Fall can be a very subtle season in the desert southwest. Unlike other regions where there is a gradual but definite change in the weather toward cool day and even cooler nights, the desert can remain warm and sunny right up to the first killing frost. While many plants begin slowing growth in response to shorter days, as opportunist, desert adapted tree species will grow as long as water, nutrients and warm temperatures persist. With this extended period of nearly ideal growing conditions desert adapted trees can be severely damaged when freezing nights do arrive if trees are not properly conditioned.
The ability of a plant to tolerate freezing temperatures is called cold hardiness. Cold hardiness is most often reported in terms of a specific temperature or over a range of temperatures (e.g. hardy to 25 F or 23 to 28). These numbers represent temperatures at which, historically, little if any cold damage has been observed but they are not a guarantee. Factors influencing cold hardiness include: maturity of the plant, the duration and intensity of freezing temperatures, rain fall, humidity, cloud cover vs. clear night, protection provided by other plants and structures, whether the plant is actively growing or dormant and hardened off and the genetic characteristics of the plant. Many popular desert landscape trees, like hybrid mesquites, will continue to grow so long as temperatures and cultural practices encourage growth. If not hardened off, succulent new wood, the result of late summer and early fall growth, is especially prone to frost injury from sudden cold fronts or rapid drops in temperatures.
Plants are damaged by low temperature because the water inside the plant freezes. As water freezes, ice crystals form within and between the tissues in the plant. Water expands as it freezes so ice crystals take up more space than did the liquid water. Ice crystals crush, pierce and irreparably damage a variety of succulent plant tissues as they form and grow.
December seems a perfect time to revisit a couple of previously published Arid Zone Times articles that deal with cold injury, how best to prevent it and how to handle the aftermath of freeze damage on desert trees. Also below we have included a link to the 2012 revision of the USDA Plant Hardiness Zone Map. You can input your zip code in the box at the top of the page or click on your individual state for detailed information.