   Cold Temperatures and Plant Growth - December 15, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County When temperatures drop in winter, plant growth and metabolism changes in many ways. Some changes are directly related to the increase in the viscosity of water as it approaches freezing. Other changes are highly complex, involve many processes, and are still poorly understood. In this column, we will explore some ways that plants respond to cold temperature and how these relate to horticultural practices. I do this at some risk of boring people to tears, but it is good basic information. Many annual and crop plants simply will not survive the cold temperatures (32 degrees F and below) we experience during winter. Many of us have planted tomatoes in April only to have a late freeze wipe them out. Seeds of many plants simply will not germinate until the environment reaches a certain minimum temperature. Other seeds need to be exposed to cold temperatures for a given time before they will germinate. These data are documented by experiments and recommendations should be provided when purchased seed is purchased from a reputable source. Cold hardiness is determined genetically within plants. Intolerance of cold temperatures is a major factor limiting native plant distributions. Poorly adapted plants do not develop cold hardiness and cannot acclimatize fast enough to survive early cold weather. Other poorly adapted plants come out of dormancy too early, begin growing and are killed by low temperatures. This is one of many reasons to landscape using native plants. Perennial plants have adaptations that allow them to survive cold temperatures. Woody plants undergo physical and physiological changes that prepare them for winter. This process is known acclimation. Shorter day lengths trigger this process in most deciduous plants. Deciduous plants form buds with many bud scales to protect the tender tissue below. At the cellular level, plant cell sap contains many dissolved compounds: sugars, salts, proteins, etc. This concentrated solution lowers the freezing point of the plant tissues. In other words, the more "stuff" dissolved in the cell sap, the harder it is to freeze it. When freezing temperatures come, properly acclimatized plants will form ice crystal in the intercellular spaces (between cells). As the temperature drops further, water moves out of the cell membrane to the intercellular spaces. Here, the cell sap has an even higher concentration of dissolved compounds. Plant tissue damage often occurs when the temperature drops so fast that the ice crystals form faster than the cell sap can migrate to the intercellular spaces. This is the type of injury that is often seen on the southwest side of fruit tree trunks. Cold injury to conifers (cone-bearing evergreens) is most often seen as shoot desiccation. Unlike deciduous trees, conifers continue to photosynthesize through the winter. Photosynthesis requires a steady supply of water. Water becomes more viscous when it is near freezing and therefore moves more slowly. Under these conditions, roots may not be able to absorb water fast enough to replace that lost to transpiration during cold, dry winter days. This is not likely to occur given the mild climate of the Verde Valley. However, this should remind you to continue watering your evergreen trees and shrubs during the winter months. To avoid cold injury, keep trees well cared for so they can acclimate properly. During winter, deep water all woody plants monthly (twice per month for evergreens). Valued specimens or fruit trees can be covered to conserve heat reradiated from the soil surface at night. Overhead sprinklers can also be used to prevent frost damage to buds of deciduous fruit trees during spring frosts. Keep on winter watering. The University of Arizona Cooperative Extension has publications and information on tree selection and planting. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |