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Irrigation in a Freeze
Confronted with approaching freezing conditions a grower has several options to mitigate the potential cold damage. There are foliar sprays like copper that can reduce the incidence and need to be applied several weeks in advance of cold. There are orchard heaters that are still allowed in some growing areas but tend to be expensive to run due to fuel costs. There are wind machines that are capable, but less effective on avocado hillsides that have natural air movement. Then there is irrigation.
Of course you need healthy, well-watered trees to protect, otherwise, it is probably not worth the effort. And you need water that is dispersed in the air. Drip is much reduced in efficacy compared to microsprinklers and much less than high pressure overhead sprinklers. And you need water volumes that can be used continuously over the protected area during the freezing period.
Using irrigation water in frost control is a delicate balance among different physical characteristics of water. When it freezes, from a liquid state to the solid ice state, heat is released. More than what is actually making up the temperature of the liquid water. But then, in its liquid state, it starts evaporating turning to a gas which cools the surroundings. Heat from liquid to solid and cooling from liquid to gas. There's another property of water which is that it conducts heat really well, better than solid earth. So, wet soil heats up better from the daytime sun than a dry one.
So, you want to make sure the soil is moist during the day to soak up the heat. Then, you don't want water being applied after sunset to avoid evaporative cooling. Then when the trigger temperature for freezing occurs, the system should be run continuously so that heat is released during the freezing cycle. If you stop the water, then evaporative cooling kicks in and it could be colder than it would be if you had not run the water. And sometimes the emitters freeze up if they are turned off, and then they don't function when you try to come back around.
So, with that, you need to decide how much water you can run continuously in a given area. So, what is the coldest spot, and can you cover that area continuously for the cold period? Or the reverse is, if you know the cold area is going to get really cold and it may not make it through the cold even with irrigation, what is the area you want to protect that you know can be helped with irrigation? The water needs to run continuously. You don't want to be turning it on and off in order to roll it over to other irrigation blocks. When the water is turned off, the air starts cooling from evaporation. So decide how much area can be watered with the given volume.
Knowing all this you start watching for the cold with low temperatures that show 32 or lower. Older trees with canopies to the ground can handle more cold than young trees with little canopy to retain heat. Watch for the dew point. If it shows something much below 32, like 25, that means the air is dry and there will be a rapid temperature drop once temperatures start heading for the low temperature.
So, you are forewarned. Get water on to the grove to make sure the trees are adequately hydrated. Then a couple of days before the freeze event, make sure the surface soil is wetted during the day to take advantage of daytime heat, but make sure the water is off before sunset. Then when the big night hits, when temperatures hit a trigger temperature like 33 or 34, you should start the water. You want to have the system going before it drops below 32. Run the system until sunrise, and then you can most likely shut down. And wait for the next night and follow the same drill. We often will have two to three nights in a row that need protection.
Hopefully there's enough water for successive nights. And hopefully, it's not so much that root rot becomes an issue. Isn't farming fun?
frost satsuma irrigation
Wild Fire and Soil
Low-severity wildland fires and prescribed burns have long been presumed by scientists and resource managers to be harmless to soils, but this may not be the case, new research shows.
According to two new studies by a team from the University of California, Merced (UCM) and the Desert Research Institute (DRI), low-severity burns - in which fire moves quickly and soil temperature does not exceed 250oC (482oF) - cause damage to soil structure and organic matter in ways that are not immediately apparent after a fire.
"When you have a high-severity fire, you burn off the organic matter from the soil and the impact is immediate," said Teamrat Ghezzehei, Ph.D., principal investigator of the two studies and Associate Professor of Environmental Soil Physics at UCM. "In a low-severity fire, the organic matter doesn't burn off, and there is no visible destruction right away. But the burning weakens the soil structure, and unless you come back at a later time and carefully look at the soil, you wouldn't notice the damage."
