Seminole County Residential Horticulturist
Citrus trees do not enter a dormancy period like temperate-zone deciduous trees such as peaches or apples. They do, however, develop some cold tolerance as temperatures reach the 40-60 degrees Fahrenheit (F) range for a period of about two weeks. This cold tolerance is influenced by environmental conditions, mainly cool temperatures, but also by tree health, its rootstock and the citrus specie.
How trees freeze: As a general rule, a citrus tree will freeze from the top to bottom and from the outside to the inside. Flowers are the first tissues to freeze, followed by tender new growth such as leaves and twigs. Younger tissues will be more affected than older tissues and smaller-diameter wood before larger-diameter wood. Young developing fruits will freeze before mature fruits. The smaller the size of the fruit, the faster it will freeze. Fruits with thin peel will freeze faster than fruits with thick peel. Freeze damage occurs when ice forms inside the citrus tissues.
Predisposing factors: Citrus trees are more susceptible to cold damage during their first five years. Stress factors such as lack of water, diseases, insect damage or nutritional deficiencies can increase the damage caused by low temperatures. Trees are more susceptible to cold damage during growth flushes, which may be induced by warm temperatures during the winter or by heavy pruning of the trees during the late fall or winter months, reducing their heat retaining capacity. Heavy fertilizer applications can also result in vigorous growth that may lead to freeze damage.
Critical temperatures: The critical temperature for ice forming in citrus tissues is about 28 degrees F. Citrus flowers will freeze at about 28 degrees F, and fruit damage will occurs when temperatures fall below 28 degrees F for at least four hours. If mature fruits freeze, they can still be used for juice if harvested promptly.
Site selection: Trees should be located on the south and west sides of the yard. The reasoning behind this recommendation is that south is the warmest area and west is the next warmest area in a given landscape. Other places to locate trees are near or between buildings, south facing slopes, next to other large trees or near large bodies of water such as lakes.
Cold protection with sprinklers: The principle behind the use of sprinklers to protect trees from freezing is that heat is released when water changes from liquid to ice, a phenomenon known as heat of fusion. When water is freezing, its temperature will be near 32 degrees F; therefore, the heat liberated as the water freezes maintains the temperature near 32 degrees F. This temperature, known as the triple point, is in equilibrium between vapor, liquid and ice. If sufficient water is applied, and all leaves and branches are covered with ice, protection can be expected. If only partial coverage of leaves or branches is accomplished, damage can occur, and the damage will be more severe than if water had not been applied. Do not stop sprinklers until the temperature is 32 degrees F or above and water is dripping from all parts of the plants.
Soil banking: One of the cheapest methods to protect the trunk of young citrus trees from cold damage is the practice known as soil banking. It consists of mounding soil around the trunk of the tree as high as possible to protect it from cold temperatures even as low as 12-15 degrees F. Banking is basically a method to protect the graft union. Soil banks must be made before freezes occur and should be removed as soon as temperatures start to warm up because it can predispose the trees to diseases and pests problems. Tree wraps are also effective in protecting young trees from freezing damage; the effectiveness of various wraps depends upon its insulation capacity. Polyurethane wraps have been found to be effective protecting the trees, but this method is less effective than soil banking. Unbanking should be done around the end of February.
Horticultural practices: Trees should be watered during the winter to reduce water stress. Water-stressed trees do not recover as quickly from freezes as non-drought-stressed trees. If freeze damage occurs, do not prune immediately after the freeze, dead tissues will protect the tree from further damage. Pruning can be done by mid-spring or early summer when tree damage can be determined. Fertilization should be initiated after the chance of freezing temperatures is over.
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