Posts Tagged: freeze

Frost in some part of Southern CA

Protecting Avocados from Frost and Then What

The long-range forecast is for no or little rain in Southern California. A dry winter often means a cold winter. A winter when frost and freeze can occur, And over the years, it seems like if there is a freeze in Chile in their winter, we can get one here, but 6 months later. Chile had a freeze this year.

The following is an outline of how a grower can prepare for cold events. Some require considerable advance preparation – plant the right variety – and some can be instituted right away – protection of young trees with palm fronds. The guidelines here are specific to avocado, but for the most part apply to lemon and other cold sensitive citrus.

Frost damage reduces tree growth and fruit yields. Thus, it presents a problem to the avocado grower who must develop and maintain high production to have a profitable business.

Before choosing frost protection equipment, however, the individual grower should ask the question: “Will the fruit and foliage saved pay for the trouble and expense of protection?” In some places, a small amount of damage may not fully justify the installation and operation of frost protection equipment. In other places, avocados can be grown only if expensive heating is done. Nearly all locations have sufficient cold to cause the grower to take into account probable frost damage, possible methods of protection, and the long-range economic effect of his frost situation.

How Frosts and Freeze Occur

Before considering the methods of frost protection, one should understand the conditions under which low temperatures occur. To differentiate between the two major sources of the cold, the terms local radiation frost and freeze are used.

Local radiation frost

Frosts are caused by a cooling of the objects on the earth's surface during the night. This loss of heat from the foliage and soil is called radiation. Heat accumulated from sunshine during the day is radiated to the sky during the night. The cooled objects chill the surrounding air and usually the coldest zone is near the ground surface. Air temperatures are warmer as one goes higher until a height is reached where they are at a maximum for that location. This is called the ceiling. The temperature and height of the ceiling is different for each place and situation.

When the ceiling is low, warm air is nearer the ground than when the ceiling is high. This phenomenon, occurring frequently in Southern California, is called temperature inversion and accounts for this area's ability to protect its orchards.

Colder air settles near the ground and then moves (flows) down the slope to still lower ground resulting in what is called the drift.

Terrain features and winds influence its direction and velocity. A strong drift maintains air temperatures higher than those existing when little or no drift is present.

Freeze

The other type of freezing weather occurs when a large mass of cold air moves in from the north. Fortunately, these freezes have not occurred frequently. The major ones were in 1913, 1922, 1937, 1949, and 1950. Under freeze conditions, temperatures usually go below those experienced during local radiation frosts. When this cold air moves in, high ground is no warmer than lower ground and there is little or no ceiling. Orchard protection at such time is difficult, and methods employing the addition of heat are most effective.

Measuring Temperatures

Orchard temperatures are usually measured with a specially designed self-registering minimum thermometer mounted in a standardized shelter. In this way, readings are comparable and experience allows the grower to use these air temperature readings in judging his hazard and operating his equipment. The height of the thermometer above ground should be five feet in mature orchards and two feet in nurseries and recently planted orchards.

Most orchards have areas which are colder than other areas. You should have enough thermometers to measure temperatures in each of these areas. In establishing thermometer locations in a new orchard, set up one thermometer in a permanent reference location. Move the other thermometers around every few nights until you find the cold spots. With this experience, you can locate and determine the number of thermometers needed. The range in numbers will be from a minimum of two or three for small groves to one for each three to five acres in larger orchards.

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 ‘Hass', the first evidence of damage will show up as a browning on the stem a few days after a frost. If this is the only damage, mature fruit might be picked and marketed. This all depends on the packer. The problem often is assessing how extensive the damage is. 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.

Stem Damage

 

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.

Water soaked and frosted neck, Mary Lu Arpaia

Brown vascular bundles, Mary Lu Arpaia

 

 

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

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. It is still not clear how much cold ‘GEM' variety can handle.

 

Table I – 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, Edranol, Rincon, Lamb Hass, Sir Prize	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.

Methods of Protection

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 installation and operation. 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 grove.

Their disadvantages are:

• Smokiness. However, certain types have low smoke output when properly operated.
• Relatively high costs of investment and operation (Table II).
• 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. Pie 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 advantages are:

• More economical than orchard heaters (Table III)
• 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.
• Increase 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.

