Cold Injury in Desert-Adapted Landscape Trees
Unlike other regions of the country where fall is marked by gradual but definite change in the weather and day length, toward cooler shorter days and even cooler nights, the desert southwest can remain warm and sunny right up to the first killing frost (if there even is one that year). Most plants respond to this seasonal change by slowing their growth in response to shorter days and begin hardening off their trunks and branches in anticipation of cold winter weather. The desert’s “shoulder” seasons, spring transitioning to early summer and early fall till the first frost, are periods of very active and rapid growth among desert-adapted trees species. In both native and landscape settings, these species will exploit these seasons and continue to grow so long as water, nutrients and warm (non-freezing temperatures) persist. The typical care and maintenance in most landscape settings ensures that moisture and nutrients are available year-round. This unique growth habit can leave desert-adapted trees particularly vulnerable to severe damage from sudden hard frost if they are not properly conditioned
Plants are damaged by low temperatures because the water inside the plant tissues freezes. The ambient air temperature that leads to freeze damage varies from one tree species to the next. The full extent of the damage is generally driven by the duration (length of time) of the freezing temperature. As liquid water is transformed to ice it forms crystals within and between the cells and tissues in the plant. Ice crystals expand as they grow taking up more space than did the liquid water. This expanding ice crushes, pierces and irreparably damages vulnerable plant tissues.
The inherent ability of a plant to tolerate freezing temperatures is called cold hardiness. Cold hardiness is most often reported in terms of a specific ambient air temperature or over a of range 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. Several factors influence cold hardiness: 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 tree. 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 a sudden drop in temperatures.
What to do with damaged Trees? Trees that are freeze damaged should not be pruned until new growth has appeared, usually late spring or early summer of the year following the injury. In spring, with the appearance of new, healthy growth, you can more accurately detect the extent of damage and better limit pruning to damaged areas only. Good pruning techniques should always be used to prevent stimulating excessive or unwanted new shoot growth.
September and October are the best months to begin winterizing desert-adapted landscape trees for the approaching colder temperatures. The simplest and most effective method to slow growth is by gradually reducing irrigation and halting fertilizer application by September 1. This will serve to reduce the amount of new, terminal (tip) growth that is the most susceptible to cold injury. Growth management of this sort can be complicated in landscapes where under-story plantings or winter and fall color plants are added at the end of the summer. Trees and shrubs planted in lawns that are over-seeded with winter grasses pose special challenges. Over-seeding requires that large amounts of water and fertilizer be applied during a season when trees should receive little of either.
A survey conducted by William Kinnison in 1978 (published in "Desert Plants") at Central Arizona College after a hard freeze (24-25 F) showed that the following desert-adapted trees were hardy: Acacia aneura, A. berlandieri, A. craspedocarpa, A. stenophylla, Prosopis chilensis, Pithecellobium flexicaule, P. mexicana. Warren Jones writing about the effects of the same freeze in northern Sonora Mexico (also published in Desert Plants"), reported that Lysiloma thornberi was damaged by temperatures below 25F and Olneya tesota were damaged at 20F. Prevention remains the most effective method of reducing cold injury. Judicious landscape tree selection for your region and proper horticultural practices keeps the landscape vigorous while minimizing injury from cold temperatures.
If you have questions, please contact Kevin Salamandra or Ed Mulrean at info@aridzonetrees.com.