Young Citrus Trees Need The Most Protection
By Larry Jackson and Dr. Larry Parsons
University of Florida's Institute of Food and Agricultural Sciences
Protecting young trees from cold damage is a difficult task that has been complicated by several factors in the last decade. These complications include a significant increase in the number of young trees planted over the past several years, increases in cost of fuel, equipment, and labor, and an increase in the number and severity of freezes. Many young citrus trees are placed in a situation where active cold-protection measures are difficult, if not impossible, and growers have to rely upon passive means of cold protection.
Comparison Of Methods
Because of the cost of fuel, microsprinkler irrigation is rapidly becoming the preferred method for providing cold protection. This type of irrigation works particularly well for resets and the lower trunk and branches of young trees. Banking very effectively provides cold protection to the trunks of young trees. However, banks are time consuming to create and can produce some pest and cultural problems. The grower can avoid some of these problems by using tree wraps, which can be left on for an extended period of time once installed.
Soil banking consists of placing a mound of soil around the tree’s trunk to protect the bud union and trunk from cold. Since the soil stores heat from the sun during the day and releases it at night, heat deep in the soil moves up to the surface by conduction and is lost to the air by radiation. By mounding soil around the trunk of a tree (banking), heat is conducted through the soil and into the protected area of the young tree. Thus, banking protects by both conduction and insulation.
This method would be most efficient if trees were banked the day before a freeze, but the state of the art of weather forecasting does not permit this luxury. A good rule of thumb is to try to have all trees banked by Nov. 15 for the northern areas and no later than mid-December for the rest of the state.
Tree wraps are most useful in protecting young citrus trees during mild to moderate freezes or in traditionally warmer locations within the state. Tree wraps protect only the trunk, and, consequently, leaf loss can occur during moderate or severe freezes. Wraps work by delaying — but not preventing — heat loss from the tree trunk as air temperatures decrease. Temperatures under tree wraps generally are up to 6°F higher than air temperatures, depending on the type of wrap. Wraps are most effective during short-term freezes where temperatures drop rapidly.
Most tree wraps, unlike soil banks, can be attached anytime during the year and left on the tree throughout the year — even for several years. However, some types of wraps, such as those made of poor insulating materials or clear plastic, may damage or kill the tree due to excessive daytime trunk temperatures during the summer.
Microsprinkler Irrigation, or small-nozzled, high-volume sprinklers, has been used successfully in citrus nurseries for years as a means of cold protection. Recently, there has been interest in using low-volume microsprinklers to protect young trees in the field. However, success varies with the type of system, application rates, type of freeze, and severity of the freeze.
Water protects young trees by transferring heat to the tree and the environment. The heat is provided from two sources, sensible heat and the latent heat of fusion. Most irrigation water comes out of the ground at 68°F to 72°F, depending on the depth of the well. As the water is sprayed into the air, it releases this stored (sensible) heat. The major source of heat from irrigation, however, is provided when the water changes to ice. As long as water is constantly changing to ice, the temperature of the ice-water mixture will remain at 32°F.
Wind machines offer some excellent advantages in cold protection because they minimize labor requirements, consume less fuel per acre protected, and require less fuel storage than heaters. Wind machines are dependent on having an inversion — which is warmer air at about 50 feet above the grove. A temperature inversion of at least 5°F difference is necessary and an inversion of 10°F to 15°F makes the wind machine very effective. They are most beneficial when located in low pockets where they mix cold, heavy air, which settles there, with warmer air above. Usually, one wind machine is required for each 10 acre block. Start wind machines when temperatures are two to three degrees above the critical temperature.
|
