Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: frost

New Avocados for the Valley?

California avocados are the best in the world. So says downtown restaurant manager Daniel Avalos in a Valley Public Radio story by reporter Ezra David Romero.

The fact that they currently thrive only on a small swath of coastal Southern California is being challenged by UC Cooperative Extension specialist Mary Lu Arpaia. She is on a mission to find avocado varieties that withstand the hot summers and cold winters of the San Joaquin Valley, where irrigation water and crop land are more abundant and cheaper. 

She hopes to find avocado varieties that ripen at various times of year, and varieties that might be an alternative crop for citrus growers should huanglongbing, a disease that has devastated the Florida citrus crop, take hold in Central California.

"There's a void of California fruit on the market in the months of November, December and actually early January," Arpaia said. "So if we can find different selections that maybe are unique that fit into that window, then we help the entire California avocado industry."

An as-yet unnamed avocado variety. 

Romero visited the UC Lindcove Research and Extension Center to see the trees in Arpaia's study. Currently, the vast majority of California avocados are the Hass variety. The goal is to breed varieties with similar eating quality that grow to a moderate height and have high yield. One potential that is already being produced by nurseries is called "gem."

"This is gem," said Eric Focht, a staff research associate in Arpaia's lab. "You can see it's a little more oval or egg shaped than Hass. It has the speckling on the skin. Now as this ripens, it will turn dark and a lot of times the speckled lenticels with get a yellow kind of golden color it it."

Another promising variety is called "lunchbox" because of its small size. According to Focht, it "just falls out of the skin." Arpaia said, "It makes wonderful guacamole and I found, with a non-replicated test in my refrigerator, the fruit doesn't brown."

Arpaia's favorite guacamole recipe is featured at the end of the story on the KVPR website. And there is more on this story at:


Mary Lu Arpaia

Mary Lu
Mary Lu

Posted on Friday, May 12, 2017 at 5:46 AM
  • Author: Jeanette Warnert
Tags: avocado (276), California (12), citrus (311), freeze (9), frost (17), heat damage (4), irrigation (78), lemon (94), new crops (6), new variety (1), nutrients (21), nutrition (5), oranges (6), temperature control (1)

Cold Nights in Ventura

By way of Ventura grower Sandy Hedrick.  This is a compilation of nights of "firing" done by Farm Advisor Bob Brendler from 1923 to 1988 that was collected by Terry Schaeffer of The Fruit Frost Service.  Can you imagine over 70 nights of freezing temperatures when the smudge pots had to be lit?  Now we don't normally "fire" the pots, but use water and wind more commonly.  And it's only for a few nights at the most during the winter. In the "old" you couldn't plant avocado or lemon east of Sycamore Rd, just east of Santa Paula.  Now there are avocados planted in Fillmore.  And apple bloom erratically out there for lack of chill hours. And in summer it's getting too hot to grow strawberries even on the coast because with temperatures above 80 deg there is poor pollination.

Brendler loved weather information  and irrigation issues. He was the first person in the Ventura office to use a computer and he was the oldest Advisor in the office at the time.  The weather data chart was the first type of chart that could be done on a computer.

Do you think something has changed?  By the way Terry is still running a private weather service for growers in Ventura.

Click on the link: Temp Chart002     



Temp Chart003
Temp Chart003

Posted on Friday, May 27, 2016 at 6:21 AM
Tags: avocado (276), citrus (311), cold weather (1), frost (17), orchard heaters (2), smudge pots (2), winteer (1)

Frost in the Forecast?

It is that time of year and we should be alert to threat of freezing weather and damage to trees. Last winter was one of the warmest on record, but there was still a sneak cold blast around the New Year that caused some problems in some areas. Wet winters tend to have lower frost threats, and even though wet is forecast for this winter, that rain is not forecast until late in January. That still leaves December and early January which historically when most of our damaging frosts occur. Fox Weather on the CA Avocado Commission is forecasting some cold weather coming up, so growers need to be prepared for the worst.


Here are some links to frost information, preparing for frost and managing frost damage to trees.

A Frost Primer

Methods of Frost Protection

Protecting Avocados from Frost

Rehabilitation of Freeze-Damaged Citrus and Avocado Trees

The forecast is for north winds, which often means cold, dry air and often with winds. Winds mean no inversion and no warm air that can be introduced at ground level to warm trees. If this occurs, running a wind machine can make the damage worse. Wind machines and orchard heaters work on the principle of mixing that warmer air higher up – 20-100 or so feet higher than ground level which has colder air. When temperatures drop, the air is dry (wet-bulb temp below 28 deg F) and there is no inversion, running a wind machine can just stir up cold air and cause worse conditions (freeze-drying). It's better to not run the machine. The only thing left to do is to run the microsprinklers during the day so that the water can absorb the day's heat. Then turn the water off before sunset so that evaporative cooling from the running water isn't accentuated. Then when temperatures drop near 32 at night and the dewpoint is much below that, it's time to start the water again and let it run until sunrise (when risk is less). Running water works even if the water freezes. This is due to the release of heat when water goes from liquid to frozen state. This 1-2 degrees can mean the difference between frost damage and no damage. Also, ice on fruit and leaves can insulate the fruit. As the ice melts at the surface of the plant, it releases heat, protecting the plants. If there is not sufficient water to run the whole orchard, it's best to pick out the irrigation blocks that are the coldest or the ones you definitely want to save and run the water there continuously. Running the water and turning it off during the night to irrigate another block can lead to colder temperatures in both blocks.


Keep warm this winter.

freeze damage citrus 3
freeze damage citrus 3

Posted on Thursday, December 10, 2015 at 9:01 PM
Tags: avocado (276), citrus (311), damage (23), freeze (9), frost (17), lemon (94), orange (67), winter (6)

Rehabilitation of Freeze Damaged Subtropical Fruit Trees

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?


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.


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 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:

Light Damage Medium Damage Severe Damage Extreme Damage
Where only the foliage and small twigs are injured,pruning is not required Where a considerable part of the top has been killed but the trunk and main crown limbs show little damage, branches should be removed back to living wood above vigorous sprouts

Where the top and crown limbs are severely damaged but there are sprouts above the bud union, the tree should be cut back to the uppermost sprout

Where trees are killed to the bud union or the rootstock has been girdled, the trees should be removed and replaced with new trees


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 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.


citrus frost
citrus frost

Posted on Tuesday, January 13, 2015 at 5:16 PM
Tags: avocado (276), citrus (311), cold (3), freeze (9), frost (17), subtropicals (8)

Frozen Citrus Fruit and Young Trees

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.


citrus frost
citrus frost

Posted on Wednesday, December 11, 2013 at 7:48 AM
Tags: citrus (311), cold (3), damage (23), freeze (9), frost (17), fruit (18), winter (6)

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