When your trees and shrubs show symptoms of water stress, particularly in late winter or early spring, but you’re sure you’ve irrigated them, you might be looking at winter salt damage.
Salt is really good at keeping roads and sidewalks clear of winter snow and ice, but it brings with it some side effects. The most visible side effect is damage (and even death) to both deciduous and evergreen trees and shrubs.
In this article, we cover how to identify salt-related injury to plants, why it happens, how to prevent salt damage to your trees, and what to do if your trees and shrubs have been affected by winter salt or de-icers.
What does salt damage look like in trees or shrubs?
Typically, salt damage looks like water stress on foliage. Conifer needles turn brown from their tip backwards to the needle’s base. Broadleaf evergreen and deciduous trees and shrubs develop brown leaf margins or entirely dead leaves. Branches may also develop witches’ brooms in response to salt damage.
More severe salt damage can kill branch tips and whole branches, stunt plant growth, destroy leaf and flower buds and, in extreme conditions, kill whole trees. Salt can accumulate in soil and in plants, and high levels in either can prevent other nutrients from being accessed by your plants.
Along roadways and embankments, you might see trees and shrubs with their road-facing sides looking scorched while the back face of the plant looks healthy. This is from the arcs of water sprayed by car and truck tires as they drive by. If you’ve ever stood at a crosswalk in the rain, you know how far cars can spray water!
Why do salt-damaged leaf edges turn brown first?
Damage is seen first along leaf edges because a plant’s vascular system pulls water up from the soil through its roots and then pushes excess water out through tiny pores on leaves. When the water moving through a plant is too salty, salts collect at leaf edges and kill the leaf tissue.
If salt is so harmful to plants, why do we use it on roads and walkways?
Salt and saltwater have pretty much always been known for their toxicity to plants. Seacoast plants and plants native to areas with saltier soil or water have developed a tolerance for salty conditions, but most species used in landscaping haven’t.
However, salt is very efficient at banishing snow and ice, making roads, sidewalks, driveways, parking lots and other paved areas safer for drivers and pedestrians.
Road salt dissolves in water to make brine. Brine has a freezing point lower than water (snow), so when it’s spread on a paved surface it keeps snow from freezing and accumulating. There is usually an abrasive or gritty ingredient added to road salt, such as sand, to give shoes and tires extra grip on wet concrete or asphalt.
What is de-icing salt made out of?
Sodium chloride (rock salt or table salt) and calcium chloride (a salt from limestone) are the two most commonly used salts for roads. They are easy to store and apply. Calcium chloride is less damaging to plants than rock salt, but it and other less damaging salts are more expensive than rock salt.
If you get to choose the de-icing materials that will be applied to areas near your trees and shrubs, choose one such as CMA that is not as corrosive as road salt.
How can I protect my plants from road salt and ice-melt products?
Water is the best antidote to salt build-up in soil. If you irrigate or leach your soil with water, it will lower the concentration of salt near plant roots. Enough water applied to the soil will wash salt away from your plants’ root area by moving it deeper into the soil and spreading it away from the plant.
If there are times during winter when temperatures are above freezing, you should repeatedly irrigate “low and slow” to allow as much water as possible to penetrate the soil without causing erosion. But avoid flushing low lying or poorly drained soils; water will collect in the soil and can drown plants’ root balls. And even if you do irrigate during winter, you’ll still want to flush the ground well in spring.
Wash the Leaves
If your plants and trees are in areas where road salt is sprayed on them from cars and trucks, wash off the foliage with water as often as you can. Temperatures need to be above freezing for this and need to be above freezing long enough to prevent ice from forming and building up on leaves and branches.
Wrap Them Up
You can also wrap plants in burlap or other materials to protect foliage from salt spray—just remember to unwrap as soon as road salting ends. This isn’t the most aesthetically appealing way to protect your plants, but it can help save their foliage. Wrapping has the additional benefit of protecting your plants from cold, drying winter winds, so leaves retain their water longer.
If you want to help protect your evergreen plants like holly, boxwood, and rhododendron, you can use an anti-desiccant or anti-transpirant spray to coat leaves and needles. Remember that these sprays must be applied before snowfall and road salting begins, and before temperatures drop below 32F and the soil freezes.
Place Snow Somewhere Else
Finally, if you shovel or plow snow that has been in contact with road salt, don’t leave snow piles near your plants. If you can, pile it elsewhere. If there’s nowhere else to put snow, wash it with water as soon as temperatures rise to dilute its salt concentration.
If my trees are damaged by salt, when will I see symptoms?
Salt damage usually appears in late winter and early spring, as plants emerge from dormancy. It can also show up later in summer, when plants are growing most vigorously and taking up the most water and nutrients from soil.
When your plants’ roots start taking up water and nutrients from soil for their leaf and flower budding, they also take up the de-icing salt that has built up in the soil. That’s when you’ll typically see symptoms of salt burn.
Are there any plants in Connecticut that are more tolerant of salt?
Choosing native or salt-tolerant plants is a good way to minimize salt damage to areas of your landscape near roads and walkways that are salted in winter.
UConn has a list of native, salt-tolerant plants, and Purdue has a list of landscape plants and their salt tolerance. You’ll find many tree species commonly found in Connecticut have some tolerance of salt, such as ginkgo, red oak, and London plane trees.
Can I do anything else to protect my plants from road salt?
If you’re designing a landscape near road salt areas, pay attention to slopes and keep your plants out of low-lying areas where salty runoff can accumulate. Raising up the grade where plants will grow will help to keep road salt from reaching them and will help to wash salt out of the soil when you irrigate.
And, as always, soil health is key. Well-drained and amended soils help to keep your plants vigorous and better able to withstand damage from road salt. A thick layer of mulch can help hold existing (non-salty) moisture in the soil, where it’s available to plant roots.
What should I do if my tree or shrub looks badly injured from salt?
Call the arborists at Rayzor’s Edge Tree Service! We’re trained to assess tree and soil health, and can recommend a course of action to help your plants recover (or, if the damage is to severe, we can remove the tree quickly and safely).
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