Tender Plants and Cold Temperatures - October 29, 2014
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Fall is here and overnight temperatures are beginning to drop. Once freezing temperatures occur, they 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. Lantana, Gazanias, etc.). Others have frost tender plants such as lemon trees and tropical cactus in pots and need to move them into protected locations when winter sets in. Various other frost protection strategies can be employed, such as mulching, floating row covers, and hot caps.

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 gradual changes in metabolic processes. This gradual acclimation to cold is sometimes called “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. 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 can 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 dry winter periods. Last winter, many landscape pine trees were not irrigated adequately and the effects were not evident until they declined or died in June and July.

Large, evergreen, trees provide a little bit of frost protection, as do eaves, porch roofs and a location next to the house. Frost tender potted plants should be placed inside a garage, shed or house if a hard freeze is expected, since they are much more susceptible to freeze damage. People with frost sensitive cacti (Saguaro and others) planted in the native soil often put a Styrofoam cup on each growing tip to protect these most susceptible areas. Remove plants from covered areas (if light will be excluded) the next morning once the sun is up and the temperature is above freezing. Never water succulents directly in advance of a freeze. Most hardy succulents survive freezing temperatures best if the soil around them is dry.

Lastly, regular readers know that I do not recommend nitrogen fertilization of woody ornamentals at any time unless they are showing signs of nutrient deficiency. Plants should enter the autumn season as healthy as possible, but not rapidly growing, or acclimation may be affected.

Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at verdevalleymg@gmail.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

Additional Resources

Frost Protection and Extending the Growing Season
Colorado State University Extension


Frost Protection
University of Arizona Cooperative Extension


Prevention and Care of Freeze Damage
Desert Botanical Garden, Phoenix, AZ

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: October 23, 2014
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