GET AVOCADOS AND CITRUS READY FOR WINTER

General Cultural Practices
There are cultural practices that can enhance the possibility of tree survival. Three to four degrees F of higher temperatures can mean the difference between a bad burn and death of a tree. The following practices can help protect your orchard from frost damage; they all have in common some method of retaining heat in the orchard.

During the day, an orchard soil acts as a reservoir for heat absorbed from the sun. Some of this heat returns back into the atmosphere during the day and at night (reradiation), warming the orchard. Moist soils will store more heat and are better heat conductors than dry ones, and a compacted soil - one that forms a firm surface - is a better conductor of heat than a soil that has been recently cultivated. Therefore, when there is danger of frost, do not cultivate your orchard soil, but do keep it moist to a depth of at least one foot.

The insulating effect of weeds, sod and leaf litter between the rows reduces soil absorption of heat during the day and reduces reradiation. In an experiment, an orchard that was weed-free with a compacted soil showed 3.8°F more warmth than an orchard with sod. The same experiment demonstrated that an orchard with rain-compacted soil was warmer than one that had been recently cultivated. If you have a ground cover, mow as short as possible to minimize the insulating effect.

The size of the tree canopy in many ways determines the amount of reradiated heat that is captured by the trees. A large, dense canopy can intercept more heat leaving the orchard than a small, sparse one. A healthy tree has been shown to have a higher canopy temperature than a sparsely foliated tree. For example, lemon trees with sieve-tube necrosis are frequently more frost-damaged than healthy ones, partly because of the sparseness of the foliage. Proper pest and disease control, irrigation and fertilization are all important in maintaining a healthy tree more resistant to frost damage.

A most important factor in determining how damaging a frost may be is the time of year at which it occurs - early or late winter. If the frost comes early, before temperatures have a chance to drop gradually, trees have not had time to acclimate themselves, and the damage can be severe. If, however, the frost hits later in the season (after a period of gradual temperature decline), trees can develop a cold hardiness and better tolerate the cold weather. To this end, late applications of nitrogen that cause excessive growth may delay this acclimation, thereby further reducing the tree’s cold tolerance.

In most cases, frost protection can be improved by improving air drainage. Brush, tall weeds and trees can impede the movement of cold air out of an orchard. Remove these obstacles (or at least mow the weeds or prune up windbreaks), creating channels for air movement.

Managing Irrigation Water for Tree Protection
Irrigation water releases two types of heat when applied during a frosty night: the heat released by going from the water temperature at the nozzle down to 32° F (0° C), and another type called the heat of fusion, which is how most of the warming energy is released. This is the energy that is obtained by water going from a liquid to a solid state. For every degree of temperature drop (Celsius), there is one calorie of heat released per gram of water. A gram of water with a nozzle-exiting temperature of 65° F will release about 18 calories of heat when it cools to 32° F, but in changing from liquid to ice, 79 calories are released. The more ice is made, the more heat is released. This heat helps raise the temperature of the orchard.

In furrow irrigation it is best to place the furrows as close to the dripline as possible and make the furrows as wide as possible to increase the transfer of heat into the air. With sprinklers, the more output the better. At Lake Alfred, Florida, the guidelines call for at least 20 gph sprinklers, with lower rates giving less effect. If you do not have the capacity to irrigate your entire orchard, it is advisable to choose one area and protect it, rather than stopping in the middle of the night to irrigate another area. This is because the resultant evaporative cooling will take 607 calories per gram of water, and this heat loss comes at the expense of the leaf. Therefore, an area where the water has been turned off prematurely will rapidly cool.

The rate of cooling is related to the dewpoint. A high dewpoint indicates moist air, and a low dewpoint relatively dry air. The drier the air, the more rapid the cooling. When temperatures are low with extremely dry air conditions, irrigation can be a hazard to trees. If the predicted dewpoint is well below (at least more than 5° F) the predicted minimum temperature, sprinklers can cause excessive evaporative cooling. With evaporative cooling, orchards with running water can actually be cooler than dry orchards. Therefore, if trees are sprinkled and ice forms, in order to protect the trees, the sprinklers should continue in operation until the ice begins to melt. To be certain, sprinklers should continue until the ice is completely melted.

Use of Wind Machines
In most situations, wind machines can be both effective and economical in providing protection; however, in some situations, as we found in an advective freeze (cold polar winds), they can exacerbate the situation. The dry cold air drawn in by the machines will intensify the cold damage.

The San Joaquin Valley and Southern California more commonly have radiation frosts (heat loss under conditions of clear skies and light wind conditions) which commonly have a temperature inversion. For a machine to do its job properly, this temperature inversion is needed so that warmer temperatures at higher elevation can be mixed with cold air near the ground.

The number of engines, power requirement and area protected vary with terrain and manufacturer’s brand. Generally 7 to 10 brake horsepower are required per acre, more in cold spots, less if there is a slope. They should be started before temperatures drop below 35° F, since it is difficult for a wind machine to warm an orchard that has dropped below this critical temperature.

Ice-Nucleating Bacteria
For some years, it has been known that bacteria residing on cold-sensitive plants initiate ice formation. The ice formed in or on the plants spreads rapidly, causing mechanical injury. Copper-containing sprays that are applied 10 days before cold weather, kill off these bacteria and allow enough time for their decomposition.

By controlling the ice-nucleating bacteria (INB), tests have shown temperatures may drop 2-4° F below critical levels without plants showing damage.

Recent Research and Frost-Protectant Sprays
There are many products on the market today which claim to protect plants from frost and freeze. A research plot to evaluate the efficacy of Kocide®, a copper spray; Anti-Stress 2000®, a plastic polymer coating; Frost Shield®, another polymer; and Frost Guard®, a systemic/contact material affecting plant nutrition and INB.

A Lisbon lemon grove in the ground for six months was sprayed with the materials, the check trees were sprayed with only water. Fifteen single-tree replicates were randomized throughout the plot. Frost Guard was applied to a light wetting, as directed; the other products were sprayed to runoff. Four days later on December 23, a frost hit with temperatures below 32° F for four hours and two hours at 28° F.

Trees were evaluated for damage two weeks later. Only the trees sprayed with Anti-Stress 2000® showed any significant protection over the water-only treatment; other products showed no aid in cold tolerance. Why Kocide® showed no improvement may have been due to not allowing enough time for bacterial cell decomposition between the spray treatment and the onset of cold temperatures.

Perhaps if colder temperatures for longer hours had occurred, some of the other products would have shown value. More tests need to be conducted before conclusive remarks can be made.

To simplify information, trade names of products have been used. No endorsement of name products is intended, nor is criticism of similar products which are not mentioned.