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

Use of palm fronds for both sun and frost protection, Leo McGuire.

 

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 tree s 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 latex paint or a whitewash. The paint needs to be white enough to reflect the light, but thin enough to flow through a sprayer. With latex paint, a 2-part latex to 1 part water is usually adequate.

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 cinnamon 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. New 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.

 

Care of young trees

Badly frozen young trees usually develop strong sucker growth which 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.

Whitewashed and topworked frozen tree

 

 And a lot more information from the CA Avocado Commission

 

Frost and Frost Protection Source of Information

You wanted to know all about Frost, well it's all here.  Emeritus Rick Snyder put it together over the years and it's at your finger tips.

English Frost Protection Training Units (videos)

Spanish Frost Protection Training Units (videos)

When to Turn Sprinklers On and Off for Frost Protection

FP001 Quick Answer—This quick answer provides information on using a psychrometer or temperature and dew point data to determine when to start and start sprinklers for frost protection.

Predicting Temperature Trends during Freeze Nights

FP002 Quick Answer—This quick answer gives a method for predicting the change in
temperature during a calm, radiation freeze night.

A Simple Method to Measure the Dew Point Temperature

FP003 Quick Answer—This quick answer provides information on how to measure the dew point for use in estimating minimum temperature and for starting and stopping
sprinklers for frost protection.

Sprinkler Application Rates for Freeze Protection

FP004 Quick Answer—This quick answer provides information on the sprinkler
application (precipitation) rates needed to protect crops from freezing.

Principles of Frost Protection

Frost Protection Training Units (videos) on measuring temperature, frost protection with water, frost protection with wind machines, and passive protection. Traning units are availble in English and Spanish .

FP005 Quick Answer—This quick answer provides information on the general principles of well-known frost protection methods.  A PDF file of the WEB page can be uploaded from this Quick Answer.  In addition, a shorter version is available.

Programs for Estimating Frost Night Minimum Temperatures and Temperature Trends (new 7 Mar 2007)

The FFST Excel application programs FFST_E.xls and FFST_M.xls are available from this link. The FFST application helps users to determine an empirical equation for estimating minimum temperatures during radiation frost nights. Note that the program will provide good estimates if there is little or no wind, no significant cold air drainage, and no clouds.

The FTrend Excel application programs FTrend_E.xls and FTrend_M.xls are available from this link. The FTrend application program is used for estimating changes in air and wet-bulb temperatures during a frost night. The application is useful for predicting when to start sprinklers for frost protection and for other methods as well.

Estimating Risk of Frost Damage from Climate Data (new 8 Jul 2006)

One of the programs included with the book Frost protection: fundamentals, practice, and economics. Volume 2, which is available from the UN FAO is the Frisk program. Information on how to obtain the book is presented above on this web page. Based on reader feedback, some improvements have been made to the Frisk application since the book was published. Updated versions of the application are made available to the public here.  The Frisk program is an MS Excel application that is designed to help growers to make decisions on the risk of frost damage in a particular location.  The user inputs the local climate data and a critical damage temperature, and the application computes the probability of temperature falling below the critical temperature on each day of the year. These data are then used to estimate the length of the growing season relative to the critical temperature by presenting a graph of the probability that the growing season will be less than a specific number of days.  The application also computes the certainty that no air temperature will fall below the critical damage temperature after a selected date in the spring or before a selected date in the fall for design periods of 5, 10, …, 30 years. This certainty of no events is equivalent to the risk of having one or more events during the same time period. Although Celsius temperature was used in the example application programs, the program works equally well using Fahrenheit temperature. To obtain a copy of the application, click on FriskNH.xls for the northern hemisphere or FriskSH.xls for the southern hemisphere. Note that metric units were used in the Frisk application sample data.

 

Public Value: UCANR: Promoting economic prosperity in California

 

It Could Still be a Cold Winter

A moderate La Niña climate phase is expected through spring 2021. This indicates that slightly above average temperatures and slightly less rainfall than normal can be expected in California's avocado/lemon growing areas.

