by D.A. Salazar, MG Class of 2009This winter we’ve seen some pretty weird weather combinations around here: dry air with freezing temperatures, bursts of sleet or hail falling from the clear blue sky, bright sunny days suddenly overtaken by hammering cold rains. The promise of a warm, crystalline Thursday is usurped by a weekend of steadily dribbling snow.
By this time, your plants are probably as confused as you are. Such schizophrenic winter temperatures can test the fortitude of even the healthiest plants – especially with the sudden freezing and thawing that our soils have been experiencing. Winter injury, frost injury, and desiccation from cold and wind can result in a smorgasbord of damage to plant life – including stem and bud death, frozen roots, sun scald and wind scald, bark splitting, leaf droop and roll, leaf burn, and limb breakage. And sometimes the damage isn’t evident until spring.
Well, at least it keeps things interesting.
Causes of Winter Injury
On sunny winter days, south- and southwest-facing plants are warmed by winter sun, causing cambium and phloem cells to deacclimate (i.e., lose cold hardiness). The warmed-up tissues become susceptible to cold injury when the sun goes down or when a sudden drop in temperature occurs. Leaf and stem tissues can be damaged when warm weather in early winter suddenly turns very cold. Likewise, extended mild winter temperatures followed by an extreme cold spell can also predispose normally hardy plants to cold injury.
- Sun scald occurs during periods of severe or extended cold weather, combined with bright sunshine. Scalded leaves turn brown, starting with brown edges or needle tips and progressing between the veins or down the needles.
- Bud and stem death are caused when the tissue is not able to withstand cold temperatures. A partial kill of buds or tissues also is not uncommon.
- Drooping and rolling of leaves are natural protective reactions to cold. They reduce the amount of leaf surface exposed to cold or drying winds. Leaves that survive the cold usually return to normal as temperatures warm.
Our neighbors in the Southwest might think they’ve got the lock on killing plants through lack of water — but drought is not the only thing that causes plants to dry up.
In winter, plants become desiccated (dehydrated) when they are unable to take up sufficient moisture from frozen soil to replace water lost through the leaves and stems. This can happen on sunny, cold winter days when the ground is frozen, as well as on days of below freezing temperatures and heavy winds, as both direct sun and wind cause plants to lose moisture.
- As with sun scald, wind scald occurs on sunny winter days when the bark of trees and shrubs has been warmed by the sun, warming the cambial tissues. When a sudden, cold wind blows through, causing an abrupt drop in temperature, the bark can crack open or separate from the tree.
- Frozen roots are not uncommon in container gardens, but can also happen to in-ground plants, as well – and you won’t know it until spring. The plant may leaf out normally only to wither and die. Take a minute to examine the roots: Dead roots are usually brown to black, and either mushy or completely dry. Live roots should be firm but pliable and may have white growing tips.
- Leaf burn is generally seen in evergreens. Injury is found on the outer portion of the branches and is often most severe on the side of the tree facing the wind or a source of radiated heat, such as a wall or street. The top is most likely to dry out first as it is the farthest away from the roots and therefore receives water last.
Frost can affect new growth and flowers in the spring. Its severity and distribution depends on the stage of the plant’s development and where the plant is growing; low-lying areas can act as frost pockets. Symptoms may develop some time after exposure and typically include wilting and dieback of affected tissues. Damage is often seen to buds, flowers, and at stem tips, where tender young tissues are more susceptible to injury.
- Most common (but not limited) to thin-barked trees, bark splitting can be the result of extreme cold temperatures at the soil surface followed by a rapid thaw. The bark will split at or near the point where the stem meets the roots (the a.k.a. “the crown”). This can lead to the eventual death of the roots and the entire plant; a plant this deeply affected may leaf out in spring, but it won’t be long for this world.
- Sometimes, instead of splitting, the damaged bark adheres to the wood and dries up, forming a sunken area known as a “frost canker.”
- Many days of heavy frost or snow can cause limb breakage on woody plants. Water in plant cells freeze, forming ice crystals that expand as they freeze and rupture cell walls, collapsing the cells and weakening the plant.
Man, oh, man — look at that mess! Is there anything I can do?
While it’s true that some of your plants may be goners, there are a number of tried-and-trues that you can put into play to mitigate the damage.
From now until the end of winter (or until the temperatures moderate):
- Apply loose organic mulch over root zones to maintain soil moisture and give protection from cold temperatures.
- Resist temptation: Do not prune yet. Pruning winter-damaged plants might cause further stress if the temperatures stay low. And as painful as it may look now, the damage may not be as bad as you think. Better to wait for the new growth in spring, so you can identify the dead wood and prune it out.
- Periodic slow, deep watering, at times when the ground is not frozen, can be very beneficial. When dry soil freezes, it has less insulative ability than moist soil, causing freezing damage to smaller roots. Stress from lack of water can limit the plant’s biochemical and physiological changes needed for maximum cold hardiness.
- Provide shading or a windbreak for your evergreens during the remaining cold months.
- Cover susceptible plants to protect against frost. Light-weight floating row cover fabrics, blankets, newspaper, or other materials may be used. (Do remove frost covers when warmer temperatures arrive, as frost covers left on plants long-term can create moist conditions that invite disease.)
In the spring:
- On damaged fruit-bearing trees and shrubs, remove as much developing fruit as possible. This allows the plant to put its energies toward recuperation.
- Prune out only dead and severely damaged wood on your trees and woody shrubs. Prune back to live, green, healthy wood; prune to a bud, stem, or trunk.
- On split bark, don’t cover the wound with paint or tar, as this will interfere with the tree’s natural ability to form a callus. Most trees that were healthy to begin with will survive a split by forming that callus; you can (carefully) help the process along with a procedure called bark tracing.
- Look for green shoots at the base of perennials, and cut away any dead foliage at the soil line. Dig up and discard any plants that do not produce new growth.
- Water properly throughout the growing season. Try this little test to measure your soil’s drainage. Don’t forget to check on your evergreens and established plants!
- Remember to select plants hardy for the local climate and soil conditions, especially native plants.
For more info on winter plant damage and treatment, visit our sources:WSU Extension-Skagit County; WSU Extension’s Garden Tips; Landscape Trades; HortSense; and WSU Extension-Puyallup’s Horticultural Myths.
Photo credits (top to bottom)
D.A. Salazar; U. of Arkansas Division of Agriculture; Cornell University Department of Horticulture; Viette Gardening; WSU Extension; KLRU.org gardening blog; Purdue University Cooperative Extension; Montana State University Extension; University Of Missouri Extension; Bugwood; WSU Extension-Skagit County; Missouri Botanical Garden; WSU Extension.