The cold weather we typically receive now through January can cause problems for home and commercial citrus in our area. It is important to know what the critical temperatures are and the factors affecting them.

Among the citrus types that are most easily killed by freezing temperatures are citrons, lemons, and limes. Temperatures in the high 20s will readily kill or severely damage these plants. Sweet oranges and grapefruit are somewhat more cold hardy and usually require temperatures in the 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 before significant wood damage occurs. But among the edible types of sweet citrus, the satsuma has the greatest degree of cold hardiness. Kumquats are also among the most cold hardy.

Properly hardened bearing trees will withstand temperatures as low as 20 degrees F without appreciable wood damage. Temperatures at ground level can be several degrees colder than temperatures around the canopy of the tree, especially if there is no wind.

Keep in mind that the temperature ranges given above refer only to leaf or wood damage. Citrus fruits easily freeze at 26 to 28 degrees F when these temperatures last for several hours. Further, a longer duration of freezing temperatures is required to freeze fruits of grapefruit as compared with sweet oranges. And tangerines and satsuma fruits are the most easily frozen of the common citrus.

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. Some of the more important are the following:

• The freezing temperature reached
• The duration of the minimal temperature
• How well the plant became hardened or conditioned before freezing temperatures occurred (The freezing point of tissue of a hardened citrus plant may be 5 to 6 degrees lower than an unhardened plant.)
• Age of plant (A young plant cannot withstand as much cold as a more mature tree.)
• Healthy trees are more hardy than diseased trees.  

Another complicating factor contributing to observations by some that citrus plants seem to freeze at higher temperatures in some years than others is the difference between air (ambient) temperatures and leaf (tissue) temperature.

On a windy night with clear or cloudy skies, leaf temperature will be approximately the same as air temperature. However, on a cold, clear night with little or no wind movement, leaf temperature can 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 itself. Trees with a good fruit crop are less hardy than those with no fruit.

Research from the LSU AgCenter has noted that trees growing on bare ground have a higher probability of survival as opposed to trees growing in turf areas. The heat from the ground can radiate up on bare areas as opposed to being trapped in by turf areas. The difference in the canopy of the tree can be up to five degrees F.

We generally recommend you protect your citrus when the temperatures will be below 27 degrees F for an extended period of time. This protection must:

• Entirely cover the plant trying not to touch the foliage
• Extend all the way to the ground
• Must be removed the next morning once temperatures begin to rise  

You can also install small lights such as Christmas lights on the trees to increase the temperature around the trees. Freeze damaged trees should not be pruned until the extent of the cold damage has been determined. Normally, the damage is not evident until mid spring.

Question of the Week: What evergreen perennials should bloom this time of year?

Answer: Camellia sasanqua’s are beginning to bloom this week and the Sweet Olive (Osmanthus fragrans) will start blooming in the coming weeks. In addition, Cassia (Cassia splendida) plants are finishing up their blooming period.

Information provided by Bobby Fletcher, LSU AgCenter. For more information on these and other horticultural topics, call 985-446-1316, email at bhfletcher@agctr.lsu.edu, or visit the LSU AgCenter website at www.lsuagcenter.com.