The fact that the winter weather pattern is expected to be warmer than usual doesn't rule out the possibility of a freeze. A freeze can occur any winter, regardless of the climate phase. And dry winters are often susceptible to sudden cold spells because of the lack of tempering effect of soil wet from winter rains

La Niña is one of three climate phases that are part of the El Niño Southern Oscillation (ENSO) climate pattern. Others are El Niño, during which colder and wetter conditions are expected in California, and neutral, when conditions are neither El Niño nor La Niña.

California's most severe freezes have occurred in weak La Niña, weak El Niño or neutral ENSO phases. Severe freezes in Decembers of 1990, 1998, 2001 and 2006 occurred after droughty years and years of low rainfall. In 2015, it snowed in Temecula and again in 2020 – in February!!! So cold weather is still waiting out there.

The most damaging freezes for avocado and citrus are advection and radiation freezes. In advection freezes, cold fronts move arctic air through the region. Radiation freezes feature overnight clear skies and light to no winds with periods of calm. Cold pockets and cold locations will have lower temperatures during radiation freezes. Break freezes and unsettled freezes are the other types of freezes.

During a radiation freeze, cold air drains down and pools in low areas. Know the cold pockets in your grove, and keep that air flowing with a wind machine, if possible.

Read the collected works on frost protection, frost damage and frost recovery here:

https://www.californiaavocadogrowers.com/growing/cultural-management-library/freeze-protection

It's also a good idea to be ready for fires in years of low/no rainfall.

 

The 2021 Weather Forecast from Fox Weather provided by CAC is below:

 

 Image: Temecula snow on avocados in February

avocado snow Temecula

Frost in Your Future?

Protecting Avocados from Frost and Then What

The long-range forecast for no or little rain in Southern California. A dry winter often means a cold winter. A winter when frost and freeze can occur - https://www.californiaavocadogrowers.com/sites/default/files/greensheet-pdfs/Greensheet-Nov-2-2020.pdf

The following is an outline of how a grower can prepare for cold events. Some require considerable advance preparation – plant the right variety – and some can be instituted right away – protection of young trees with palm fronds. The guidelines here are specific to avocado, but for the most part apply to lemon and other cold sensitive citrus.

Frost damage reduces tree growth and fruit yields. Thus, it presents a problem to the avocado grower who must develop and maintain high production to have a profitable business.

Before choosing frost protection equipment, however, the individual grower should ask the question: “Will the fruit and foliage saved pay for the trouble and expense of protection?” In some places, a small amount of damage may not fully justify the installation and operation of frost protection equipment. In other places, avocados can be grown only if expensive heating is done. Nearly all locations have sufficient cold to cause the grower to take into account probable frost damage, possible methods of protection, and the long-range economic effect of his frost situation.

 

How Frosts and Freeze Occur

Before considering the methods of frost protection, one should understand the conditions under which low temperatures occur. To differentiate between the two major sources of the cold, the terms local radiation frost and freeze are used.

 

Local radiation frost

Frosts are caused by a cooling of the objects on the earth's surface during the night. This loss of heat from the foliage and soil is called radiation. Heat accumulated from sunshine during the day is radiated to the sky during the night. The cooled objects chill the surrounding air and usually the coldest zone is near the ground surface. Air temperatures are warmer as one goes higher until a height is reached where they are at a maximum for that location. This is called the ceiling. The temperature and height of the ceiling is different for each place and situation.

When the ceiling is low, warm air is nearer the ground than when the ceiling is high. This phenomenon, occurring frequently in Southern California, is called temperature inversion and accounts for this area's ability to protect its orchards.

Colder air settles near the ground and then moves (flows) down the slope to still lower ground resulting in what is called the drift.

Terrain features and winds influence its direction and velocity. A strong drift maintains air temperatures higher than those existing when little or no drift is present.

 

Freeze

The other type of freezing weather occurs when a large mass of cold air moves in from the north. Fortunately, these freezes have not occurred frequently. The major ones were in 1913, 1922, 1937, 1949, and 1950. Under freeze conditions, temperatures usually go below those experienced during local radiation frosts. When this cold air moves in, high ground is no warmer than lower ground and there is little or no ceiling. Orchard protection at such time is difficult, and methods employing the addition of heat are most effective.

