   Cold Temperatures Can Harm Plants - December 6, 2006 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Low temperatures can harm plants in several ways. Freezing temperatures will kill all warm season annuals and many poorly adapted perennials and woody plants that are left outdoors unprotected. Many gardeners treat these perennials and woody plants as annuals and replant them each spring (i.e. some Lantanas, Gazanias, etc.). Others have frost tender plants such as lemon trees and tropical cactus in pots and move them to protected locations when winter sets in. Various other strategies can be employed, such as mulching, using row covers, and hot caps can increase chances of survival for these cold sensitive plants. Other plants are said to be “cold hardy” and have various adaptations that allow them to survive low temperatures. This is often a period of dormancy when physiological processes are modified and metabolic activity is drastically reduced. Native cold hardy plants have been genetically selected over millennia to survive the climate in which they have evolved. Landscape plants are often non-native and gardeners like to experiment with species they had enjoyed in other gardening climates. However, each plant species has a cold tolerance threshold (exposure time and temperature) below which permanent damage and/or death will occur. The cold hardiness of a species is usually considered to be the lowest midwinter temperature plant tissues can endure. This is not the only consideration. Plant injury frequently occurs during autumn or spring when the plant is not at its maximum hardiness. Minimum temperatures can only be tolerated if the plant has had time to acclimate through processes compatible with its genetic make up. This is often a gradual period of acclimation that many gardeners call “hardening off”. Most landscape plants acclimate or develop hardiness to freezing temperature in response to changes in day length and temperature. Acclimation is a two-stage process. The first stage is initiated by shorter days and results in partial hardiness. 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 second stage is initiated by cold temperatures and results in full hardiness and acclimation. Cool temperatures initiate 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. Cold damage can be caused by a variety of processes. Low temperatures can cause 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. Desiccation injury occurs when water is lost from plants faster than it can be replaced through absorption of water by roots. Desiccation often occurs when leaf and air temperatures are high and relative humidity is low. Wind movement across plants often increases the rate of moisture loss. These conditions regularly occur during winter in northern Arizona. This is why it is critical to for us to provide some irrigation to landscape evergreens during extended winter dry periods. Gardeners can assist, to some degree, in plant acclimation to winter conditions. The amount of nitrogen fertilizer applied to turf and fruit trees should be reduced after mid-July and stopped by late summer. Regular readers know that I do not recommend nitrogen fertilization of woody ornamentals at any time unless they are showing signs of deficiency. Plants should enter the autumn season as healthy as possible, but not rapidly growing, or acclimation may be affected. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |