Recognize Winter Cold Damage

Cold Injury on Landscape Plants

The magical process of cold hardening enables plants to withstand extreme freezing temperatures in the winter. In an unhardened plant, tissue death occurs when the water inside the plant cells freezes and forms crystals that rupture the cell walls. Many factors weigh in to determine whether a plant will handle winter conditions. Find out which plants are most at risk from cold damage, and learn how to recognize cold injury in different types of plants.

The magical process of cold hardening enables a plant to withstand extreme freezing temperatures in the winter when a borderline freeze could kill the same plant in summer. These changes are brought about by gradually decreasing temperatures and shorter day lengths beginning in fall. Different species harden to varying degrees in winter and do so at different rates. An early freeze can have substantial consequences for some plants if they haven’t had time to cold harden. Plants may also deharden at different rates. Plants that tend to come out of dormancy at temperatures that are relatively low or are only following an internal clock may be damaged by late cold snaps.

Plants going into winter are physiologically slowing down. Some plants will “die back” so that only below-ground or surface-level tissue, or the seeds, remain to persist in the protection of the ground. Deciduous plants and trees lose their leaves and stop photosynthesizing; this also protects them from water loss by transpiration. Evergreen plants and trees (both needled and broad-leaved) continue to photosynthesize and take in water and expel it through their leaves. Winter may be especially hard on broad-leaved evergreens with their greater leaf surface area allowing substantial water loss through transpiration. Both deciduous and evergreen types significantly reduce metabolism and growth to manage the changes needed for winter survival.

Cold Hardening - The Process

A woody plant doesn’t need to protect all its cells from freezing. Woody plants contain many dead cells that assist with functions during the growing season, such as the transport of water and nutrients by xylem cells. And dead cells in the bark can serve as first-line protection against the cold. In winter these cells can freeze with no damaging effects. Tissue death of living cells occurs when the water inside plant cells freezes and forms crystals that rupture the cell walls. To avoid this damage, plants respond to cold at the molecular level by removing water from their tissues. Starting in the fall, trees begin converting starches into sugars; the concentration of sugars lowers the freezing point inside the cells. The sugar-free water between the cells is allowed to freeze. Also, cell membranes become more pliable to allow the water in the cells to pass through to the spaces between cells. These 2 mechanisms cause progressive cellular dehydration whereby the cellular contents become so thick they are almost solid, which avoids crystallization. This process has been studied most in needled evergreens but presumably other plants use these methods as well.

Which Plants are Most at Risk from Cold Damage? 

Cold damage occurs when plants aren’t able to fully protect themselves. These are the primary conditions related to most injury:

• Plants that are growing in a climate that is too cold for that species or cultivar

• Plants that experienced an early cold front disrupting their cold-hardening process

• Plants that are growing in an area where a winter warm spell caused an early break of dormancy followed by a late cold spell

• Plants borderline hardy for their region located in microclimates that introduce colder conditions than what is expected for the area

• Plants which have been fertilized or pruned too late in the growing season causing a push of new growth susceptible to the cold, and delaying hardening

• Plants not properly cared for earlier in the year making them more susceptible to injury, especially if the roots were compromised

• Plants stressed by insects or disease

• Newly planted (especially if they were balled and burlapped) plants with insufficiently developed roots

• Young plants, since they don’t have the hardiness that they’ll have when older. They may not have sufficiently hardened bark to avoid sunscald or frost cracks or sufficiently developed roots to sustain them.

• Plants in containers, since they don’t have the protection that the ground offers

• Plants near roads or other areas where de-icing chemicals are used, especially if sodium chloride is used

• Taller plants since they are further from any heat which may be radiated from the ground

• Plants with white rock or other reflective material around their base

• Evergreens, because the leaves remain in the winter, making them more susceptible to desiccation than deciduous plants. Of these, the broad-leaved evergreens, yews, hemlocks and arborvitae are most susceptible. 

• Evergreens sited where they will receive winter winds since desiccation is greater here

• Trees planted in a sunny southern or western exposure because bark injury is more common here

• Trees with shallow root systems, such as Norway maple, birch, silver maple, linden, Colorado blue spruce, and Norway spruce

• Fruit trees with large crops since they may have a delayed cold-hardening response

• Of the plant parts, buds, especially flower buds and newly opened flowers, young bark and the foliage of evergreens are most susceptible to cold injury.

Identifying Types of Winter Damage

Besides the influence of the above factors, the extent of damage from frost and freeze depends largely on the timing of the event, the degree of cold and the length of time that the cold event lasts. Damage can range from simple leaf-tip “burn” and discoloration to leaf loss and tissue death which can affect whole branches all the way down to the crown and roots, resulting in the death of the plant. Damage may show quickly or may not show until the next season when the plant is under further stress.

