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Cold Protection of Citrus in Georgia
An important consideration when growing any type of citrus plant in Georgia is cold protection. Young citrus plants, even the most cold hardy types, cannot withstand freezing temperatures as well as more mature, bearing trees.
Before the first freeze, trees up to four years of age should be banked with a clean soil up to a height of about 15 inches. Soil banks should be removed after the last chance of freeze in the early spring. Wrapping material with good insulating properties such as fiberglass or foam rubber also make effective protectors and may be used in lieu of soil banks. These materials should be a minimum of six inches thick and must make good contact with the soil.
When only a few plants are involved, protective covers may be used when severe freezes occur. On extremely cold nights, placement of one or two electric light bulbs beneath the cover provides good protection.
Sprinkler irrigation can also be used to protect citrus during freezes. Start aplying 1/4 inch per hour when temperatures drop below freezing and continue until temperatures rise above 32 degrees F. Support weak limbs if possible to prevent breakage from ice. The ice should be clear and icicles should be present. If ice is milky white, increase the volume of water being applied.
Among the citrus types which are most easily killed by freezing temperatures are citrons, lemons and limes. Temperatures from the mid to high 20s F. will readily kill or severely damage these plants. Sweet oranges and grapefruit are somewhat more hardy and usually require temperatures in the low to mid 20s before incurring major damage to large branches. Tangerines and mandarins are quite cold hardy, usually withstanding temperatures as low as the low 20s F. before significant wood damage occurs. But among the edible types of sweet citrus, the satsuma (also called the satsuma mandarin or satsuma orange) has the greatest degree of cold hardiness. Properly hardened bearing trees will with stand temperatures as low as 20 degrees F. without appreciable wood damage.
Keep in mind the temperature ranges given above refer only to leaf or wood damage. Citrus fruits easily freeze at 26-28 degrees F. when these temperatures last for several hours. Further, a longer duration of freezing temperatures is required to freeze fruits of grapefruits as compared with sweet oranges. And tangerines and satsuma fruits are more easily frozen than either of the former.
The particular temperature at which tissue of a given plant will freeze and the degree of the damage sustained are functions of a number of factors in addition to the species and variety involved. These factors include:
- the freezing temperature reached
- the duration of the minimal temperature;
- how well the plant became hardened or conditioned before freezing temperatures occured (the freezing point of tissue of a hardened citrus plant may be 5 to 6 degrees lower than an unhardened plant
- whether the plant is wet or dry (the killing temperature is 2 to 4 degrees F. lower for a dry citrus plant)
- age of the plant (a young plant cannot withstand as much cold as a more mature tree).
Citrus plants seem to freeze at higher temperatures in some years than others. The contributing factor seems to be the difference between air (ambient) temperature and leaf (tissue) temperature. On a windy night with clear or cloudy skies, leaf temperature will be approximately the same as air temperature. On a cold, clear night with little or no wind movement, leaf temperature may easily drop several degrees (3 to 4 degrees F.) below air temperature because of radiation heat loss. Thus, under the latter circumstances, while the minimum air temperature on a given night may have only been 25 degrees F., actual leaf temperature of the plants may have reached 21 to 22 degrees F. The critical temperature is that of the leaf or fruit and not the air temperature per se.