Freezing and Frost
Cold temperature can kill bark, buds, flowers, and shoots. Cold injury is most likely to occur during autumn and spring, the coldest times during winter, and when temperatures decline rapidly after warm weather.
Often injury is not apparent until days after a freeze and when temperatures rise. However, by the summer, new growth often replaces tissue damaged by cold during spring. Plants adapted to the local environment usually are not permanently harmed.
Symptoms resembling freezing and frost injury are also caused by anthracnose and other leaf and shoot diseases, gas or mechanical injury to roots, phytotoxicity, and water deficit.
Cold temperature damage causes buds, flowers, and shoots to curl, turn brown or black, and die. Foliage appears scorched because low temperatures severely dehydrate plant tissue. Bark and wood can crack or split, and whole branches or entire plants may be killed if temperatures are below those tolerated by the plant.
Frost and freezing produce the same damage but occur under different conditions, and some of their management strategies differ. Freezing occurs when air temperatures are 32°F or colder. Frost occurs when air is warmer than 32°F but plant tissues drop to 32°F or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.
The time of year, minimum temperature, duration of cold, the rate at which temperatures drop, and plant characteristics (e.g., age, hydration, part affected, whether it has acclimated) influence the severity of damage. Plants that are gradually exposed to increasingly cool weather during the fall become acclimated (hardened) and tolerate more cold than during spring and summer.
Broadleaf evergreens and plants not in dormancy generally are the most sensitive to cold. Buds, flowers, younger leaves and shoots, and especially succulent tissues in the spring are the parts most susceptible to freezing and frost injury.
Choose species well adapted to the climate and seasonal temperatures for the location. To increase a soil's ability to absorb heat and warm plants, control weeds and during winter rake away mulch and keep soil bare. When frost or freezing are expected, irrigate dry topsoil at least 3 days before the cold weather to increase the soil's ability to retain heat.
When frost is expected, cover sensitive plants overnight with cloth or similar material other than plastic to reduce heat loss to the atmosphere, but leave covers open at their bottom so heat from soil can help warm plants. Remove covers during the day.
During freezing, covering plants is of little help unless a heat source is provided. Placing incandescent lights designed for outdoor use in the canopy may generate enough heat to prevent plants from freezing if plants are also covered. Be sure not to create electrical shock or fire hazards.
Operating misters or sprinklers to wet foliage continuously and flooding beneath plants during freezing are used in some commercial situations, but generally not in landscapes. With sprinklers, the irrigation system and plants must be able to tolerate being covered with ice, and flooding soil can damage plants. Do not combine the use of outdoor lights with flooding or sprinkling.
Do not prune freeze-damaged plants until after you are certain what tissues are dead, preferably by waiting until spring or summer after new growth begins. An exception is limbs or trunks that are hazards and may fall. Replant with more cold-tolerant species.