TYPES OF INJURY
Fruit damage consists of frozen stems, frozen fibers inside the fruit, or frozen flesh and skin, depending on the temperatures and their duration. This damage appears slowly and is difficult to appraise accurately.
For some varieties, principally in the case of the Fuerte, the first evidence of damage will show up as a browning on the stem a few days after a frost. Mature fruit with only the stems frosted can be picked and marketed. Nearly every year some of the crop from frosted Fuerte groves is salvaged by picking all the fruit showing frosted stems as soon after the frost as possible. Unless this fruit is harvested, it will fall from the tree. Frozen fruit of other varieties, particularly the Guatemalan types, may never show the browning, but will drop anyway.
Frozen fruit fibers (vascular bundles) turn black and California law prohibits the marketing of all seriously damaged fruits. Before you pick mature frosted fruit, you should call in your marketing organization representative and local agricultural inspector to help you determine how much of the fruit should be harvested. By cutting representative fruit, they can tell whether it is within the legal tolerance or not, and guide you in marketing your fruit.
Severely frozen fruit may show brown water-soaked spots in the skin, gray areas in the flesh, and may even crack or split. It is unmarketable and will drop from the tree in time.
Frozen blossoms, leaves, and twigs turn black within a few days. But larger wood, although frozen, may never turn black. Great variation occurs in the location of tree damage. In some cases, only the lower part of the tree will be frosted, while in other cases only the top part is affected. Sometimes, only a limb on one side will be frosted. Thus, it is impossible to evaluate accurately the extent of wood damage until new growth comes out the following spring and summer.
Trees suffering moderate wood damage usually will not bloom properly and will produce a reduced crop the following season. Occasionally, trees have been killed all the way to the ground by severe freezes.
When tender growth should be delayed until early spring
The resistance of the fruit and trees to freezing temperatures helps determine whether a grove must be protected or not. Then too, both the minimum temperature and the duration of damaging temperatures affect the amount of injury. For example, three or four hours of 29 degrees may injure an orchard more than a brief drop to 25 degrees. Frost susceptibility varies greatly from situation to situation. Adequate appraisal of each situation should include consideration of the following variables.
Age and Crowding
Young trees (up to 3 years of age) and recently top-worked trees of all varieties may be killed by temperatures which would only cause minor damage to nearby mature trees. Protection for such trees is a wise practice, even in warm locations.
In general, older trees of a given variety are more resistant, except in orchards where they are crowded. Where the soil is shaded during the day, it cannot absorb the sun's heat and thus has little heat to give off during the night. Crowding also restricts the drift through the grove and increases the difficulties of orchard heating. For these reasons, thinning groves is often advisable to reduce their frost hazard.
Weak trees are more subject to frost damage than healthy ones. Be sure your trees go through the whole year with an adequate supply of moisture; withholding irrigation water in the fall to "harden them up" can actually weaken trees and make them more susceptible.
Occasionally avocado trees are in a growth flush (a period of rapid new growth) when freezing temperatures occur. This is not usual, but unseasonably warm weather may stimulate a tree into abnormal growth in the winter. To avoid stimulation, pruning should be delayed until early spring. When tender growth occurs in the winter, growers may have to provide additional protection or sustain damage.
Size of Crop
Trees with large crops are more susceptible to frost. To save this fruit, temperatures would have to be maintained one or two degrees higher than would otherwise be necessary.
Varieties of Avocado
A wide range of susceptibility occurs among avocado varieties. To classify them according to their resistance is difficult because of the variations already mentioned. But the races of avocados do exhibit differences and can be used as frost resistance indicators.
The Mexican race is the most resistant, and is able to withstand temperatures about as well as orange trees.
The Guatemalan race is the most susceptible grown in California. Some varieties are slightly more resistant than others, so the race can be divided into tender and very tender classifications. Because the fruit is immature during the winter, the crop may be lost while the trees will suffer only minor damage. Lower temperatures will seriously damage trees and affect their future productivity.
The hybrids, as typified by the Fuerte, are intermediate in their frost resistance, and are in about the same class as lemon trees. Because of the Fuerte's importance, it has become the standard measure of frost resistance. Protection is usually provided to save the Fuerte crop since the fruit is mature in the winter months.
As a guide to inexperienced growers, Table I gives a basis for establishing frost protection practices. The critical temperatures listed are for mature healthy trees. Other tree conditions might move these figures one or two degrees higher. Also, the duration of the damaging temperatures would affect the degree of injury.
|Variety Frost Resistance|
|Race||Typical varieties||Critical temperature below which fruit and/or trees are subject to damage|
|Mexican||Duke, Topa topa, Mexicola, Zutano, Bacon||25 Degrees F|
|Hybrids||Fuerte, Puebla||28 Degrees F|
|Ryan, Hass, MacArthur, Nabal, Endranol, Rincon||29 Degrees F|
|Guatemalan (Very Tender)||
Anaheim, Dickinson, Carlsbad, Challenge, Hellen
|30 Degrees F|
Need for Protection
Frost protection is an expensive and disagreeable job which requires careful consideration by all growers. Each grower must determine his own needs. No one can predict accurately the frost hazard for a given location. Information can be obtained by installing the proper thermometers on the property. Observing adjoining orchards and talking with neighbors will give good background information.
The topography of the orchard and the surrounding land enters into the evaluation. Land on hillsides with steeper slopes is usually warmer than lower valley land. Locations with strong prevailing drifts, or exposed to winds, usually are warmer than sheltered areas having little air movement. Tall dense windbreaks can make a grove one or two degrees colder, but the lower branches of the windbreak can be trimmed to overcome this. Coastal areas usually have less frost hazard because the relative humidity is higher, and ceilings are lower than in interior areas.
