When to protect citrus from the winter’s coldDec 15th, 2011 | By William Dilella | Category: news
Louisiana has its coldest temperatures in January and protection of citrus trees becomes important to homeowners across the state. However, it is difficult to pin point a “thresh-hold” temperature at which it becomes necessary to protect citrus trees. The biggest difficulty in making such a determination is the difference between various citrus species 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, theses 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.
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 very slight.
Three factors which are basically involved in determining freeze susceptibility of citrus trees are:
1) 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 slight less amount of cold hardiness. Citrus increases in hardiness as they get older.
2) Threshold temperatures are approximately 20 degrees for satsumas and kumquats, and about 26 degrees for all other citrus.
3) The duration of the sub-freezing temperature is very significant. If the temperature is below freezing for 24 to 36 hours, damage can usually be expected.
Generally, when freeze injury occurs, damage is inflicted mostly to cell membranes by freezing and thawing. The tenderest tissues of citrus trees are new buds, flowers, and small fruit. The coldest hardy part of the tree is dormant wood. If a citrus tree is gradually exposed to cooler temperatures, a process called hardening occurs and there is a decrease in the freezing point of the plant tissues. Citrus trees are evergreen and never become full dormant, as do trees that lose their leaves (deciduous). Trees that are slightly dormant (pre-conditioned by gradual cold) are less likely to be damaged by cold.
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. Seven steps are suggested to reduce freeze damage:
1) Clean cultivation, mechanically or by herbicides, prior to a freeze susceptible period is recommended. Grass, weeds, and straw mulches prevent heat from entering the soil during the day; therefore, less energy is stored for release at night.
2) Banking the trunks of young trees with soil and using tree wraps of plastic, rubber, Styrofoam, aluminum foil, or rope will help prevent cold damage to trunks, and root systems. This must be done before the first killing freeze. Trunks should be treated with a neutral cooper spray or fungicide before wrapping or banking to prevent foot rot. 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 to prevent development of Phytophthora (foot rot).
3) Several days in advance of a cold front, the soil beneath citrus trees can be irrigated. Good soil moisture acts as a cold buffer. If this is done at the time the front arrives, evaporation may occur and result in colder temperatures near the tree. In Louisiana, this practice is risky due to intermittent warm periods in winter that may result in flushing (new growth). The new growth can be damaged by subsequent cold. Also associated with cold fronts, larger amounts of rainfall may occur which could cause saturated soils and root pruning.
4) Trees should be pruned in late spring to maximize tree growth and hardening before winter. Cuts should be made at branch crotches, leaving no stubs, and should be treated with water-repellant tree paint. Good leaf canopies should be left to cut wind speed through citrus trees and thus 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 when a thick canopy exists.
5) Fertilizer should be applied to citrus trees in late January or early February. A complete fertilizer 8-8-8 at the rate of two pounds per year of tree age should be used. If using 13-13-13, the rate is 1 1/2 pounds of fertilizer per year of tree age. 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 stimulate new growth, which is easily killed by cold. Some evidence exists from tests that navel orange trees receiving complete fertilizer (8-12-8) are less susceptible to late winter freezes than are trees receiving only nitrogen as fertilizer or no fertilizer. Also, trees that had been limed to a soil pH of 6.5 suffered more damage than unlimed trees with a pH of 6.
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 encases the tree in translucent plastic with one or two layers. In our areas, such an extreme practice would be needed only on a few severely cold nights and cloudy days. Incandescent light bulbs can raise the temperature in the frame a few degrees. Fresh air is a must for such frames, and air temperatures within should not be allowed to go above 85 degrees to 90 degrees F. Venting should be provided to make it easy to maintain a constant supply of fresh air and to control temperature. Care should be taken to prevent the bulb from coming in contact with the plastic wrap.