Many plant problems can look similar to cold injury, however. To complicate matters, damage can reveal itself even months after cold injury and cold injury can make plants more susceptible to secondary problems such as twig dieback (a fungal cause) or insect infection. You might be able to rule out cold damage when trying to diagnose a problem. Plants native to your area and properly sited, established plants and those which should have survived your winter temperatures based on their cold hardiness rating are less likely to have winter damage. Did the fall season have gradual cooling to induce proper cold hardening? Were your plants well-hydrated going into winter? Was there snow cover to insulate the ground if this is usual for your area? Several different kinds of plants showing damage in the same location may have some other physical damage such as spray from an herbicide. If the temperature during a cold spell remained far below freezing throughout the entire period so that warming and expansion of wood was unlikely, then sunscald or frost cracks will also be unlikely. Descriptions of cold injuries can help identify the problem.

Chilling Injury

Chilling injuries occur above the freezing point (32℉) and mainly involve plants of tropical or subtropical origin. In their native surroundings these plants do not normally experience freezing temperatures and do not cold harden as temperate plants do. Injured foliage can appear off-color, purple or reddish, and sometimes wilted. Besides outright damage, chilling can delay blooming and reduce growth. Chilling can also happen with temperate plants as well, depending on the physical status of the plant, and such variables as timing and duration of low temperatures. Plants may become wilted, display discolored, yellowed or browned foliage, drop their leaves, grow new misshapen foliage or stunted growth, or even lose whole portions of the plant.

Freeze/Frost Injury

These injuries involve temperatures at or below 32℉, resulting from a frost or freeze event. In a plant that isn’t sufficiently cold hardened, freezing temperatures cause physical damage as expanding rigid ice crystals puncture cellular membranes, and dehydration damage as the water within plant tissues freezes and later drains out. Generally, damage is first noticeable on young leaves and shoots on the outermost parts of the plant. Freeze/Frost injury includes low temperature damage, frost cracks, sunscald, and frost heaves. The following describes low-temperature damage in annuals, herbaceous perennials, deciduous plants, broadleaf evergreens, needled evergreens and succulents. 

1.  Low Temperature Damage

 

         Annuals and Herbaceous Perennials - With light frost, the tips or entire leaves of annuals can wilt as if water-stressed. With greater cold, herbaceous and tender plant parts may wilt or become dark green and water-soaked. Within a few hours or days, this will become browned or blackened, mushy and necrotic, or in some cases curled and crispy. Stems will collapse. Eventually, the injured leaves and stems will disintegrate. Crispy or mushy plant parts are dead.

 

The foliage of most herbaceous perennials will usually die back to the ground as part of the winter process. But unlike freeze damage, the plant appears to slowly dry and atrophy or be crispy. The stems and foliage of some herbaceous perennials, such as ornamental grasses, will dry out and brown but remain standing. Unless freeze damage was extreme, most herbaceous perennials will resprout from their crowns or roots in the spring. Some, like hellebores, may be unscathed by an early hard freeze and may already show new growth at their bases.

 

         Deciduous Shrubs and Trees - If a freeze in early spring damages new growth on deciduous plants, leaves may become limp, shrivel and brown or blacken. There may be scorching or pale brown patches between the leaf veins. Eventually these leaves will drop from the tree or shrub. Unless the plant is in poor health, it will leaf out again within a few weeks, having suffered a small setback in wasted energy. 

 

Freeze damage to vegetative or flowering buds may not show until spring when buds dry up, don’t open or do open and quickly die. Tip or branch die-back usually indicates a more severe event and might be seen early on with the scratch test or later when branches fail to leaf out. New branches may even succumb later in summer after beginning to wilt and die back once the damaged tissue can’t keep up with the stress of summer. 

—----------------------The Scratch Test—----------------------

You can use the Scratch Test to determine whether a shoot or branch is still alive but it may be best to perform this once spring has arrived (when frosts are likely over according to your average last frost date), and you are evaluating which branches need to be pruned. A branch may appear alive early on but die by spring. Use your thumbnail or small knife to gently scrape the bark from a small area of the branch or stem to reveal the underlying cambium. Green underneath means the stem is alive and brown or black means it has died. 

—------------------------------------------------------------------------------------------------------------------

——--------------------The Flower Bud Test—-------------------

Of the plant parts, flower buds, opened flowers and the flowering stage following petal shed are the most susceptible to cold damage. To check the amount of damage your flowers (and eventual fruit) sustained, wait several days after a freeze and remove a few flower buds from different areas of the plant. Cut a cross section off the top of the bud. If there’s any discoloration, that bud would not have produced fruit. Damage may be localized on the plant, however.

