By Sarah Browning
From the December 2024 Issue
Cold temperatures, drying sun and wind, road salt, heavy snow and ice loads can all take a toll on landscape plants during winter. Every year when Spring arrives, many landscape managers get a nasty surprise when they find damage in their landscapes which looked great the previous Fall. Let’s look at these problems and prevention/ management strategies for each.
Winter Desiccation
Both deciduous and evergreen plants can be damaged by dry conditions in Winter. Bright sun and drying winter wind remove moisture that plant’s roots cannot replace. This is especially true for plants with low internal moisture due to dry late Summer and Fall conditions. Plants particularly susceptible to winter desiccation include broadleaf evergreens.
Symptoms of winter desiccation include leaves, needles or twigs drying out and dying. Leaves may be only partially scorched around the edges or completely brown, while needles usually dry from the tip back, often resulting in partially brown needles. However, evergreen needles often hold their green color until warmer temperatures arrive in Spring, delaying the appearance of browning.
Sometimes the pattern of damage is directional, being more severe on one side of the plant than the other. Wind accompanying dry periods accelerates water loss and results in more extensive damage on the side facing prevailing wind. Injury can also be more severe on the side of the plant facing a source of radiated heat, such as a south or west-facing brick wall or street.
Several strategies can be used to eliminate or minimize winter desiccation. Read on.
Fall irrigation. Ensure plants are well-hydrated throughout fall, enabling them to recover from Summer drought stress and enter Winter with a good level of internal moisture. Provide deep, infrequent irrigation, ideally moistening to a depth of 12-18”. Plan on twice monthly irrigations from August through October, with at least one deep soaking per month in November and December when conditions are dry. Watering can continue until soil freezes. Focus on underneath a tree’s drip line at a minimum, but further out if possible. Frequent and shallow irrigations are not recommended.
Mulch. Conserve soil moisture by applying a ring of mulch around the base of trees and shrubs. Use 3-4” of organic material, like arborist wood chips. Apply the mulch in a wide, flat pancake-like layer – not piled up around the tree’s trunk, like a mulch volcano. Pull the mulch back a few inches so it doesn’t touch the tree’s bark.
Anti-transpirants. Products can be used as a short-term solution to minimize Winter desiccation are anti-transpirants. The best long-term strategy for plants with a history of severe Winter desiccation is moving them to a more favorable site.
Anti-transpirants are liquid products sprayed on plants, forming a clear flexible coating on leaves and stems and slow the rate of water loss through evaporation and transpiration. The coating gradually weathers and degrades, requiring reapplication.
Use the Winter application rate and make the first Fall application after plants have hardened off and become dormant. Allow at least 3 to 4 hours of drying time, with air temperatures above freezing to apply. Spray plants completely, covering both upper and lower leaf surfaces to the point of runoff. When the application is complete, clean the sprayer thoroughly to prevent product residues from gumming up sprayer components.
The oil base of these products removes the waxy coating on Colorado blue spruce needles, changing the foliage from blue to green. Next Spring’s new growth will emerge with its normal blue coloration; older needles present at the time of application will remain green. Read and follow label directions and carefully, particularly re-application recommendations to avoid plant damage and maintain product effectiveness throughout Winter. Watch for use restrictions on specific plants. For example, Vapor Guard should not be used on arborvitae, juniper or cedar.
Salt Injury
Supporting Salt damage to turf, trees and shrubs occurs in three ways:
- Salt sprayed directly on plant foliage, stems, and buds reduces cold hardiness, making them more susceptible to freezing damage. Often, this occurs when salty slush on roadways is splashed by passing cars.
- Salt accumulation in soil destroys its structure by breaking down soil aggregates. This results in heavy compacted soil with reduced pore space, reduced water percolation, and creates a poor environment for root growth.
- Salt also changes soil chemistry. Water is pulled out of roots toward the higher salt concentration in soil water, resulting in root dehydration and death. Chloride ions are absorbed by roots, transported to leaves and accumulate there to toxic levels, often resulting in marginal leaf scorch.
To determine if salt is causing damage to plants, note where the damage appears. If salt is the problem, damage will be more severe on sides facing a road, sidewalk or other hard surfaces where salt was applied.
Also make note of where snow is piled in the landscape. Avoid piling snow containing salt around sensitive plants, including: Ash (white), Barberry, Beech, Black walnut, Boxwood, Filbert, Balsam and Douglas Fir, Hemlock, Hickory, Linden (American and littleleaf), Red and Silver Maples, Pine (Norway, red, and white), Poplar, Privet, Spirea (Vanhoutte), and Spruce (Colorado and Norway.)
In evergreens, damage may not be evident until plants become physiologically active in late Winter or Spring. Needle and foliage browning will appear usually starting at branch tips. However, branches covered by snow may have been protected from salt exposure and show little damage compared to those exposed. Plants repeatedly damaged by salt exposure over the course of several years may develop a witch’s broom twig growth pattern and become stunted on the exposed side.
Although salt is applied throughout Winter, most plant damage occurs in later Winter months and early Spring when they are beginning to actively grow, and excess salts are pulled into them. It is particularly important to protect plants during this time, and limit salt use in late Winter. During that time, when the soil in affected areas is no longer frozen, leaching salt through the soil and out of the plant’s root zone is possible to minimize uptake by landscape plants. However, this is only effective in areas with good water percolation properties, which will allow the water and salt to move down through the soil profile.
Additional strategies for minimizing salt damage include the following: Install burlap or other fabric barriers in late Fall to block salty spray from hitting roadside plantings throughout Winter; Use the smallest amount and least damaging products possible to manage ice; Use products like sand, sawdust, or cat litter on slick surfaces instead of or in combination with traditional de-icing methods to improve traction.
Low toxicity de-icers include: Potassium chloride (KCl) – lowest practical temperature 12° F; will not harm plants; Magnesium chloride (MgCl2) – lowest practical temperature 5° F; will not damage plants if used as recommended; Calcium magnesium acetate (CMA) – will work below 0° F; low plant toxicity; Calcium chloride (CaCl2) – effective to -59° F; melts ice at temperatures to -25° F; will not harm plants if used as recommended.
Physical Damage
Wet snow and ice create a heavy load on branches and can lead to limb breakage. The heavy weight can also cause less obvious internal splits or cracks in trunks or branches, which weaken their structure in the long term.
Arborvitae and pyramidal yew often benefit from additional protection from heavy Winter snow and ice loads. One way to protect branches from breaking under heavy snow loads is to tie up each plant with rope or cord.
Start by tying twine around the trunk at the base of the branches or to the base of one of the lower branches. Next, wrap the twine spirally up the plant, pulling the branches inward into a tight pyramidal shape, but don’t pull the twine so tightly that any branches break. This helps plants shed snow more easily and prevents branch damage. Remove the rope in Spring after the danger of snow has passed. As the weather warms, branches will move back to their normal position.
Shrub cover jackets or protectors are another option. Made of polypropylene or fiberglass mesh, they are available in many sizes and styles. They are designed to provide a smooth cover which sheds snow and protects the plant from wind, sunscald, and deer.
Sarah Browning is an Extension Educator, with Nebraska Extension since 1998, providing horticulture information to the general public and commercial green industry professionals. She became an International Society of Arboriculture certified arborist in 2010 and is also a member of the Nebraska Arborist Association.
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