Avocados are a semi subtropical tree that can be found in high elevations of cloud forests in Mexico and Central America. A delicious and highly nutritious fruit, the cultivation of avocado has spread around the world. In California, growers are having commercial success in areas with year-round mild climates, such as San Diego, Riverside, Santa Barbara, Ventura and San Luis Obispo counties. Avocados evolved in a very moderate climate and are vulnerable to extreme weather. As a result, they are poorly adapted to the high temperatures and low humidity that can sometimes occur in California, especially in Southern California. In July 2018, there was a historic heatwave that caught everyone off guard, some areas in San Diego reached temperatures of 118-122 °F with winds up to 30 miles per hour. Fortunately, most growers were well into harvesting or just about done during that time, so the 2018 harvest wasn't as impacted as one first thought. Parts further north with later harvests were significantly affected.
Under regular conditions, an avocado tree is extracting water from the soil through its roots, which moves through the tree and exits through leaf pores known as stomates. A stomate is a group of cells in the epidermis made up of guard cells and subsidiary cells. Guard cells are a pair of kidney-shaped cells that form the opening of the stomate. When the guard cells are turgid (full of water), they are open and allow the entry of carbon dioxide for photosynthesis. However, this also exposes the leaf to potential water loss. When the guard cells are flaccid (less water) usually due to high temperature (when temperatures usually rise above 88 °F) and low humidity, this slows the loss of water and to prevent air exchange. The opening and closing of stomates is driven by plant hydration, temperature, relative humidity and wind. The loss of water from a plant's leaves through the stomates acts as an evaporative cooler, maintaining leaf temperature close to that of air temperature. The movement of water through the roots and out the leaves also cools the rest of the tree, preventing the trunk and branches from overheating.
Unfortunately, it seems that heat waves always occur during the peak of the season. Most trees are starting to begin to produce that year's summer flush that will become the following year's bloom which will set to produce the succeeding crop load. Fruit and flowers normally drop if the temperature is 100 °F or above. Thus, the effects of a heatwave can impact several years.
The most sensitive varieties to that heat were the ‘Hass' and ‘Fuerte', whose new leaves shriveled like plastic held to fire. Less affected were the varieties of ‘Sir-Prize' and ‘Sharwil', which had blackening and curling on some new leaves. Some of the toughest were the ‘Pinkerton', ‘Lamb Hass', and ‘Reed'. They showed nearly zero evidence of having been uncomfortable under such a terribly strong sun. Keep in mind, however, that how well any variety of avocado will do under high temperatures is mainly dependent on how well-watered the tree is, in addition to how healthy its root system is overall.
According to Tim Spann from the California Avocado Commission (2018), he suggests that there should be every attempt made to harvest fruit when temperatures are below 90 °F, and no harvesting should take place when temperatures exceed 95 °F. Temperature in the shade should be monitored during harvesting and, when possible, harvesting crews should be moved to the coolest, least exposed areas of the grove. This prevents heat damage to the fruit.
Field bins should be placed under the trees while being filled to protect the harvested fruit from sunburn. Once filled, bins should be moved to a shade structure (open-sided roofed building), or covered with bin covers or light-colored tarps if they cannot be immediately transported to the packinghouse. You should never cover the bins with tree scaffolds or branches, due to the potential spread of Polyphagous Shot hole borer. The insect can potentially hitchhike in the bins and be transported to another grove or the packing house. Also, never leave filled bins exposed to the direct sun. The surface layer of fruit can easily heat up to more than 15 °F above ambient temperature when exposed to direct sun. Acute sunburn will only show on fruit after it is packed and is a major quality detractor. To avoid water loss and decreased fruit quality, do not hold fruit too long after harvest. Transport fruit to the packinghouse at least once per day, if not twice daily (Spann 2018). Bins should not be left in the grove for more than 8 hours after harvest. Cover bins during transport to avoid sunburn and to reduce water loss
Watering enough is absolutely the most important thing you can do to help an avocado tree cope with heat above its comfort zone. Growers should be irrigating their trees in advance of the heat, to ensure that their trees are fully hydrated. An additional 50% of the budgeted amount of water is recommended to be applied the day before a heat wave (Spann 2018). For extended heat waves, daily pulses of irrigation are recommended to maintain the trees' water status and reduce air temperature. A well-watered tree will tolerate the stress of a heat wave much better than a tree that is suffering from water stress.
Cover crops provide a multitude of benefits based on species selected. Growing cover crops can increase soil organic matter, improve soil structure, enhance nutrient cycling, aid in weed suppression, provide habitat for beneficial insects and pollinators, and build on- farm resiliency to climatic changes (Rowe 2019). For avocado growers in Ventura County, improving water infiltration is a noticeable benefit. All growers that had reported issues of runoff prior to cover cropping have seen dramatic improvements in retaining water in the soil. The ground cover also helps cool the soil and lower air temperature. For a drought prone area and sensitive avocado trees, this could be the difference in surviving a 120 °F heat wave.
