For the first time since the great freeze of '89-90, we have experienced a little more than minor damage to our crops. Compared to the San Joaquin Valley, Ventura country escaped without major damage; although there were some areas harder hit like the Ojai Valley and some canyons near Santa Paula. Many parts of the SJV were hard hit.

As in the freeze of 1990, your trees must be cared for in the same way during this post freeze period. In 1990, advice was issued to the grower about the rehabilitation of their trees, both citrus and avocado. We would like to review that information for you at this time. How can we best aid tree recovery so that tree growth and yield will proceed most rapidly?

FREEZE DAMAGE

Citrus and avocado leaves appear wilted or flaccid during periods of low temperature. This is a natural protective response to freezing temperatures and does not mean the leaves have been frozen. Leaves will be firm and brittle and often curled when frozen. Leaves become flaccid after thawing, and if the injury is not too great, they gradually regain turgor and recover, leaving however, dark flecks on the leaves. Seriously frozen leaves collapse, dry out, and remain on the tree. Foliage form recent flushes are most susceptible to this damage. If twigs or wood have been seriously damaged, the frozen leaves may remain on the tree for several weeks. If the twigs and wood have not been damaged severely, the leaves are rapidly shed. Trees losing their leaves rapidly is often a good sign and is not, as many growers believe a sign of extensive damage.

Cold damage to the twigs appears as water soaking or discoloration. In older branches and trunks it appears as splitting or loosening of bark where the cambium has been killed. Bark may curl and dry with many small cracks. Dead patches of bark may occur in various locations on limbs and trunk.

Sensitivity to frost is dependent upon many variables. In general, mandarins are the most cold hardy followed by sweet orange and grapefruit. Lemons are very frost sensitive with Eureka decidedly more sensitive than Lisbon. For avocados, Hass is about as cold tolerant as lemons, while Bacon is more cold tolerant. Limes are the least cold hardy. Healthy trees are more tolerant than stressed ones. The rootstock also imparts sensitivity onto the scion.

Injury to the foliage and to young trees may be immediately recognizable but the true extent of the damage to larger branches, trunks, and rootstocks may not appear for on to four months following the freeze. No attempt should be made to prune or even assess damage from the frost until spring when new growth appears.

WHITEWASHING

The only treatment that should be done rapidly after a freeze is whitewashing. Often the most sever damage following a freeze results from sunburn of exposed twigs and branches after defoliation. Avocados and lemons are the most susceptible to sunburn, oranges not as much; but, if the tree has been defoliated, applying whitewash would be precautionary. Temperatures do not have to be extremely high to cause sunburn.

PRUNING

Pruning should be carried out to prevent secondary pathogens and wood decay organisms from slowing tree recovery. Again, however, there should be no rush to prune. Premature pruning, at the very least, may have to be repeated and, at the worst, it can slow tree rehabilitation. It should be remembered that when pruning, all cuts should be made into living wood. Try to cut flush with existing branches at crotches. Do not leave branch stubs or uneven surfaces. Tools should be disinfected in bleach or other fungicide before moving on to the next tree.

The extent of pruning is dictated by the amount of freeze damage:

IRRIGATION

Irrigate carefully! Remember that when leaves are lost, obviously evaporation from leaves is greatly reduced, and, therefore the amount of water required is also greatly reduced. A frost-damaged tree will use the same amount of water as a much younger or smaller tree. Over irrigation will not result in rapid recovery. Instead, it may induce root damage and encourage growth of root rotting organisms. This is particularly true for avocados. Irrigation should be less frequent, and smaller amounts of water should be applied until trees have regained their normal foliage development.

FERTILIZATION

Fertilization of freeze-damaged trees should be carefully considered. There is no evidence to indicate that frozen trees respond to any special fertilizer that is supposed to stimulate growth. If trees are severely injured-with large limbs or even parts of the trunk killed-nitrogen fertilizer applications should be greatly reduced, until the structure and balance of the tree become re-established. Trees should be watched for evidence of deficiencies of minor elements. Deficiencies of zinc, manganese, copper, and iron are most likely to develop. For citrus, these materials should be applied as sprays, and they should be used as often as symptoms are observed. Two or more applications may be required the first year.

Mature Orchards

Only two general methods of protecting avocado groves have proved satisfactory - heaters and wind machines. A combination of these two also is used. Many makes and designs of heaters and wind machines are offered for sale and they must be compared on the basis of protection provided and cost of installing and operating. This publication is limited to their general application in avocado groves. Their actual operation is discussed in other available publications of the University of California.

Orchard Heaters

Experience has shown that a large number of small fires burning throughout the orchard provide better protection than a few large fires concentrated in spots. The value of heaters is noted below to provide a basis for selecting the general type of protection needed.

Their advantages are:

• They usually furnish more adequate protection than wind machines. However, in extremely cold conditions they, too, can be inadequate.
• Only enough heaters to maintain safe temperatures need to be lighted.
• Additional heaters and oil can be stored in the field for emergency conditions.
• They distribute heat to all parts of the grove.

Their disadvantages are:

• Smokiness. However, certain types have low smoke output when properly operated.
• Relatively high costs of investment and operation.
• Fire hazard due to mat of leaves beneath the trees.
• Trouble and work of operation and maintenance.
• High labor requirements.