 

Measuring Temperatures

Orchard temperatures are usually measured with a specially designed self-registering minimum thermometer mounted in a standardized shelter. In this way, readings are comparable and experience allows the grower to use these air temperature readings in judging his hazard and operating his equipment. The height of the thermometer above ground should be five feet in mature orchards and two feet in nurseries and recently planted orchards.

Most orchards have areas which are colder than other areas. You should have enough thermometers to measure temperatures in each of these areas. In establishing thermometer locations in a new orchard, set up one thermometer in a permanent reference location. Move the other thermometers around every few nights until you find the cold spots. With this experience, you can locate and determine the number of thermometers needed. The range in numbers will be from a minimum of two or three for small groves to one for each three to five acres in larger orchards.

 

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 ‘Hass', the first evidence of damage will show up as a browning on the stem a few days after a frost. If this is the only damage, mature fruit might be picked and marketed. This all depends on the packer. The problem often is assessing how extensive the damage is. 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.

Stem Damage

 

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.

Water soaked and frosted neck, Mary Lu Arpaia

Brown vascular bundles, Mary Lu Arpaia

 

 

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

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. It is still not clear how much cold ‘GEM' variety can handle.

 

Table I – 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, Edranol, Rincon, Lamb Hass, Sir Prize	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.

 

Methods of Protection

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 installation and operation. 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 grove.

 

Their disadvantages are:

• Smokiness. However, certain types have low smoke output when properly operated.
• Relatively high costs of investment and operation (Table II).
• 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. Pie 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 advantages are:

• More economical than orchard heaters (Table III)
• 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.
• Increase 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.

 

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

Use of palm fronds for both sun and frost protection, Leo McGuire.

 

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 tree s 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 latex paint or a whitewash. The paint needs to be white enough to reflect the light, but thin enough to flow through a sprayer. With latex paint, a 2-part latex to 1 part water is usually adequate.

 

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 cinnamon 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. New 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.

 

Care of young trees

Badly frozen young trees usually develop strong sucker growth which 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.

Whitewashed and topworked frozen tree

 

 And a lot more information from the CA Avocado Commission

Avocado Growers Gear up for the Cold - What Challenges will growers face this Season: Snow, frost, or Freeze?

More than 6 inches of snow fell on areas of Riverside County, part of the state's southern-most growing region, which produces nearly half of the nation's avocado supply. As much as a foot of snow fell on mountain ridges around Temecula and Murrieta the higher elevations of the De Luz area and surrounding region of San Diego and Riverside counties in late December 2014. The accumulation of snow snapped many limbs and branches. The cold and snow also damaged a significant amount of fruit. The De Luz area near Temecula was hardest hit in the higher elevations. It seemed. As if the storm blew in from the East. In some groves lost was estimated 80-90 percent and others we lost nothing. Over the next several weeks, some fruit on the trees turned brown or bronze indicating that it was also unmarketable. 

Frost vs. Freeze

The avocado tree, while quite adaptable to a range of climatic conditions, is considered to be sensitive to frost. Growers in regions where frost is likely will need to consider this when designing the layout and irrigation of their orchard. It's important to differentiate between a frost and a freeze. There is a lot of science behind frost, but in basic terms, frost is the formation of ice crystals on plants which more importantly can lead to the freezing of internal sap. Frost is the result of climatic conditions that lead to sub-zero temperatures. A frost is caused when objects cool during the night and radiate their heat loss, chilling the surrounding air. Cold air tends to settle near the ground while warm air rises. In Southern California, the “ceiling” is often low, which means that warm air is closer to the ground. This phenomenon is called a temperature inversion and it tends to protect orchards from the cold. That said, when forecasts indicate windy conditions this disrupts the inversion and thus the colder air is pressed to ground level. Knowing whether a temperature inversion is present or not is a key factor in determining whether to use wind machines as a preventative measure.

 

A freeze occurs when a mass of cold air moves in and the temperature of the air decreases at both ground level and in the higher air levels. Protecting groves in these conditions is difficult and usually involves adding heat to the grove with orchard heaters. When grove temperatures are cold (generally below 60-degrees Fahrenheit) and the day is sunny, avocado leaves can stress and bleach or yellow because they cannot metabolize the light being received in such cold temperatures. The leaves produce hydrogen peroxide, which damages the leaves.

