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 inherent 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 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 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 plant tissues as they form and grow.
September and October are the best months to begin winterizing landscape trees for the approaching colder temperatures. The simplest and most effective method is to slow growth 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. In these situations it's best to plant trees that can tolerate temperatures of 24-25 without significant damage. Such trees would include: A. berlandieri, A. smallii, A. stenophylla, A. schaffneri, Cercidium Hybrid 'AZT', Eysenhardtia orthocarpa, Prosopis chilensis 'AZT', P. mexicanum and Sophora secundiflora.
What to do with damaged Trees? Trees that are freeze damaged should not be pruned until new growth has occurred, usually late spring or early summer of the year following the injury, when. In spring you can more accurately detect the extent of damage and better limit pruning to damaged branches only. Good pruning techniques should be used to prevent stimulating excessive or unwanted new shoot growth that may lead to additional frost damage the following winter.
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