Cold hardiness is determined by the genetic capacity of a plant to acclimate (transformation from a non-hardy to hardy condition) to freezing temperatures. This capacity can be influenced by plant care practices. While the cold hardiness of a species is usually considered to be the lowest midwinter temperature plant tissues can endure, injury frequently occurs during autumn or spring when the plant is not at its maximum hardiness. Thus, injury can occur during the autumn, winter, or spring seasons depending on the extent of acclimation or deacclimation (process of transforming from hardy to non-hardy condition). Winter injury may be manifested as excessive browning of evergreen foliage, injury or death of flower buds, splitting of bark, or death of roots. The extent of injury is often difficult to determine, and may only be exhibited as delayed bud development or slightly reduced growth.
Low Temperature Injury
Low temperature injury, often called freeze damage, can be caused by intra- or extra-cellular ice formations within the plant. When intra-cellular ice is formed, crystals originate within the protoplasm of plant cells. This type of ice formation occurs infrequently and only when the temperature decreases very rapidly. If the ice formation is extensive or ice remains for a long period of time, cells rupture and die.
The second type of freeze damage occurs when extra-cellular ice forms during normal winter conditions. When freezing conditions exist, water moves out of plant cells in response to the low temperature and back into cells when the temperature rises above freezing. This type of freeze damage is not lethal to most woody plant species that have been properly acclimated. Injury can occur; however, if the cells are dehydrated for relatively long periods of time, or subjected to very low temperatures.
Desiccation injury occurs when water is lost from evergreen plants to the atmosphere faster than it can be replaced through absorption of water by roots.
Injury is a function of the degree and length of time that stress is imposed. When leaf and air temperatures are low and the relative humidity high, little moisture loss occurs. However, when leaf and air temperatures are high and relative humidity is low (as often occurs in winter), moisture loss can be excessive and injury extensive. Further injury can occur if water cannot move within the plant to replenish desiccated leaf and stem tissues or when insufficient water is absorbed by roots from cold or frozen soil. Wind movement across plants may increase the rate of moisture loss.
Most landscape plants acclimate or develop hardiness to freezing temperature in response to changes in light duration and temperature. Acclimation is a two-stage process. The first stage is initiated by decreasing day length and results in partial hardiness. The second stage is initiated by subfreezing temperatures and results in full hardiness and acclimation.
For many species, the shortened photoperiod (hours of daylight) of late summer initiates the hardening process by slowing vegetative growth. The time it takes for plant growth to stop differs widely. Some plants stop growing in July or August and others continue to grow into autumn. These differences are due to hormonal balances in the plants controlled by day length and modified by temperature.
Leaves are the receptors of the short-day signal. After growth stops, the short-day photoperiod triggers a hardening signal that is transferred from the leaves to the stems and branches. The short-day signal results in partial cold hardiness. The timing and rate of hardening can be altered by temperature, while day length is predictable by calendar dates. The hardening response in a single plant may vary from year to year because of temperature differences.
Cool temperature initiates the accumulation of sugars, modification of proteins and changes in cell membrane permeability that are associated with an increase in cold hardiness. While most plants require short photoperiods and lower temperatures to develop full hardiness, some harden only in response to low temperature regardless of photoperiod.
Freezing alone contributes to hardiness. Once leaves and stems of evergreens harden enough to withstand freezing, being frozen makes them hardier. The freezing response is strictly localized and is not translocated. In other words, if lower leaves are acclimated to freezing, that does not necessarily mean the upper leaves are also hardened.
Although autumn temperature above 60 degrees F reduces root hardiness, it appears that cool temperature contributes to slowing or stopping root growth. Roots cannot acclimate to the same extent as shoots, so it is fortunate they are protected by a large volume of soil which serves as insulation.
The water content of woody tissues decreases as acclimation to winter conditions proceeds. Most research, however, supports the practice of irrigation late in the growing season to assure the normal rate of cold acclimation. This practice is especially beneficial for plants, such as rhododendron, which continue growth late into the season and are susceptible to early freeze damage. Since woody plants appear to have a built-in mechanism to reduce water levels when they acclimate, reducing soil water may not benefit the development of maximum midwinter cold hardiness.
Acclimating Plants for Overwintering
Gardeners can assist, to some degree, in plant acclimation to winter conditions. The amount of nitrogen fertilizer applied should be reduced after mid-July and stopped by late summer. Commercial growers decrease the rate of nitrogen fertilizer by approximately one-half and double the rate of potassium application in late summer. Plants should enter the autumn season as healthy as possible, but not rapidly growing, or acclimation may be affected.
Tissue desiccation during the winter, especially with evergreens, is one of the most common forms of winter injury. The soil in which evergreens are being grown should be well-irrigated in mid-to-late autumn, before the soil freezes. If the landscape where evergreens are located is in a dry site, sandy soil, or under the overhang of a roof, the soil should be irrigated in midwinter when the temperature is above freezing.
Protecting Plants in the Landscape:
Apply a layer of mulch, 2 to 2.5 inches deep, after the soil freezes to keep the soil cold rather than protect the soil from becoming cold. This practice will reduce injury from plant roots heaving (coming out of the soil) because of alternate freezing and thawing. Plants that benefit from this practice include perennials, alpine, rock garden plants, strawberries and other shallow-rooted species. A mulch maintains a more even soil temperature and retains soil moisture.
Apply bark products, composts, peat moss, pine needles, straw, hay, or any one of a number of readily available materials from the local garden center. Pine boughs or remains from Christmas trees can be propped against and over evergreens to help protect against damage by wind and sun.
Multiple leader (branched) plants such as arborvitae, juniper and yew may be damaged by the weight of snow or ice. Prevent plant breakage by fastening heavy twine at the base of the plant and winding it spirally around and upward to the top and back down in a reverse spiral. This technique is needed more as plants become larger and begin to open at the top.
Narrow and broadleaf evergreens lose moisture through leaves in winter. Since the soil moisture may be frozen, plant roots cannot absorb what is lost and the foliage desiccates, turns brown, and may drop. This can be serious with evergreen azalea, holly, boxwood and rhododendron.
Applying an anti-transpirant, also called anti-desiccant, reduces transpiration, and hence, damage to the foliage. At least two applications per season, one in December and another in February, are usually necessary to provide protection all winter. A number of products are available in most garden centers for this use.
A wrap of burlap or canvas can offer protection to plants against desiccation from sun and wind and drift from de-icing salts applied to drives and streets. Wrap the “body” of the evergreens, but do not cover the top of the plant as some light is necessary during the winter.
Some landscape plants, especially during a time when there is an extended period of snow cover, become a food source for rabbits, mice, or moles. When their normal food supply is covered with ice or snow, rodents turn to the bark and young stems of apple, flowering crabapple, mountain ash, hawthorn, euonymus and viburnum, among others. Complete girdling of stems by rodents will kill the plants and partial girdling creates wounds for borers and disease organisms to enter, as well as weakening the plant itself.
Protect stems and trunks of these plants in late autumn with plastic collars cut in a spiral fashion so they can be slipped around tree trunks. Hardware cloth can also be used as a stem wrap along with aluminum foil.
Trunks, stems and lower limbs can be sprayed or painted with rodent repellents. A number of these materials are available in most garden centers. Repeat the application at least once during a warm period in midwinter. Mixing the repellents with an anti-transpirant often results in extended effectiveness of these products.