Avocado roots grow right at the surface of the soil. Mulch keeps that level of the soil comfortably cool and hospitable for avocado roots. The more roots a tree has, the more water it can draw up into its canopy to cool itself. Growers should apply between 3 and 8 inches of organic mulch, such as natural bark chips, straw, or compost, is ideal. Mulch also provides an insolation from sunlight and can reduce air temperatures.
Chalker-Scott, L. The Myth of Hot-Weather Watering “Watering plants on a hot sunny day will scorch their leaves” https://s3.wp.wsu.edu/uploads/sites/403/2015/03/leaf-scorch.pdf. Accessed 11 May 2020.
Downer, J and B. Faber (2019) Mulches for Landscape. UC ANR Publication 8672
Rowe, A. 2019. Panel Explores Avocado Cover Crops. The Grove. Fall 2019. Pp 28-31.
Spann, T. 2018. Avocado Heat Damage Follow-Up. The Grove. Fall 2018. Pp 22-25.
Image: Heat damage to avocado leaves
From Fox Weather by way of CA Avocado Commission, hot weather is forecast for mid- to late- June
This is a time to make sure that trees are adequately hydrated prior to the heat spell. Once trees start losing water through transpiration, it's hard for them to absorb water and heat stress and sunburn damage can result. The trees need to be fully water, so that they can continue to transpire to cool themselves during the heat spells.
And don't forget people in the field:
- Be sure shade is available on demand when the temperature is below 80 degrees F, shade must be provided at all times when the temperature exceeds 80 degrees F, as close as practicable to where employees are working;
- Shade must be provided to all employees on a rest or meal break,except those who choose to take a meal break elsewhere (editor's note: provision of shade as usual may not be consistent with social distancing recommended by various COVID-19 guidance; ag employers implementing heat illness shade requirements can ensure adequate shade consistent with social distancing requirements by staggering meal and rest breaks, but additional shade may be necessary);
- Fresh, pure, and suitably cool water must be made available in sufficient quantities (replenishment is permissible) to allow each employee to drink one quart per hour;
- Water is to be provided as close as practicable to location of work;
- Employees must be trained about heat illness and the Cal/OSHA Heat Illness Prevention (HIP) Standard before they work in conditions where they might be exposed to heat;
- Supervisors must be additionally trained in HIP compliance procedures, emergency responses, and ensuring effective communication to facilitate emergency response.
- A written copy of your HIP program in English and the language understood by the majority of the employees and be available to employees and Cal/OSHA inspectors on request — this is the most frequently-cited part of the HIP standard — and probably the most easily-avoided HIP citation!
- Remember: When temperatures exceed 95 degrees, employers must implement “high heat” procedures, including a mandatory 10 minute break every two hours (meal and rest periods can serve as these breaks, but if employees work beyond eight hours or waive meal or rest periods, you must still ensure the mandatory rest break occurs).
30-Day Weather Outlook for May 31, 2020, to June 22, 2020
Summary- The prevailing pattern is a high pressure ridge from N California westsouthwest or southwestward. Cold fronts coming S through California will tend to extend southwestward from southcentral-S California to the area SW – W of S California.
A long-lived pattern of troughing or low pressure will continue from southwest of Central California to about 25N then extend west toward Hawaii.
The MJO is showing a slow increase in activity over the next two weeks.
CFSDailyAI and CFSv2 suggest some rains primarily in northern California and the Sierras, and into Siskiyou Mountains and southern Oregon at times.
It is early for monsoonal showers and thunderstorms (TSTMS). However, the presence of upper lows may begin to bring tropical moisture northward into SOCAL and the Sierras, despite the lack of a usual summer monsoonal pattern.
Potential Dates of Precipitation (from Fox Weather's CFSDAILYAI system):
Salinas Valley-San Luis Obispo Co- S SierraNV:
Salinas Valley Showers: 6/2-3. Hot spells 6/4, 6/8-9, 6/11-12, 6/14-17, 6/22-27.
San Luis Ob/Edna: Hot spells 6/6, 6/9, 6/12, 6/15-17, 6/20-27th, 7/1.
Southern California Citrus/Avocado Area, San Luis Obispo Co to San Diego Co:
Southern California Citrus/Avocado Area: May 31-June 15.
Santa Barbara, Ventura to San Diego Co: No rainfall of consequence.
Santa Barbara Co: 6/6, 6/12. 6/16-17th, 6/22-26th.
Ventura Co: Hot 6/16-17th, 6/22-26th.
San Diego/Orange: Hot 6/16-17, 6/22-26.
Summary – June 15 – July 15… In Northern and Central California, Hottest: 6/14-17, 6/22,27, 7/1-2.
San Luis Obispo Co... Hottest periods 6/15-17, 6/22-27.
Southern California… Shallow marine layer and hot inland. Hottest: 6/16-17, 6/22-26, 7/1-3.
Seasonal Outlook July 15 – August 31... Northern and Central California overall pattern…. Near normal rainfall (minimal). Above normal temperatures occur during all of July and all of August. Usual thunderstorms (TSTMS) in the central and N Sierra and Plateau.
Southern California: San Luis Obispo Co, Santa Barbara Co, and Ventura to San Diego Counties east through Los Angeles to San Bernardino, Riverside and Imperial Counties…. Our latest guidance is suggesting a hot period in N and Northcentral California during mid-July, but near normal behavior of the marine layer at the SOCAL coast and valley areas. Although cloud amounts should be about normal, temperatures will drift above normal due to warmer sea surface off SOCAL and Baja. Weak troughs and upper lows will intermittently develop and deepen the marine layer as is normal for summer.
Looking further ahead into Sept – Nov, Dry and persistently warmer than normal conditions develop during the late Sept through Nov Santa Ana season.
Alan Fox...Fox Weather, LLC
Copyright © 2020, Fox Weather, LLC, Used by permission.
What are the effects of fire and smoke and ash and heat and all the other potential things that might affect plants and animal products that we eat?????? Can you simply wash off contaminants? What impact on the soil, itself? Does anything special need to be done to start producing food again? Come learn more.
For more information about the program for to:
In 2018 the Ojai Valley Land Conservancy (OVLC) accepted a grant from the Resources Legacy Fund on behalf of Watershed Coalition of Ventura County (WCVC) for a study of projected climate changes in Ventura County. OVLC contracted with Drs. Nina Oakley and Ben Hatchett, climatologists with the Desert Research Institute (DRI), to evaluate historic climate variability and projected changes in Ventura County. This information is needed to “paint a picture” of future climate in the watersheds of Ventura County (Ventura River, Santa Clara River, and Calleguas Creek) to support and inform climate change-related decision-making. This study provides important information for the amendment to WCVC's Integrated Regional Water Management (IRWM) Plan
You can find a copy of the report on the DRI website at: https://wrcc.dri.edu/Climate/reports.php.
To view presentations and other information from the two WCVC Climate workshops conducted with Drs. Oakley and Hatchett in October of 2018, and April of this year please visit: http://wcvc.ventura.org/documents/climate_change.htm
Some of those most interesting findings for me, are the historical data. For example, data for the years 1896 – 2018, show a tendency toward increasing maximum temperatures over the period, especially the last 10 years (Fig 1.2). But most interesting, is the increasing minimum temperatures (Fig 1.3) as compared to the maximum temperatures. Winter where is thy sting? The 2018-19 winter was the coldest in my memory, with the heater on full time at night, but there was no general frost damage this year. I can remember 1990 and 2007.
Precipitation in the South Coast region exhibits high interannual variability over the period examined. No notable long-term trends are observed (Fig. 1.4). Since approximately 2000, the 11-year running mean decreases, associated in part with the 2012–2019 drought. It is unclear whether this trend will continue in subsequent years.
There's a lot more information in the report. READ On.
But something to keep in mind, is that we had a terrible heat wave last July, and it could easily happen again. Growers who had their trees well hydrated before the heat arrived, sustain less or no damage to the trees and much less fruit drop. Trees that were irrigated on the day it started to get hot, never had a chance to catch up with the heat. Once the atmosphere starts sucking the tree dry, water movement through the soil, roots and trunk cant keep up with the demand. Weather forecasting is pretty accurate 3 days out, and if heat is forecast, get those trees in shape. You can run water to reduce the temperature and raise the humidity in the orchard to reduce transpirational demand which helps some.
Something we learned last year. What we saw and what to expect:
Map of elevational changes in Ventura County and how
Sometimes we don't see things that are not uncommon, but suddenly catch our eye. A recent lemon harvest of a trial in the Central Valley turned up lots of fruit with enlarged nipples on the stylar end. These are from a 'Limoneira 8A' rootstock trial. Not all of of the fruit was like this, but all of the rootstocks had these fruit, so it wasn't a rootstock effect.
On asking around it turns out, this happens in other places, for example on Spanish fruit:
And on Australian fruit:
And even in many normal years and orchards there is some of this special fruit
During the 2018 spring bloom there were several heat waves that hit citrus growing areas. Dr. Mary Lu Arpaia, UCR Fruit Specialist, surmises that high temperatures make for elongated fruit and quite likely impact cell division at the stylar end, as well. So the more heat spells during bloom, it's likely that we will see more of this fruit shape. It's still good to eat.