- Author: Ben Faber
TYPES OF INJURY
Fruit Damage
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.
Tree Damage
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.
Tree Resistance
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.
Health
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.
Growth Flush
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 |
Guatemalan (Tender) |
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.
- Author: Ben Faber
Dr. Gordon Frankie at UC Berkeley is teaming with me in Ventura County to jointly work on a pollination study for avocados. Dr. Frankie is studying how to increase native bee pollinators in avocado orchards. To do this, we plan on surveying the native bees that are in the area, identify the plants that they are foraging on and then plant those species in orchards to lure indigenous bees + pollinators in to the orchards. These plants will also be monitored for their attractiveness to beneficial insects which could control avocado thrips, persea mite along with other pests. We have applied for UC grants and will apply to CA Avocado Commission and CDFA Specialty Crops for money. We plan on planting the natives before the winter rains, if they ever come.
For a review of the native pollinators, check out the UC publication: How to attract and maintain pollinators in your garden. http://anrcatalog.ucdavis.edu/pdf/8498.pdf
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/span>- Author: Ben Faber
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.
- Author: Ben Faber
In the past avocados were rarely pruned. In fact, if the trees got very big, growers would stump them down to 3-4 feet and then let them regrow. This would often be a disaster, since the trees rapidly grew to stupendous sizes again. They also might regrow then suddenly collapse, because all that regrowth was coming at the expense of energy being sent to the roots. If the roots were compromised by root rot, they would then not have the energy to fend off the disease. So by bringing the canopy into balance with a sick root system that was continuing to die and was not being fed by a big canopy, the root death would accelerate and when the canopy and root system became imbalanced again, the whole canopy would collapse and the tree would die.
Also, this wild regrowth was wild and hard to manage. The adage of “prune avocado trees cautiously” was heard round the avocado community and as a result many growers would not do anything. The trees growing larger and larger and larger with the fruiting canopy going higher and higher and higher and picking costs and liability going up. Tree thinning was practiced, where every other tree would be removed so that light could penetrate into the orchard, encouraging more fruit production and slowing tree growth. But they would still grow and another thinning would be needed. The original commercial ‘Hass’ orchard in Carpinteria started out in 1954 with 140 trees and 40 years later was down to 17 trees and was still productive, but they were monsters that were finally felled by root rot.
Many commercial avocados are now routinely pruned to keep the trees short, so that harvesting costs and other tree maintenance expenses are reduced. Also more light shines into the trees, so that more fruit is borne on the lower branches. Light or minor pruning can be done any time of year to correct imbalances or limb breakages. However, major or heavy pruning should only be done in the early part of the year from January through April. Avocados flower and bear fruit at stem terminals, so if you give the tree and buzz cut (heading cuts), all the flower terminals will be cut off and there will be no flowering the following year. It also leads to an explosion of water sprouts that result from bud break up and down the branch because the terminal bud which control the buds lower down have been removed. Naphthalene acetic acid (TreeHold) painted on the cut end can be used to restrict some of this wild bud break.
Whenever possible, thinning cuts should be made, where the branch is removed back to a subtending branch. This results in much less wild growth. Also when there are buds that start growing into water sprouts, they can be nipped back to force lateral growth. These laterals will then slow down the growth of the sprout and the side terminal buds will also be able to grow and transition of flower buds later.
Work in Carol Lovatt’s lab at UCR has shown that terminal buds need a certain maturity to flower and the transition from a vegetative bud to a flowering bud occurs sometime in late summer/early fall. If pruning is done in July, there is not enough time for the new buds to mature by August and there will be no flowers from that branch the following spring. New vegetative buds formed on growth from spring will often have enough maturation time to make the transition to flower buds, resulting in flowering the next spring.
Again, light pruning can be done at any time of the year, but removing terminals is removing potential fruiting wood. Therefore, if heavy pruning is needed, it is best to remove one branch at a time. To reduce the height of a tree, cut out the tallest branch one year, the next tallest branch the following year, and so on until the tree is down to the height required. The process may take three to four years. By reducing the height over several years, the tree is put under less stress, less disease is likely to occur and fruit production is not drastically reduced. Pruning the sides of the tree should be done in the same way. Prune off a side branch that most impinges on a neighboring tree one year, then the next worst offender in the second year, and over the years continue this process until there is light all around the tree.
If pruning creates major open areas in the tree to sun-light where there once was shade, the exposed branches should be painted with white latex paint diluted with water so that it can be sprayed on. It needs to be white enough that it can reflect sunlight and avoid heat damage that can cause sunburn. Sunburn can utterly destroy all the work that has been done.
If the trees are really monsters, the only real alternative is to bring the whole tree down. But not stumping, rather scaffolding where much of the structure is maintained. This is where the tree is brought down to as high a height as is convenient and safe. By cutting the tree to a height of 8 feet or so, there is not so much rank regrowth because a greater portion of the tree is retained. Also many times there are leafy branches that remain that will flower and fruit and slow the wild regrowth. Water sprouts that form should be headed back to force lateral growth that encourages stems that will flower, which will also slow the wild regrowth.
And one last warning. Do not. Do not. Do not. Got it? Prune sick trees. If the roots are compromised, the regrowth is going to be hard on the roots. Get the trees perked up with one of the phophite products so that they are ready to go through this process. You may have to wait a couple years to start the pruning process until the trees are in shape for the rigors.
Image: Don't make cuts like this.
- Author: Ben Faber
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.