.

There are many strategies for dealing with the issue. Species selection is top of list. Trees such as honeylocust, hawthorn, and Norway maple are more salt-tolerant than others, and can be used in place of sensitive trees like sugar maple.

Homeowners can use low-salt recipes to reduce salt damage. Employing sand or commercially available mineral abrasives rather than rock salt, or by at least switching to mixture of salt and sand, will ease the stress on woody plants. Alternative deicing products such calcium magnesium acetate (CMA) are much less toxic to plants, though they generally cost more.

Physical barriers are another tool to combat salt damage. Constructing a fence of burlap or geotextile fabric to keep salt spray from contacting foliage will help a great deal. Protective berms to deflect and/or divert salt-laden spring runoff from root areas can assist as well.

For more information on ways to reduce the harm wrought by winter’s “a-salt” on trees and shrubs, contact your local Cornell Cooperative Extension office.

Photo by by Brian P. Whattam.

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

11 Responses

  1. John says:

    Paul, excellent article. I would add that Norway maple is considered an invasive species. It is hyper competitive in a forest, woodland setting often out competing sugar maple and other native species. I would not recommend it for that reason. Locust is considered invasive by some. As it spreads by root suckering. However, it is native to Appalachia and was brought here for its rot resistant lumber so I do not really consider it invasive being that its native to a nearby, rather than across an ocean. Locust blooms fragrant flowers in the spring and is an important species for pollinators.

  2. John, a good point about Norway maples. For many years I was strongly opposed to their use generally. However, with the loss due to EAB of the nearly bullet-proof green ash as a salt-tolerant street tree, i think Norway maple has a place in downtown street environments where the risk of its spread into natural forest areas is very low. What really bothers me is when those red-leaved Norway maple cultivars (‘Crimson King’ etc.) are planted in suburban and rural lawns–that poses more danger to our forests than putting Norway maples on streets.

    • Boreas says:

      Can these “invasive” cultivars be made sterile so they can be planted where desired but do not spread?

      • I think it would require some effort and expense on the part of plant breeders. Seems unlikely to happen until/unless Norway maple is placed on NYSDEC’s Prohibited Plant List.

  3. Tim-Brunswick says:

    Wonderful…….”what really bothers me” is people getting injured/killed on ice covered roadways when it could have been prevented by judicious application of salt/chloride to the highway surface, I mean…. hey we’ve gotta a lot of people so we can probably spare a few in deference to saving some trees and shrubs along our roadways……

    Guess it comes down to choices and I choose “people”!

    • Suzanne says:

      The salt also leaches into rivers and streams.

    • Scott says:

      People can also choose to drive slow on icy roads and not think the government needs to control everything and provide everything.

    • Tim,
      Apologies. I could have been a bit more clear. Sand/salt mixes and rock-salt substitutes have been deemed just as good as straight NaCl in reducing risk to people from icy surfaces (search for MN and MI DOT salt-substitute trials). It is a win-win; people are protected, and trees, fish, and cars last longer. In cases where salt use is unavoidable, taking measures to protect plants, as well as choosing tolerant trees, are still good ideas.
      I will amend the text for future publications so that people who read the piece hastily do not get an unintended message. I wish you well.

  4. geogymn says:

    Guess it comes down to choices and I choose “people”!

    LOL, typical, egomania leading the way.

  5. Dean Bianco says:

    To Tim-Brunswick:

    The problem with New York State, and in specific, the Adirondacks, is that they do not practice the judicious use of salt-based de-icers. They practice EXCESS use of it. Have you actually observed the trucks spreading the salt on our roads here? Have you made note of the weather conditions occurring at the time the trucks are spreading? Have you made note of the number of times the same truck will pass on the same lane-miles re-salting what had been salted not long before? Have you made note of the shoulders being heavily and unnecessarily made bare-bones dry?

    I have! And many other concerned taxpayers have, as well. NYS needs to be forced to substantially REDUCE the excessive application of sodium on our roads in order to reduce the damage it causes to cars, bridges, roadway surfaces, flora and fauna. Can lives be saved with such a reduction of salt on the highways? The answer is absolutely, definitely “Yes.” Other neighboring states have reduced their salt loads and the statistics bare out the safety numbers. (Google: “Road Salt” + “Vermont, Massachusetts, Maine, New Hampshire.”

    Your comment, perhaps, and I hope I am wrong on my guess, sounds like you are a cynical anti-environmentalist. I hope you’re not, but the tone of the comment strikes me as reactionary. I hope I am wrong about that, as I said.

    The Adirondacks are not “Anywhere, USA.” This region needs extra care, extra loving, if you will. It needs watchdogs who are not afraid to bark when they see wrongs, and, if necessary, bite the committers of the said wrongs.

    Just my opinion. Peace and Merry Christmas to you and all!

    • Greg Keefer says:

      Dean – Well said. We’ve got similar issues in Northern Virginia. Here, the Commonwealth employs private contractors who are paid by the hour, resulting in extreme application of salt on our roads,to the point of abuse. Taxpayers need to speak up and exercise logic, our politicians certainly seem challenged in this area – regardless if in NY or VA – 🙂

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