For colder locations and positive protection, orchard heating is the only proven method. Pipe line heaters, using either heater oil or natural gas under pressure from permanently installed pipelines, are efficient but very expensive. Economical and effective heating is provided by heaters with 9-gallon capacity bowls and improved designs of stacks burning a low grade of diesel oil. Usually 45 to 90 heaters are used per acre depending on the frost hazard.

Wind Machines

In certain locations, wind machines have provided economical protection. Their effect is essentially that of a large fan which mixes the air within and above the orchard so that the average air temperature near the ground is raised. Their precise effects are not completely understood, but studies are being continued by the University of California.

Their advantages are:

• More economical than orchard heaters
• Low labor requirements for operation
• Adequate protection against local radiation frosts when temperatures go only 2 or 3 degrees below the damaging point
• Useful in increasing air movement in groves where dead air occurs
• Increased effectiveness of heaters.

Their disadvantages are:

• Inadequate protection with freeze conditions or when temperatures go 4 or 5 degrees or more below the damaging point
• Inadequate protection in locations where little or no ceiling occurs
• Unequal protection throughout grove
• Less effective in young plantings.

Machines providing at least 5 horsepower per acre should be selected on the basis of cost and ease of operation. The location of the machine or machines in an orchard depends on the drift, the topography, and the other variables. Consult other University of California publications for additional information.

Combination Heaters and Wind Machines

Usually 8 to 25 heaters per acre uniformly scattered throughout the grove are sufficient when used with an effectively installed wind machine. The combination provides adequate protection for even the colder locations. The wind machine will protect the grove for some of the nights by itself, but for the very cold nights, the heaters are available to add heat and thus provide positive protection. Usually the heaters are lighted whenever the wind machines cannot maintain the temperatures above the danger point.

This combination method has the advantage over heaters used alone, in that it is cheaper, while providing as complete protection.

A great many other types and methods of protection has been tried, but because of one or more faults in each, they cannot be recommended.

Protecting Young Trees

Orchard heaters and wind machines are less effective in protecting young trees in their first two or three years in the orchard than other methods which are usually cheaper, anyway. The first consideration in protecting newly planted trees is to have the trees as large as possible. Planting and topworking should be done in the spring so the trees will make the maximum growth during the summer and fall, before low temperatures occur in the winter.

In locations having serious frost hazard, you might try the practice of planting Mexican seedlings in place in the field and permitting them to grow for 2 to 3 years before topworking. Then if the topworking is done early in the spring, the young trees will have made enough growth to better withstand the cold of the following winter.

Protection for young trees is best accomplished by sheltering them from radiation heat loss. Shelters for this purpose are of many types, the most common are sketched below.

In cold location, young trees may require several types of protection; they may be wrapped with paper, mounded with dirt, or surrounded with corn stalks.

Care of Frost Damaged Trees

Determining the amount of damage is often a difficult job and cannot be done accurately for some months following the freeze. It is usually better to let the tree recover by itself.

Sunburn Prevention

Exposed limbs can be badly damaged by sunburning. Whenever defoliated trees have not grown enough new leaves to protect the limbs before hot weather occurs, you should provide protection.

Protection is best provided by spraying or painting all exposed limbs with either a cold water white paint or a whitewash. One good whitewash formula would include 50 lbs. hydrated lime and 4 lbs. zinc sulfate to each 100 gallons of water.

Irrigation

Do not irrigate frosted trees until the soil in the root zone approaches dryness. The loss of leaves reduces the use of water so the soil will remain wet longer than with unaffected trees. Careful, frequent examination of the soil is necessary to prevent excess moisture from normal irrigations. Avocado root rot occurs in soils with excessive moisture when the cinnamomi fungus is present, and growers must guard against this disease following frost damage.

Pruning

Do no pruning until you know how much of the tree has been killed. No foliage will grow from the remaining live wood and the tree will recover better without pruning.

When new shoots are at least two or three feet long, you can remove the dead wood. This will usually be mid-summer, 6 to 8 months following the frost. At the same time, suckers should be thinned out to select the new limbs to replace those lost. Large pruning cuts (3 inches or more) should be painted with an asphalt emulsion, or other sealing paint, to prevent drying and infection.

Care of Young Trees

Badly frozen young trees usually develop strong sucker growth that can be used to form a tree as good as a replanted tree. If these suckers are from above the bud union, you can develop a new top by thinning and training. On young trees frozen back to below the bud union, strong root suckers can be budded or grafted to the desired variety the following spring. If the sucker growth is weak, the tree should be removed.

Severely Frozen Mature Trees

The handling of severely frozen mature trees where they have been killed back to the large scaffold limbs, to the trunk, or to the ground, presents many problems. Each tree should be considered separately. Often growers should topwork badly frozen trees to a more resistant or productive variety. Ask your Farm Advisor to assist you in determining the best procedures.

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.

You see the darndest things in orchards, aside from all the junk that people throw out of their cars.  Here's a case where someone had a little time on their hands and tattooed some fruit in the field.  Apparently it was some pointed object that scared the skin and then it healed over. Maybe this is a new marketing opportunity for Meyer lemons.

photo by S. Caughey