Cold weather is on its way, and many people are concerned about their citrus trees. One of the most common questions I get from gardeners is, "How much cold can citrus trees tolerate?" Because there are a number of variables, it's difficult to pin point a "threshold" temperature at which it becomes necessary to protect your trees.
There are three basic factors in determining freeze susceptibility of citrus trees.
The type and age of the citrus: Satsumas are the most cold hardy of the commonly grown citrus species in Louisiana. Kumquats follow with just a slightly less amount of cold hardiness. The order from most cold hardy to least cold hardy are: satsuma, kumquat, orange, grapefruit, lemon and lime. A citrus tree increases in hardiness as it gets older. Older trees with larger, denser canopies deal with the cold better as they trap more heat than smaller trees or unhealthy trees with thin canopies.
How cold it gets: Threshold temperatures are approximately 20 degrees for satsumas and kumquats and about 25 degrees for all other citrus trees. Trees will be damaged or killed by temperatures in the teens.
The duration of the sub-freezing temperature: If the temperature is below freezing for 24 hours or more, you should expect damage. All ripe fruit should be harvested from trees prior to a significant freeze. Temperatures cold enough to damage the tree also will ruin the fruit. It takes temperatures in the mid- to low 20s for five to 10 hours to freeze the fruit. The most cold hardy parts of the tree are the mature wood of major branches and the trunk.
If a citrus tree is gradually exposed to cooler temperatures, a process called hardening occurs, and trees become more tolerant to freezes. Chilly but above-freezing nights (in the 30s and 40s) that occur during the fall and early winter make the trees better able to handle freezes.
Severe freezes taking place when only mild weather has occurred previously are more likely to cause significant damage. This has been a relatively warm fall so our citrus trees are not well hardened at this point. Still, the temperatures this weekend should not produce severe damage.
The best way to lessen cold damage to citrus is to maintain healthy trees. Cultural practices that tend to induce and maintain dormancy in winter should be used. These methods include avoiding late summer or fall fertilization or pruning. Vigorous trees may recover from cold injury. Weak trees that are showing disease, insect damage or nutritional deficiencies are the ones most severely damaged and are the slowest to recover after freezes. fore covering, the tree could be generously wrapped with small, outdoor incandescent Christmas lights to provide additional warmth and increase the level of protection. Incandescent Christmas lights will not damage the tree even if they come into contact with it.
The frame and cover could stay in place indefinitely, but will need to be vented. Air temperatures within should not be allowed to go above 85 to 90 degrees. Venting should be provided on sunny warm days to prevent overheating and to maintain a supply of fresh air.
For trees too large to cover, banking the lower trunks of trees with soil or using tree wraps of bubble wrap, foam rubber or Styrofoam will help prevent cold damage to the trunk. This must be done before the first killing freeze and can be left on through the winter. Trunks should be treated with a copper fungicide before wrapping to prevent disease. Or the coverings could be applied during freezes and removed during mild weather.
Although tree tops may still be lost during freezes, a tree can recover if its trunk and root system are intact. Banking or wraps should be removed in the spring.
If the weather has been dry for a week or more in advance of a cold front, water your citrus trees. Trees that are drought stressed may experience more cold damage. This should be done a day or two before the freeze is expected.
Do not prune in the late summer or fall. Late pruning stimulates new growth that may not mature before winter, making it more prone to cold damage. In addition, pruning reduces the size of the canopy going into the winter, and this also reduces cold tolerance.
If pruning is needed, it should be done in early spring. Cuts should be made at branch crotches leaving no stubs. Prune to maintain a full, dense canopy. Trees need good leaf canopies to cut wind speed through the canopy and reduce the rate of cooling.
Leaves radiate heat to each other. Outer leaves may be lost to a freeze, but complete loss of inner leaves is averted by a thick canopy.
Fertilizer should be applied to citrus trees in late January or early February. A general-purpose fertilizer or citrus fertilizer may be used following label directions. Spread the fertilizer around the edge of the branches in the area of the feeder roots. Late summer or fall applications of fertilizer should be avoided as they can reduce the cold hardiness.
Avoid using oil sprays to control insects during the fall and winter. Horticultural oil sprays may decrease cold tolerance.
We potentially have a lot of cold weather yet to come this winter. Cover smaller trees as needed to protect them, and protect the trunks of larger trees if practical.
Keep things in perspective. We generally do not see significant damage if temperatures stay in the mid- to upper 20s and only stay below freezing for a few hours
http://www.nola.com/homegarden/index.ssf/2016/12/how_to_protect_citrus_trees_fr.html
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