Excerpt from LSU AG Center
BATON ROUGE, La. – Louisiana citrus tree owners need to take precautions to protect their trees during cold snaps this winter, according to LSU AgCenter extension horticulturist Dan Gill.
Gill said it’s difficult to pinpoint a "threshold" temperature at which it becomes necessary to protect citrus trees because the various citrus types differ in cold hardiness.
“Satsumas, for example, do not need protection until the temperature drops below 25 degrees F. Lemons, limes and oranges generally need to be protected when the temperature dips below 26 degrees F. However, these trees can be killed or damaged at higher temperatures if they are not sufficiently hardened with enough pre-conditioning cold temperatures to halt their growth” Gill said.
The length of time citrus trees are exposed to sub-freezing temperatures is also significant. Exposure to sub-freezing temperatures for more than 24 to 36 hours will harm them. But, if the temperature moderates to above freezing prior to this time, damage is usually slight, he said.
Gill strongly advises harvesting all ripe fruit before a significant freeze event. Temperatures cold enough to damage the tree will also ruin the fruit. It takes temperatures in the mid- to low 20s five to 10 hours to freeze the fruit.
Satsumas are the most cold-hardy of the commonly grown citrus species in Louisiana. Kumquats follow with just a slight less amount of cold hardiness. In order from most cold-hardy to least cold-hardy: satsuma, kumquat, orange, grapefruit, lemon and lime.
A citrus tree increases in hardiness as it gets older. Trees with larger, denser canopies deal with the cold better as they trap more heat, Gill said.
Generally, when freeze injury occurs, damage is inflicted mostly to cell membranes by freezing and thawing. The least cold-hardy tissues of citrus trees are new buds, flowers and small fruit. The more cold-hardy part of the tree is 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 freezing temperatures. Citrus trees are evergreen and never become full dormant, but trees that are slightly dormant – that is, pre-conditioned by gradual cold – are less likely to be damaged by cold,” Gill said.
Chilly but above freezing nights, mid to upper 30s and 40s, that occur during winter before major freezes make the trees more cold tolerant. Severe freezes taking place when only mild weather has occurred previously are more likely to cause significant damage, Gill said.
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 no 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.
Alan Vaughn, extension agent in Plaquemines Parish, where Louisiana’s commercial citrus growers are, suggests these seven steps to protect citrus from a freeze:
1) Clean cultivation under the canopy of a tree, mechanically or by herbicides, prior to winter is recommended. Grass, weeds, and straw mulches prevent heat from entering the soil during the day; therefore, less heat energy is stored in the soil under the tree for release at night.
2) 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 or banking to prevent foot rot. Or, the coverings may 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.
3) If the weather has been dry, several days in advance of a cold front the soil beneath citrus trees can be irrigated. Good soil moisture acts as a cold buffer, and trees that are drought stressed may experience more cold damage. This must be done well in advance of the freeze. If this is done at the time the front arrives, evaporation may occur and result in colder temperatures near the tree.
4) If pruning is needed, it should be done in spring to allow tree growth to mature before winter. Do not prune in the late summer or fall. 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.
5) Fertilizer should be applied to citrus trees in late January or early February. A complete fertilizer such as 8-8-8 at the rate of two pounds per year of tree age may be used. Stop increasing the rate when you reach 15 pounds. If using 13-13-13, the rate is one and one-half pounds of fertilizer per year of tree age. Stop increasing the rate when you reach 10 pounds. Spread the fertilizer around the edge of the branches in the area of the feeder roots. Apply a subsequent application of nitrogen when good soil moisture exists in June. Late summer or fall applications of fertilizer should be avoided as they can reduce the hardiness.
6) Oil sprays used to control insects and mites decrease cold tolerance and should not be used later than August 15.
7) To protect a single tree, some home gardeners have constructed frames over trees and encased the tree with one or two layers of translucent plastic. This is generally most practical for smaller trees. In southeast Louisiana, such an extreme practice would be needed only on a few severely cold nights and cloudy days.
Before covering, the tree could be generously draped and 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 can stay in place indefinitely, but will need to be vented. Air temperatures within should not be allowed to go above 85 degrees to 90 degrees F. Venting should be provided on sunny warm days to prevent overheating and to maintain a supply of fresh air.