As mentioned in the beginning, your final evaluation should answer this question: "Will the fruit and foliage I save pay for the trouble and expense of protection?" Often it is possible to take some damage every few years and still have a more profitable orchard than if equipment were purchased and operated. On the other hand, an investment in equipment, if properly operated, will save your trees and crops. The decision should be based on a long range economic estimate, balancing the money lost by damage against the costs of protection. This same evaluation will determine the type of protection to provide, if needed.
In some counties and communities, smoke ordinances regulate the type of equipment and methods of operation to reduce smoke output. When you purchase new orchard heater equipment, it should be the most smokeless available in order to comply with possible future expansion or tightening of these ordinances. The trend toward eliminating smoky orchard heaters in most areas will affect the grower's choice of equipment.
Citrus and avocado are of subtropical and tropical origin are cold-tender plants that have not developed the effective cold hardening process of deciduous trees that drop their leaves and go dormant. Even within deciduous, temperate tree species there are ranges of frost tolerance. Subtropicals do have the capacity to develop some cold tolerance and this is by going through quiescence, a sort of resting condition of no or slow growth when cooler temperatures arrive. Quiescence is induced several weeks after 40-50 degree F temperatures arrive. Cold tolerance develops most when trees are not flushing. The healthier and less stressed the tree, the more responsive it is to the cooler temperatures that induce quiescence. A tree that has been recently pruned or nitrogen fertilized is more likely to continue flushing through the cold-induction of early winter and is more subject to cold damage. Unfortunately along the coast, there can be warm temperatures that occur during the winter. This can break quiescence and the trees start flowering or flushing and when cold weather arrives again, the trees are now more susceptible to cold.
Because an evergreen canopy like avocado continues to transpire, the roots of subtropicals continue to operate to deliver water to the leaves. The leaves though are not doing much other than losing water. Therefore, there is not much demand nutritionally on the part of the tree. In fact, fertilizing with nitrogen can break the quiescence and make the tree more susceptible to cold. The tree already stores the bulk of its nutritional needs in the roots, branches, stems and leaves and can call on nutrients if it needs them. Supplemental fertilizer at the time of quiescence, though, can result in an excess of the tree’s needs and induce flushing. The goal is to go into winter with adequate storage, so that when spring comes, the nutrients are there for the demanding flowering period. Adequacy is based on previous fertilizer applications, tree condition, leaf analysis and crop load. Nitrogen applications applied in winter are also susceptible to high losses from leaching and volatilization, resulting in environmental problems.
So fertilizer timing starts with when the first frosts might come. Along the coast in Southern California, the last nitrogen application should be no later than October 1, in preparation for frosts than can occur in December. Nitrogen can resume March 1 in most cases. Then the next issue is how frequently to apply nitrogen after that. The more frequent/small applications that are made, the more efficiently it is taken up, so the less that is required. Continuous injection is ideal, but most operations are not equipped to do it this way. Setting a monthly time for injection, such as the first of the month, is the next most efficient. Whatever timing you use, though, nitrogen applications should be confined to the spring and summer months with some possible in early fall.
As for potassium, it is not so liable to cause flushing and is not so susceptible to leaching and has no volatilization. It can be applied most any time and will not go anywhere other than through erosion, waiting for the tree’s roots to absorb it. Microelements such as iron and zinc though, need an actively growing root system for uptake. Their soil application should be limited to the summer time.
There is a running debate about whether avocado canopies should be skirted up, raised up so that you can see under the canopy. In doing so, the tree’s tendency is to maintain its bearing volume by increasing a similar amount in height that is lost by removing the bottom layer of canopy. A tree with a full canopy is more cold resistant because it traps heat inside the canopy and is not so prone to cold winds. In an inversion freeze, though, warming air from irrigation, wind machines and orchard heaters is less likely to circulate when the skirts block air movement. A low skirt also impedes a uniform application of water from microsprinklers, and hence fertilizer distribution. A low skirt also has more fruit lying on the ground which is more uneven in coloration and more prone to disease and possibly food safety issues. A raised skirt also promotes more air circulation within the canopy which can reduce the incidence of some other diseases of both fruit, stems and branches.
A raised skirt, though exposes the base to light, and if there is no leaf mulch, there are more weeds to control. In the case of hillsides, because of gravity and wind exposure, leaves tend to blow away. The roots are now more exposed to drying because of increased evaporative loss. Loss of leaves is also a major disease problem, since leaves and organic matter are the first lines of defense (after proper irrigation management) against Phytophthora root rot. It is the microorganisms breaking down the leaves that create a hostile environment for the Phytophthora pathogen. In fact, in releasing enzymes to break down organic matter, the microorganisms also break down the cell walls of Phytophthora which are made of the same material as leaves. An orchard with no leaves is wide open to root rot infection.
So I propose something modest. On flat ground where trees are more prone to frost damage, and less subject to winds blowing away leaves that the trees are skirted. On slopes, though where winds blow away leaves and the trees are less subject to low lying cold, that the skirts are left. To maintain a more even water distribution, though, windows are cut into the canopy on the side facing the microsprinkler so that the canopy does not interfere with water spray.
Gary Bender has made his manual on avocado production available on his website. And it's free. Take a look at it to see if you might be missing something in your orchard:
UCCE Farm Advisor Gary Bender finally has his 14 chapter book on avocado history, botany and cultural practices on the San Diego County web site. Check it out: