Plants and Cold Temperatures
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Most of Eastern NC lies within USDA hardiness zone 8a, which has an expected average minimum temperatures of 10 to 15 degrees F. While it is rare, temperatures can fall below this expected minimum and cause damage to some landscape and garden plants. Records from the National Weather Service show only three accounts of record cold: March of 1998, March of 2004 and January of 1985.
Many factors impact whether or not a specific plant is damaged by cold temperatures, including snow cover. Extremely cold temperatures often follow a winter storm. If the storm left behind a blanket of snow, plants are less likely to be damaged, including low growing plants, bulbs, and dormant perennials covered by snow.
The time of year that the unusual temperatures occur also makes a difference. When extreme cold occurs in the early part of winter like January and February, most landscape trees and shrubs, fruit trees, and berry plants are fully dormant and less likely to be damaged. If cold temperatures occur later in the year (March or April) many plant parts are more susceptible to temperatures below freezing. These freezes are particularly damaging if they are preceded by a warm spell when flower buds are forming.
Trees and shrubs that are marginally hardy in our region are more at risk of cold injury, especially non-native varieties rated as hardy to zone 7b or 8. Gardenias and figs are plants frequently damaged by single digit temperatures in the landscape. Most are not completely killed by cold temperatures. Their stems may freeze causing all above ground parts to die back to the ground. Watch for new sprouts that often emerge from the base of the damaged plants in the spring. Flower and fruit production are likely to be delayed until plants recover.
Winter damage can occur on many plants. A rapid temperature drop following a mid-winter thaw can cause bark splitting. Dead twigs and branches in the spring may be the result of ice and snow damage from the winter. Injury during the winter or early spring season can be from frost or freeze injury. Some evergreens exhibit yellowing or bronzing of the needles when exposed to winter sun and wind, but return to normal color when growth resumes in the spring. Winter injury may be confused with early stages of some fungal diseases. Needles turn from bronze to reddish brown or brown, as a result of exposure to cold, dry winter wind.
Permanent damage occurs when conditions are severe, prolonged, or when temperatures change suddenly. Tissue death is caused by the removal of water in the needles faster than the plant can replace it through root uptake from frozen water in the soil. Winter scorched needles eventually drop off.
De-icing salt can accumulate on leaves, causing leaf scorch similar to winter damage and may kill buds and branch terminals. Leaf-scorch from de-icing salt is prevalent in the winter by simply being cautious of the amount and location of products used.
Cultural practices that conserve soil moisture, prevent root damage and promote “hardening off” prior to winter will reduce winter damage. Avoid fertilization or pruning in late summer, which stimulates late season growth that does not have time to “harden off” properly and is much more susceptible to winter injury. When watering, soak the soil several inches deep, and then allow to dry between waterings. This encourages deeper rooting. Avoid frequent shallow sprinklings, which encourage surface roots that are easily injured by drought and cold. The use of mulches conserves soil moisture and prevents temperature fluctuations. Mulches also keep the soil cold in early spring, which helps to reduce premature bud break.
Watch for part two of Plant Cold Injury.
Part 2 of Plant Cold Injury
Even hardy trees may develop sunscald or frost cracks. Tree bark warmed by the sun in winter, can reach a temperature as much as 18 degrees warmer than the air temperature. The cambium layer beneath is damaged. This type of freeze damage is called sunscald.
Frost cracks occur when temperature fluctuations are extreme. Water in the cells of the tree trunk freezes and moves out of the cells, causing the wood to shrink. Tension between the frozen and unfrozen layers of wood is so great that the wood separates, causing a crack. The crack can form suddenly, and is often combined with a loud cracking sound. When temperatures warm, the wood absorbs moisture and the crack closes. Frost cracks can reopen and enlarge in subsequent winters and may extend to the center of the tree. Damage to tree trunks is most likely on the south and west sides of the tree where the sun is strongest.
Frost cracks may begin in previously wounded or pruned areas. Proper pruning and avoidance of injury may help to prevent some frost cracks. Tree species prone to frost cracking (those with thin or smooth bark) may benefit from applying white latex paint to the tree trunk. The light color reflects light and helps to reduce temperature fluctuations. The following species are more likely to develop frost cracks: apple, beech, crabapple, elm, goldenrain tree, horse chestnut, linden, London plane, maple, oak, walnut, and willow.
The best prevention of winter injury is to select plants that are hardy in our area. Winter damage can be reduced by locating plants in partially shaded areas protected from winter winds. Shade in the early morning sun is especially important. Plants that freeze slowly and thaw slowly will be damaged the least. Maintain adequate soil moisture in the fall to prevent winter desiccation. Inspect plants for winter damage in the spring and prune out affected areas.
Knowing how and when to offer first aid to an ice, snow, or wind damaged plant will often save the plant from future decay and possible loss. Do not be in a hurry to start pruning a branch which is bent out of shape. Often in a few days following the damage the plant will straighten up on its own. Broken limbs can be pruned immediately using a clean cut with sharp pruners. If the plant is completely misshapen after the corrective pruning – consider pruning the entire plant where the subsequent growth will be in balance.
The signs of cold damage vary, since some damage may not be evident until months later. Leaves and tender shoots subjected to freezing temperatures or chilling damage appear water-soaked and wilted. These tissues will usually turn black within a few hours or days. The tips of narrow leaved evergreens, such as junipers, may turn uniformly brown. Broad-leaved evergreens, such as hollies, often have marginal leaf burn. Reduced flowering is common during the following season.
The extent of winter damage can best be determined after new growth starts when all danger of frost is past. For most shrubs, prune the tips of plants back to a healthy outward growing bud. Scratching the surface of a live twig or young branch should reveal green below the bark. At this point you can cut damaged wood back to the first healthy bud. A general rule is not to prune a plant by more than a third in any given year.
Pay extra attention to plants that have had winter injury by providing adequate water and ideal growing conditions in the stressful summer months.