—------------------------------------------------------------------------------------------------------------------

Hardiness zone ratings are critical when choosing plants for your region but they don’t necessarily correlate with cold damage caused by early hard freezes. Some plants harden more quickly in fall than others so there can be a disparate outcome between 2 plants hardy to the same zone. The following is the type of damage you may expect in various deciduous plants following an early advective freeze in USDA zones 6-8:

Thin barked trees and shrubs - Japanese maples, azaleas, young redbuds and dogwoods may develop bark splitting, which may take time to appear. This would be more likely if the cold spell had fluctuating temperatures near or above freezing. Deciduous trees - may leaf out later than usual.

Hydrangeas - it is likely most will survive, including oakleaf hydrangeas. Paniculata types, considered the most cold-hardy, will likely still be green underneath the bark. Macrophylla types are generally more marginal for bud hardiness and may appear dead mostly to the ground by a scratch test but they will probably come back from the roots. Since they flower on old wood their flowering will be impacted the following year.

In slightly colder areas, burning bush, butterfly bush, oakleaf hydrangeas, roses and figs may suffer.

         Broadleaf Evergreens - It may be difficult to tell the difference between freeze damage and desiccation (winter drying damage) in broadleaf evergreens. Damage may happen more quickly from low temperatures than desiccation and occur all over the plant instead of only those parts exposed to wind and sun. The outer and youngest leaves and those most in contact with frost are usually the hardest hit. They become discolored, reddish-brown or black, sometimes with brown spotting. They may dry and die with enough injury while shoots and branches could remain unharmed. However, the entire plant could die with extensive enough injury. Dead and damaged leaves often, but not always, stay on the plant until spring growth begins and fresh new leaves appear. Some plants may die back partially and grow new stems and foliage from lower portions. And still others can resprout in spring after dying to the ground. 

The following is the type of damage you may expect in various broadleaf evergreens following an early advective freeze in USDA zones 6-8: 

Schip laurel - may have dried brown leaves, and stems dead all the way to base; it is uncertain if it will regrow.

Illicium -  will likely have browned leaves but green stems.

Leatherleaf viburnum - may defoliate.

Boxwood - may take several weeks to show orange-brown lesions followed by the leaves turning brown and defoliating.

Hollies -  some varieties may be less affected than others. ‘Nellie R Stevens’ may stand up to the cold better. Hollies may take as long as June to leaf back out.

Camellias - those at the edge of their hardiness will likely defoliate but may retain some green stems.

In slightly colder areas, magnolias, heaths, heathers, and rhododendrons, mahonia, loropetalum, nandina and lavender may also be regularly damaged by cold.

         Needled Evergreens - The needles of evergreens will brown or yellow, and possibly drop, when injured by low temperatures. Damage will appear uniformly around the circumference of the entire plant and be most pronounced on the outer parts of branches where outer needles are discolored and less-exposed inner needles remain green.

Needle drop also occurs to a lesser extent on healthy evergreens as part of their annual shed. Occuring in late summer to fall in most species or spring and summer for others, older needles will drop from the interior part of the tree. The needles towards the ends of the branches should still be green, flexible and firmly attached. This isn’t as dramatic as deciduous leaf fall and is often not even noticed by many gardeners. 

—-—----------------------The Candle Test—--------------------------

Candles are the growths on the tips of a needled evergreen from which new needles emerge. You can check the health of the new buds (developing candles) by gently pinching the buds. A healthy bud is firm and somewhat moist while a dead bud is dried out and will crumble when handled. 

—------------------------------------------------------------------------------------------------------------------

The following is the type of damage you may expect in various needled evergreens following an early advective freeze in USDA zones 6-8:

Cryptomeria, Leyland cypress and Hinoki cypress may be very hard hit while the similar Sawara cypress including ‘Golden Mop’ may be unaffected.

Juniper and pines - may fair well. 

Blue atlas cedars and deodar cedars - will likely have needle browning and needle drop. Needle drop at the tips or in areas of new growth is of concern. Some discoloration may only indicate slight damage.

In slightly colder areas, juniper, arborvitae, microbiota, and yew may also be regularly damaged by cold.

         Succulents - Succulents are a special case as their tissue is different from woody plants and most perennials. Since they store water in their tissues most are at risk in a freeze event. There are quite a few hardy succulents that can handle freezes, however. These include many sempervivums, agaves and sedums, some of which are even hardy to USDA Zone 2 or 3. Light frost for under an hour may not kill even vulnerable succulents. Their leaf tips may become wrinkled and wilted or thin and dried. On small succulents, the entire plant may develop a dry, crispy appearance. If the frost was considered a hard frost and below freezing temperatures lasted throughout the night, the entire plant may collapse and turn dark. 

