Late fall arrived with snow and lower than normal temperatures. This was followed by warmer than average temperatures in early winter. Late winter brought the coldest temperatures in recent years. These fluctuating temperatures made it difficult for plants to acclimate.
Many broadleaf evergreens may look green and healthy as the coldest of winter temperatures transition into the warmth of spring. Unseen is significant damage to the xylem cells (long tubes that conduct water upward). Up to this point the individual plant has been able to supply its foliage with sufficient water. But, the limited amount of healthy conductive tissue has been working at maximum efficiency to supply the plant’s water needs. After a few days of 80°F in early spring, the plant is unable to absorb and translocate water as rapidly as it is lost. The result is leaf and stem death as if it were much hotter and drier.
Snow and ice are a common form of winter precipitation. Many gardeners are afraid that the slight bending will result in stem breakage. If the xylem in a branch freezes and then bends downward as a result of the ice or snow load, the ice crystals can result in the rupturing of the xylem cells. This type of damage is made worse by strong wind or when the owner of the plant shakes the snow or ice off of the plant thinking that they are helping the plant.
The least hardy part of any plant is its root system. Roots grow in the ground where the insulating effect of soil buffers the roots against extremes of heat and cold. Plants growing in above ground containers and plants being transplanted (balled and burlapped, bare root or container grown plants) lack the temperature moderating protection of surrounding soil. When roots are subjected to low temperatures, they can be killed even though the above ground portions of the plant are hardy and remain alive. As a rule of thumb, roots are two USDA Plant Hardiness Zones less hardy than the rating assigned to the above ground portions. As spring growth begins the buds begin to pop open but fail to put out new foliage. The green stems quickly turn brown, and die. This occurs because the roots were killed by cold and were unable to absorb water essential for growth.
Winter injury to landscape plants appears with multiple visual symptoms. While these injuries are associated with low temperatures, injury is usually the result of a combination of different environmental and cultural conditions (low temperatures, duration of cold, lack of soil moisture, low humidity, wind and sun).
Rule number one in diagnosing winter injury and making recommendations is don’t be impatient. If the foliage or the tips have been damaged but the stems and buds are still green, give the plant the opportunity to put out new growth. Sheering dead foliage will immediately improve the appearance of the plant but can change the habit. Pruning should not be done until after the chance of the last frost has passed.
Spring fertilization is not recommended, especially for plants suffering winter injury. The addition of nitrogen can encourage more spring growth than the damaged stems can supply with water when the hot, dry summer months arrive. The addition of water during dry periods is more beneficial than fertilizer. When necessary, fertilization of woody landscape plants should occur in late autumn after leafdrop (e.g. Thanksgiving to Christmas).
Broadleaf evergreens that are established and exposed to winter sun can be protected from the intensity of winter sun and wind. Cover these plants with light-colored cloth or burlap prior to the onset of winter. Spray moisture on the cloth prior to the onset of extremely windy sub-freezing temperatures. Water frozen on the cloth will further reduce the effect of the wind. The best long-term approach is to match the plant to the site. This can involve using hardy needled evergreens where evergreens are desired and deciduous species that originated in our climatic zone.
Healthy landscapes are not an accident. It is important to always match the plant to the site conditions. This helps ensure that your investment will have every opportunity to thrive and return aesthetic dividends for years.
Submitted by William M. Fountain, PhD, Extension Professor in Arboriculture, University of Kentucky