With the arrival of spring’s warmer weather, many landscape plants in Kentucky are showing the effects of a winter that broke records. The winter of 2014-15 approached all-time historical lows in many parts of the Commonwealth. Now landscapers and homeowners are wondering which plants might recover and which must be removed. Particularly hard-hit were evergreens and marginally hardy plant species.
“Evergreens” Not So Green This Spring
Broadleaf and needled evergreen plants retain foliage during winter, thus they continue to lose water through their leaves—although at a reduced rate—throughout the winter months. During the growing season, plant roots take up water from the soil, where it is pulled upward through the xylem (water conducting tubes inside stems). Some of this water is used by the plant, but much of it exits the plant as water vapor through pores in the leaves. Deciduous plants drop foliage in the fall to reduce water loss during the winter when soils are frozen. During sunny winter days with wind and low humidity—which were prevalent during the past two winters—the rate of water loss from evergreen foliage increases, leading to brown, scorched leaves. The pull of transpiration may even result in air pockets developing in the xylem, similar to air pockets in a siphon. Plants are unable to move water through these xylem tubes. Further injury may occur when frozen stems are bent by snow or physically shaken to remove ice and snow, and xylem cell walls are fractured.
Certain landscape plants, both deciduous and evergreen, are considered to be marginally hardy in Kentucky. Plant hardiness is based on the lowest temperature that the plant can tolerate under optimal growing conditions. The USDA Plant Hardiness Zone Map places most of Kentucky in Zone 6 with the far western counties in Zone 7a. Temperatures associated with the hardiness map are based on the 30-year average of the single lowest winter temperatures recorded each year. This is not the absolute lowest temperature experienced over a 30-year period, just the average. The map also does not take into account the duration of cold, soil moisture, humidity, solar radiation, topography, or wind. While there are shortcomings to this map, it is still a valuable aid in deciding what to plant and where it should be located.
Problems arise for marginally hardy plants when winters are colder than average. Examples include boxwood, cherrylaurel, southern magnolia, Leyland cypress, crapemyrtle, and nandina. Some of these plants had survived previous milder winters but now show symptoms ranging from moderate leaf burn or twig dieback to death of the entire plant (Figures 1 & 2).
Managing Winter Injury on Landscape Plants
Rule number one in addressing winter injury is to be patient. If the foliage or the tips have been damaged but the stems and buds are still green, wait until the plant puts out new growth before deciding if the plant should be pruned or removed. Sheering dead foliage will immediately improve the appearance of the plant, but 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 growth than the damaged stems can supply with water during hot, dry summer months ahead. The addition of water during dry periods is more beneficial than the addition of fertilizer. When necessary, fertilization of woody landscape plants should occur in late fall.
Established broadleaf evergreens in exposed locations 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, however, 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.
By William M. Fountain, Extension Horticulturist and Julie Beale, Plant Disease Diagnostician