Evaluating Damage to Baby Trees Requires Patience

Earlier temperatures and forecasted temperatures do not appear to be cold enough to freeze baby trees in the citrus belt. Time will tell. Semi-dormant wood in the winter looks dry even when it is healthy, so any final evaluation should be conducted in the warmth of spring. With warmer temperatures, frozen bark will peel easily from the young trunk and the degree of damage easily estimated. A tree, even those frozen down to the top of the wrap, can make an amazing recovery. Growers, in the summer after the 1990 freeze had some success budding onto the rootstocks that remained after the scions were killed by frost.

It may be better to replace a tree if it is still alive under the trunk wrap but badly damaged. Badly frozen trees regrow fairly slowly, and often are not able to resist pathogens that grow into the wood such as fungal Fusarium species causing dry root rot. Slow growing Fusarium in the wood can take up to 10 or 15 years to kill a tree.

Badly Frozen Young Trees

Badly frozen fruit may start dropping from the tree shortly after the freeze, but other fruit may hang on the tree longer than unfrozen fruit. Many growers resist picking or dropping frozen fruit in that it is another expense, at a time of little income. Reasons for dropping the fruit, even if it cannot be sold for juice, include:

Ensuring that the frozen fruit does not interfere with spring fruit set. Navel oranges, for example, will not set as much fruit if last season’s fruit remains on the tree.

Old frost-damaged fruit may harbor fungal pathogens that may infect the new crop, such as clear rot (Penicillium sp.), tear staining (Colletotrichum sp), brown rot (Phytophthora sps.) or Septoria organisms.

Avoiding having to separate last year’s partially frozen fruit from the new crop at harvest next year.

Preventing partially frozen fruit from providing habitat for insect pests.

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.

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.

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.