For California avocado growers, January is the month that groves tend to be susceptible to frost and freeze damage. As the threat of colder weather arrives, it's important for growers to understand the mechanics of frosts and freezes, and what they can do to protect their groves from unseasonably cold conditions. Be aware of temperatures falling below 30-degrees Fahrenheit, regardless of weather forecasts. Rather than rely on forecasts, establish a frost-management plan, and follow it. 

Protecting Groves

To prevent frost damage in a mature grove, the two most-relied-upon methods are orchard heaters and wind machines.

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

 

Orchard heaters tend to distribute heat to all parts of the grove when well distributed but can be expensive to run and can present a fire hazard.

Wind machines are economical and less labor intensive and, when used with orchard heaters, can increase the effectiveness of the heaters. Wind machines should only be used in frost conditions — not during freeze conditions in which temperatures fall 4 or more degrees below damage points. In addition, wind machines should not be used in conditions (usually windy) in which no temperature inversion is present. If wind machines are used in these conditions, they can cause more damage because they simply stir up the cold air at tree level.

In situations in which there is no temperature inversion, the best practice is to run microsprinklers during the day and then turn off the water prior to sunset. As water freezes it gives off heat, so by providing water to freeze, you are generating heat. However, water cannot flow when it is frozen, so any under-tree irrigation systems must be started before the temperature gets to a point when the water freezes in the pipes or emitters. If temperatures drop below 32° during the night, the water should be started again and run until sunrise. Since it can be expensive to run water in San Diego County, growers can opt to choose those portions of their grove that tend to be the coldest in order to save money. When the temperature is below freezing, Dr. Faber recommends against running water in a block during the night and then turning it off to water another block as this will lower the temperature in both blocks and lead to damage. It should be noted that these methods are less effective for protecting young trees. To protect young trees, insulative wrap or frames with a burlap cover may be needed. Side and top cover is required to protect from frost, trunk guards can help prevent total tree death by protecting the main trunk.

Location, location, location! Choose a good growing location to minimize the chances of cold air. Cold air is heavier than warm air, so planting on a slope allows cold air to flow down and out of your groves. It is important to have space available for downhill air drainage of cold air. Trees that are planted more towards this area are more prone to damage, especially when the trees are young (up to 3 years of age) or if they have cold-sensitive cultivars. Orchard design is another important factor, aligning your rows to run up and down slopes rather than across slopes to allow effective air drainage. Cultivation practices can also help with battling frost. Maximizing tree health can also be beneficial for a grove susceptible to frost, a healthy, leafy tree will generate some internal heat and reduce the depth of frost burn into the tree. Unhealthy trees should be treated for root rot at the beginning of spring.  If not properly treated, these trees will flower excessively in spring; but fail to grow new shoots. If the leaves yellow during winter, consider reviewing your nutrient program. Weed control is an important pest management routine that should be taken seriously. A strip of bare, moist earth under and around the trees will absorb and release heat more than a weedy or dry site.

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.

Caring for Frost Damaged Trees

Immediately after a frost event the main thing to do is carefully monitor your soil moisture. As the tree may have been significantly damaged, the amount of water it will be using will be reduced. The last thing you need is to damage the root system as well through waterlogged conditions. Trees stressed by wet soils may have premature leaf drop, bear small and pale green leaves, and show signs of avocado shoot die back. Too much water during this time can cause root rot. If trees lose leaves to frost/freeze, it's important to protect exposed branches from sunburn with whitewash. Initially all you can do is to let the trees start to recover naturally. Once they have started to re-shoot in spring, then you can begin the task of removing dead wood and shaping/balancing the tree.

 

Post Freeze Grove Management and Harvest

According the California Avocado Commission (CAC), after a freeze, California avocado growers should look for the following freeze indicators:

• Firm, brittle, curled leaves
• Water-soaked or discolored wood or twigs
• Older branches and trunks splitting and losing bark (this happened a lot during the snow event in 2015)
• Discolored avocados, with hues ranging from barely visible bronze to black
• Brown buds and flowers

• Fruit stems can be killed or ring barked, causing heavy-fruit drop

Soon after leaves fall, whitewash defoliated trees with white-latex or lime-based paint that is water diluted (not too diluted, as paint should appear white on tree). This will protect damaged wood from sunburn, which can cause further damage. Do not apply dressings — or additional paint-on sealants — to cracked bark, as it might strengthen bacterial or fungal infections. Pruning can be postponed until spring or summer, when new growth develops. Survey damaged trees before applying fertilizer, to identify what minerals are lacking, as nitrogen applications are likely to trend downward. It is suggested that avocado growers withhold nitrogen fertilization until midsummer (or longer). However, zinc sprays often are applied to expanding young foliage.