 

Other specific injuries related to freezing temperatures include frost cracks, sunscald and frost heaves.

 

2. Frost Cracks 

 

Frost cracks are longitudinal splits in the bark on the trunks of trees due to rapid changes in winter temperatures. Most explanations involve the bark and underlying wood warming to above freezing on sunny, cold winter days followed by the bark cooling and contracting more quickly than the underlying tissue when the sun sets. This causes the bark to split and later in the year it may also peel. The south and southwest sides of trees are most often affected. Once damaged, the area is prone to further splitting even if some healing takes place in subsequent seasons, and is at risk of insect and disease entry. Hardwood trees with thin bark are most susceptible.

 

In actively growing trees (ones which haven’t become dormant yet or have come out of dormancy early), cracks may be caused when the water in cells just beneath the bark freezes. When the sun hits the plant the next day, the ice quickly thaws, killing the cells and splitting the wood. Azaleas are prone to this injury. On small stems it may be referred to as bark splitting.

 

3. Sunscald

Sunscald is another type of damage that can occur on trees with sudden drops in temperature. Like frost cracks, it appears on the sunny side of the tree. Young hardwood trees with thin bark are most susceptible and include honeylocust, maple, apple, linden, ash, mimosa, walnut, pecan, mountain-ash, birch, crabapple, flowering cherry, aspen, tuliptree, Japanese snowbell, willow and deciduous fruit trees. Evergreen trees with lower branches to shade the trunk are less susceptible. 

Mild cases may appear as discolored reddish or brownish bark. With increased damage, bark will appear sunken where cells have died and may dry, split and peel back to expose the wood in the trunk. Minimal damage can slow growth and hamper establishment. Trees will respond by forming callous tissue over the damage but this is a slow process and the tree may eventually decline and die.

—--------------------------The Sunscald Test—--------------------------

To test if sunscald damage has occurred, make a small cut into the suspicious area with a sharp, clean knife. If the inner bark is moist and green, then it is still alive. If it is brittle and falls off, exposing the underlying wood, then damage has occurred.

—------------------------------------------------------------------------------------------------------------------

4. Frost Heave

 

Repeated freezing and thawing of soil can cause the water in the soil to expand and contract, swelling the ground and displacing plants. The soil action damages roots and may elevate crowns and roots where they are exposed to cold temperatures and drying winds. Frost heave is most common in silt or silty-clay soil. The small soil particle size promotes the movement of water by capillary action through its pores. Plants may be seriously damaged, stunted or killed.

If a cold-hardy perennial is damaged in cold weather, it is usually because of frost heave. Other victims are perennials with shallow root systems such as strawberries, heuchera, scabiosa, leucanthemum, galliardia, and bergenia, and those recently planted which haven’t had time to establish adequate root systems.

Desiccation

 

Besides injury from cold temperatures, desiccation (the drying out of plant tissue) can be a major problem in winter. Broadleaf and needled evergreens keep most of their leaves during winter and are still photosynthesizing, which requires water. Frozen ground that won’t release water, and drying winds that create water loss through transpiration can lead to water stress and desiccation when the amount of water lost exceeds the amount picked up by the roots. Other conditions can contribute to desiccation too. The hardening-off process removes water from plant tissue. Sunny conditions create more drying and increase photosynthesis. Snow cover helps keep the soil moist so if it is lacking then desiccation may be more of a problem. Deciduous plants can also experience desiccation in frozen or dry soils. 

 

Desiccation, because of its appearance, is also called leaf burn. It is probably the primary cause of winter damage to broadleaf evergreens. Damaged foliage appears reddish-brown, yellow, white or gray, often with browning along leaf margins, and the leaf may curl and eventually become dry and brittle. Symptoms may show early on or as distant as the following year. 

 

Needled evergreens are very susceptible to desiccation too. Often, only the tip of the needle will be discolored brown or yellow while the base of the needle remains green. The leaves may also show some chlorotic flecking, and as damage is increased, needle drop and tip or twig dieback may occur. Needles may also dry out and die but remain green until spring or an extended warm spell. Injury is usually found on the side of the plant exposed to drying winds, the sun or other sources of radiated heat. If severe enough, whole shrubs or trees may turn brown or off-colored.

 

Evergreens often affected by desiccation are spruce, boxwood, azalea, holly, cedar, cypress, juniper, rhododendron, arborvitae, yew, and magnolia.

 

In summary, winter cold damage can mimic many other plant problems. Recognizing winter damage is the first step to treating it and averting future occurrences. Please also see our blog “Treat Winter Cold Damage” and all five blogs in our winter series for a full understanding of how to deal with the cold.