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

Posts Tagged: frost

Netting in Your Orchard's Future?

At a recent meeting for current and prospective avocado growers near Visalia, Yosepha Shahak a retired researcher from Israel's Volcani Institute presented information on photo-selective netting. This netting was an outgrowth of netting that is used in the Mediterranean region to protect crops from frost damage and the unpredictable hail storms that can occur just as fruit might be coming to harvest. Netting is currently used in commercial orchards and vineyards throughout Europe. San Joaquin Valley growers like the idea of frost protection.

Netting over loquats (nespero) in Spain (Espana)

Netting over apples in Australia

Netting for light modification in Israel.  Tractors can work here.

Photo-selective netting refers to covering crops by nets having the capacity to selectively filter the intercepted solar radiation, in addition to their protective function. The technology is based on plastic net products into which light dispersive and reflective elements are introduced during manufacturing. These nets are designed to screen various spectral bands of the solar radiation, and/or transform direct light into scattered light. The spectral manipulation intends to specifically promote desired physiological responses, which are light-regulated, while the scattering improves the penetration of the modified light into the inner plant canopy. So, depending on the crop, more and better fruit set, bigger fruit and some other desirable properties. The netting can also substantially reduce evaporative demand and wind damage.  This can lead to not only lower water use, but also such water stress related diseases, such as blight caused by Botryosphaeria fungi.  Lower evaporative demand and less water application can lead to less salt damage.

A recent additional aspect to the photo-selective nets refers to their effects on pest behavior. The photo-selective netting concept was developed and tested in Israel in ornamental, vegetable and fruit tree crops. It is gradually spreading all over the world, for implementation in different crops, climatic regions and cultivation methods. Applying it to avocado orchards is going to require pruning and keeping trees so that they can be picked and pollinated.  And would probably lead to high density orchards.  

And how we do pest management – more or less, and maybe not by helicopter?

This might also be the future for how citrus is grown in an HLB environment.  24 sprays a year to control ACP in Florida -Yikes.


A link to a Shahak talk that she gave to Washington state apple growers can be found at:


Posted on Monday, December 11, 2017 at 7:39 AM
Tags: ACP (86), asian citrus psyllid (57), avocado (289), citrus (338), exclusion (1), frost (19), hail (1), HLB (71), huanglongbing (66), lemon (100), light modification (1), netting (1), orange (69)

Central Valley Avocado Meeting


 Interested in San Joaquin Valley Avocados?

 When:  NOVEMBER 28, Tuesday, 1PM


UC Lindcove Research and Extension Center Conference Room (22963 Carson Ave, Exeter, CA 93221), Central Valley.


Tentative Program:

Welcome and Introductions – Mary Lu Arpaia, UC Riverside

Challenges to growing avocados in the Valley

Irrigation, Fertilization and Soil Requirements – Ben Faber, UCCE, Ventura County

Avocado Root Rot and how to manage – Greg Douhan, UCCE, Tulare County

What is the California Avocado Commission and the Hass Avocado Board? – Tim Spann, California Avocado Commission, Irvine, CA

Results from the Tier 3 varietal evaluation block at UC Lindcove REC – Mary Lu Arpaia and Eric Focht, UC Riverside

Ideas for the Valley Avocado Industry – Group Discussion


Walkthrough of the Tier 3 varietal evaluation block



RSVP to Diana Nix (

For more information contact Mary Lu Arpaia (


avocado cluster
avocado cluster

Posted on Wednesday, November 15, 2017 at 6:08 AM
Tags: avocado (289), Central Valley (2), cold tolerant (1), freeze (10), frost (19), subtropical (2)

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 (289), California (13), citrus (338), freeze (10), frost (19), heat damage (4), irrigation (79), lemon (100), 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 (289), citrus (338), cold weather (1), frost (19), 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 (289), citrus (338), damage (24), freeze (10), frost (19), lemon (100), orange (69), winter (6)

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