Stem-damaged fruit will drop within seven to ten days of a freeze, it should be picked first, assuming it is mature and not otherwise harmed by the cold. While stem damage may be apparent, fruit might be unscathed.  When stems are completely girdled by a brown band, fruit will drop. Drops typically occur within seven to 10 days, following a freeze. Also, do not pick any discolored avocados (barely visible bronze to black).

 

Work Cited

1. California Avocado Society. The Weekly Newsline: Market news, opinions & Statistics for Avocado growers. [http://www.californiaavocadosociety.org/] Accessed 7 January 2015.
2. Dreistadt S. H. (2008). Integrated pest management for avocados (Vol. 3503). UCANR Publications.
3. Witney, G. W., & Arpaia, M. L. (1991). Tree recovery after the December 1990 freeze. California Avocado Society 1991 Yearbook, 75, 63-70.

 

Valley Lemons?

What about Planting Lemons in Kern County?

By Craig Kallsen, UC Cooperative Extension Advisor, Kern County

     Kern County is located at the southern end of the San Joaquin Valley of California. Over the past couple of years, I, as the citrus Farm Advisor for the University of California Cooperative Extension in Kern County, have received an increasing number of enquiries about the feasibility of growing lemons here. The answer is “yes” we can grow lemons here and according to the latest Kern County Agricultural Commissioner's Report (2017) we have 4010 acres of bearing and 10 acres of non-bearing lemons in the county.  Those 10 acres of non-bearing lemons indicate that fairly recently someone decided lemons were the way to go.

    These inquiries as to the feasibility of growing lemons are understandable. The price and demand for lemons in the U.S. and worldwide is increasing.  Depending upon where you get your statistics the retail prices of lemons was something like $1.50 per pound from 2011- 2013 to something like $2 a pound from 2015 – 2017.  The statistics show 2018 was even a better year for selling lemons.  Consumption of lemons in the U.S. was less than 1 million metric tons in 2011 to about 1.25 million metric tons in 2017.  Worldwide consumption has increased from about 4.5 million metric tons in 2011 to 5.5 million metric tons in 2017.  If you add in other factors such as a heat wave, which, for example, hit Ventura County production hard in July 2018, or extreme winter freeze events, and sometimes-erratic supplies from other lemon producing areas of the world, prices can skyrocket 40% or more in a month. Being able to sell a carton of lemons for excess of $55 can be very attractive to prospective growers. Not surprisingly, if you compare the cost and returns of growing lemons with those of oranges, a person might wonder why anybody would choose producing navels over lemons (see https://coststudies.ucdavis.edu/ ).

     Planting lemons is riskier. In the San Joaquin Valley, the major consideration is the greater frost sensitivity of lemons as compared most other citrus crops.  Not only do lemons freeze at a higher temperature, so do its branches.  A freeze, which can spoil orange or mandarin production for a year, can devastate lemon production for three years due to increased damage to the lemon canopy and the older branches of that canopy. If your tree freezes back to the major scaffold branches, you are out of business for a while. An important question is how often does it cold enough to destroy my lemon production capacity for three years or more?  Industry wide, for the last 30 years we have had three freezes where lemon leaf canopies, even in the warmer areas of Kern County, were severely damaged –  December 1990- January 1991, December 1998, and January 2007.  Not to be an alarmist but, in looking at these dates, it would appear that we may be overdue for an extreme freeze.  We flirted with one in early December of 2013.  Over the years, I have noticed that as the time interval increases from the previous frost event, citrus orchards move further and further down onto the valley floor, only to retreat to higher ground after the next severe event.

