By Michelle Sutton
Sodium chloride (NaCl), the same salt that we use to season our food, has been used to deice roadways and sidewalks since World War II. It’s cheap, and has made our roadways safer; but its downsides are many, including damage to aquatic life, infrastructure and our trees. Harm to trees results from NaCl accumulation in the soil and/or from salt spray. When you salt a slice of eggplant, you can observe how, via osmosis, water is drawn out of the cells. Salt desiccates trees in the same way.
Soil salinity and salt spray damage plants in different ways. In clay soils, salt can contribute to soil compaction because of the way salt molecules bind with clay particles. Compaction reduces the ability of water and oxygen to move through soils and be taken up by roots. Also, salt in the soil is taken up by tree roots with myriad negative consequences — from failure to leaf out to marginal leaf necrosis. By contrast, salt spray most commonly kills buds, resulting in twig or branch dieback or “witches broom,” which occurs when side shoots emerge to compensate for apical bud death.
Many environmental groups and municipalities are seeking alternatives to NaCl — from potassium chloride to urea to beet juice. Dr. Glynn Percival, plant physiologist and technical support specialist at the Bartlett Research Laboratory in Reading, U.K., has researched and published papers on many facets of salt damage. (Surprisingly, given the extent of damage NaCl can do, Percival has very few colleagues doing this kind of research). As to NaCl alternatives, Percival said, “There really is not much research in this area that I am aware of. The use of calcium magnesium acetate has proved effective and far more environmentally benign than salt, but it is expensive and therefore rarely used as an option.”
Percival is currently evaluating the salt tolerance of many ornamental trees whose salt tolerance was yet unknown — trees that could prove useful in urban situations. “This is a joint research trial with Barcham Trees (a nursery in Cambridgeshire, U.K.) who have a phenomenal collection of trees with which to use for experimental purposes,” said Percival. “The system we are using is a laboratory-based test that is rapid and effective. Data will be available very soon.”
Commercial arborists can’t control how their clients’ municipalities run their deicing programs, but things arborists can do to help prevent or mitigate damage to trees are as follows:
Educate yourself, your clients and local officials
Arborists can educate themselves and others about soil salinity and salt spray damage to trees. Percival recommends the bulletin, “Deicing salt damage to trees and shrubs: Forestry Commission Bulletin 101,” published most recently in 2011 and freely available on the web. “This is essential reading for anyone involved in selecting trees for salt tolerance and/or undertaking any form of salt related research,” he said.
Select salt-tolerant trees
“Select the right tree species for the site,” said Percival. “Salt tolerance varies massively between tree genera and even between species within a genus.” For example, red and sugar maples (Acer rubrum and A. saccharum) are very sensitive to salt spray, but hedge and sycamore maples (A. campestre and A. pseudoplatanus) are not. Japanese black pines (Pinus thunbergii) can tolerate saline soils and salt spray, but white pines (P. strobus) tolerate neither. English oak (Quercus robur) is much more tolerant of salt that pin oak (Q. palustris).
Percival describes an interesting dichotomy: “You generally find that tree species that are very tolerant of salt applied to the roots tend to be sensitive to salt applied to the foliage and vice-versa; so, when selecting a species for planting, make sure there is an appropriate species/site fit.” He said that in his observation, street trees tend to suffer more from salt applied via the roots, while in more open/exposed areas, trees are more susceptible to foliar salt spray. The Virginia Cooperative Extension (VCE) bulletin, “Trees and Shrubs that Tolerate Saline Soils and Salt Spray Drift,” provides a long list of trees that teases out the kind of salt tolerance each species possesses.
Site the sensitive ones
The VCE bulletin advises arborists to plant trees sensitive to soil salinity on berms or uphill so that gravity works to keep the salty water away from tree roots. Trees that are sensitive to both soil salinity and spray should also be planted at least 50 to 60 feet back from surfaces that are commonly deiced. Arborists can group salt-tolerant species in such a way as to protect the salt-sensitive species behind them from salt spray. To accomplish this, there is also the option of fencing to provide a physical barrier between salted roads and treasured trees.
Coat the leaves
Percival and a colleague have done research on film-forming polymers. “These are specially derived polymers that are designed to be sprayed onto tree foliage that, in essence, act as a salt-protective barrier across the leaf or conifer needle surface, and prevent salt ions entering the plant,” he said. Field trials by Percival and others showed that these newer polymers, unlike those of the past, also have no detrimental effects on tree biology, and that their protective effects last up to three months per application.
Apply sugar before salt
In another study, Percival and Al-Habsi Sulaiman found that the salt tolerance of trees can be artificially enhanced by applying sugars to their root systems before salt is applied. “Admittedly, it’s a little out of left field,” said Percival; but for the two species they studied, application of sucrose, which can protect leaf cellular structure, induced tolerance to and recovery from de-icing salt damage. It’s an intriguing finding worthy of more study with more tree species.
Improve soil and increase soil volume
Where there is more organic matter, there is better soil structure, and where there is better soil structure, there is better drainage. Better drainage allows salt to more freely leach out. A larger rooting volume also brings better drainage and salt flushing. Also, healthy soil leads to healthy plants that are better able to withstand salt exposure.
Flush the soil and/or the tree
Thorough, deep irrigation can help flush salt out of the soil. The VCE bulletin recommends “applying 2 inches of water over a 2 to 3 hour period, stopping if runoff occurs. Repeat this treatment three days later if salt levels are still high.” To deal with salt spray, the bulletin advises to “rinse salt spray off trees and shrubs after storms and high winds and again in early spring to remove salt residue from tender buds and leaves.”
Hold the fertilizer, but pass the mulch
Since synthetic fertilizers are salt-based, one should use them judiciously, if at all. Mulching is a good idea, because it helps the soil to retain moisture, which helps move salt out of the soil.
“Trees have a great capacity to store chemicals,” said Percival. “Consequently, trees may store salt over a number of years until eventually a threshold is reached, after which the tree quickly spirals into a salt toxicity decline.”
Also, a little salt is good! Percival said that spraying trees with a weak salt solution (1-3 grams in a liter of water) actually toughens them up. “We call it pre-conditioning,” he said. “Several nurseries now pre-condition their nursery-grown trees (which previously have in essence been ‘pampered’ throughout their growing lives) before planting into an urban landscape where salinity represents a major issue killing hundreds of thousands of trees annually.” Transplant losses tend to be far lower in pre-conditioned trees.
Michelle Sutton (michellejudysutton.com) is a horticulturist, writer, and editor.
Above: Marginal leaf necrosis/”burn” can be caused by soil salinity or salt spray.
Photo by Michelle Sutton