      Well, what about global warming?  Shouldn't Kern County be getting to be a safer place to grow lemons?  In answer, predictions can be difficult, and according to baseball legend Yogi Berra, this is especially so if they are about the future. Winter air temperatures have been climbing over the past 30 years in the southern San Joaquin Valley. With our Mediterranean climate in the SJV, most of our rain falls during the fall and winter.  Drought years, which means drought winters, have become more common. The higher winter temperatures are good news for citrus growers, but the droughts have been bad news in that dry air in not conducive for fog formation.  Fog, historically, is our winter blanket, that holds temperatures above freezing when conditions are ripe for rapid drops in temperature associated with clear, windless nights following cold fronts that move into the valley from Alaska and other points north.  

       The risk in growing lemons can be mitigated. As with any real estate endeavor, the three most important factors governing the value of a prospective lemon property are location, location and location. When we are talking about cold temperatures, we are talking about nighttime low air temperatures. Daytime winter temperatures, once we get into mid-morning, usually, are more than warm enough to keep lemons from freezing.  The major mitigation factor under human control is to plant lemons in the areas of Kern County that have the warmest nighttime temperatures.  These areas tend to be on the lower slopes of the foothills on the eastern and southern areas of the SJV.  Cold air is much heavier than warm air and runs like a river downslope. Good cold drainage is necessary.  If lemons are planted too far out onto the valley floor, they end up at the bottom of a lake of cold air during late fall and winter freeze events. The area where citrus is grown, often, is referred to as a belt along the lower foothills of the SJV.  Not only is this belt characterized by more fog than higher up in the foothills, but also it is close to the atmospheric inversion layer that forms in the SJV during the winter. The SJV is at the bottom of a large deep bowl formed by surrounding mountain ranges, and the depth of this bowl makes the air more difficult to disturb by wind. This still air, on cold, clear nights during the winter, allows heat radiating into the sky from the ground to warm a layer of air, usually located from 500 to 1000 feet above the valley floor. The idea of using wind machines successfully is to move this layer of warm air down to the trees on the ground.  If you are down on the valley floor, on most nights the warmer air is way too high up to bring it down to the ground with wind machines.  If an orchard is 500 feet above the valley floor on the side of a foothill, you might already be in the inversion layer and won't even need to start your wind machines, or at worst, the inversion layer is close enough to bring that warm air down to the trees with wind machines.  Unfortunately, the amount of land winter-warm enough for growing lemons in the foothills is very limited, and, currently, is occupied by other crops, probably citrus. We cannot grow lemons too high up in the foothills, because these areas are above the inversion layer and winter temperatures there will always be too cold for lemons.  Kern County, in general, appears to be colder than its neighbor Tulare County to the north, and usually suffers more in terms of fruit and tree losses during extreme frost events.

    Those bold enough to grow lemons appear to have more choices on which lemon to grow now than in the past. Some newer seedless or lower-seeded lemon varieties are available (https://citrusvariety.ucr.edu/ ). The Lisbon lemons, of which there are several selections, is an old Kern County standby, and appears to have better frost tolerance than the Eureka, commonly grown in the central and southern coastal areas. The Improved Meyer lemon is a hybrid, apparently, with citron, mandarin and pummelo heritage, and has excellent frost tolerance.  However, the fruit does not hold up well on the tree, in storage or ship very well, and few commercial groves exist. It remains a very popular and successful backyard tree for homeowners.

     With the threat of the Asian Citrus Psyllid (ACP) and the Huanglongbing disease it spreads, the feasibility of growing citrus under protective screens (CUPS) is under investigation.   These protective screens, in addition to keeping ACP out, would likely provide additional frost protection as well.

      The other obvious concern related to the number of enquiries I have received, is that even if lemons are not widely grown in Kern County now, worldwide demand suggests that there are likely many new acres of lemons in the ground now or in advanced planning stages in other locations in California, Arizona and the world.  In the past, we have seen the acreage of a number of crop commodities rise and fall with the laws of supply and demand. We have planted and then pulled lemons in Kern County before based on market conditions.  At some point, even unfrozen lemons will not sell if there are too many out there.

Figure 1.  Frozen mature lemon trees in photo background, after the 1998 freeze in the Edison area of Kern County.  Juvenile, undamaged navel orange trees in foreground (photo by Craig Kallsen).

 

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