DRI researcher Markus Berli, Ph.D., Associate Research Professor of Environmental Science, became interested in studying this phenomenon while visiting a burned area near Ely, Nev. in 2009, where he made the unexpected observation that a prescribed, low-severity fire had resulted in soil structure damage in the burned area. He and several colleagues from DRI conducted a follow-up study on another controlled burn in the area, and found that soil structure that appeared to be fine immediately after a fire but deteriorated over the weeks and months that followed. Berli then teamed up with Ghezzehei and a team from UCM that included graduate student Mathew Jian, and Associate Professor Asmeret Asefaw Berhe, Ph.D., to further investigate.
Soil consists of large and small mineral particles (gravel, sand, silt, and clay) which are bound together by organic matter, water and other materials to form aggregates. When soil aggregates are exposed to severe fires, the organic matter burns, altering the physical structure of the soil and increasing the risk of erosion in burned areas. In low-severity burn areas where organic matter doesn't experience significant losses, the team wondered if the soil structure was being degraded by another process, such as by the boiling of water held within soil aggregates?
In a study published in AGU Geophysical Research Letters in May 2018, the UCM-DRI team investigated this question, using soil samples from an unburned forest area in Mariposa County, Calif. and from unburned shrubland in Clark County, Nev. to analyze the impacts of low-severity fires on soil structure. They heated soil aggregates to temperatures that simulated the conditions of a low-severity fire (175oC/347oF) over a 15-minute period, then looked for changes in the soil's internal pore pressure and tensile strength (the force required to pull the aggregate apart).
During the experiment, they observed that pore pressure within the soil aggregates rose to a peak as water boiled and vaporized, then dropped as the bonds in the soil aggregates broke and vapor escaped. Tensile strength measurements showed that the wetter soil aggregates had been weakened more than drier soil samples during this process.
"Our results show that the heat produced by low-severity fires is actually enough to do damage to soil structure, and that the damage is worse if the soils are wet," Berli explained. "This is important information for resource managers because it implies that prescribed burns and other fires that occur during wetter times of year may be more harmful to soils than fires that occur during dry times."Next, the research team wondered what the impact of this structural degradation was on the organic matter that the soil structure normally protects. Soil organic matter consists primarily of microbes and decomposing plant tissue, and contributes to the overall stability and water-holding capacity of soils.
In a second study that was published in Frontiers in Environmental Science in late July, the UCM-DRI research team conducted simulated burn experiments to weaken the structure of the soil aggregates, and tested the soils for changes in quality and quantity of several types of organic matter over a 70-day period.
They found that heating of soils led to the release of organic carbon into the atmosphere as CO2 during the weeks and months after the fire, and again found that the highest levels of degradation occurred in soils that were moist. This loss of organic carbon is important for several reasons, Ghezzehei explained.
"The loss of organic matter from soil to the atmosphere directly contributes to climate change, because that carbon is released as CO2," Ghezzehei said. "Organic matter that is lost due to fires is also the most important reserve of nutrients for soil micro-organisms, and it is the glue that holds soil aggregates together. Once you lose the structure, there are a lot of other things that happen. For example, infiltration becomes slower, you get more runoff, you have erosion."
Although the research team's findings showed several detrimental effects of fire on soils, low-severity wildfires and prescribed burns are known to benefit ecosystems in other ways -- recycling nutrients back into the soil and getting rid of overgrown vegetation, for example. It is not yet clear whether the negative impacts on soil associated with these low-severity fires outweigh the positives, Berli says, but the team hopes that their research results will help to inform land managers as they manage wildfires and plan prescribed burns.
"There is very little fuel in arid and semi-arid areas, and thus fires tend to be short lived and relatively low in peak temperature," Ghezzehei said. "In contrast to the hot fires and that burn for days and weeks that we see in the news, these seem to be benign and we usually treat them as such. Our work shows that low-severity fires are not as harmless as they may appear."
The study, "Soil Structural Degradation During Low?Severity Burns," was published on May 31, 2018 in the journal AGU Geophysical Research Letters and is available here: https:/
The study, "Vulnerability of Physically Protected Soil Organic Carbon to Loss Under Low Severity Fires," was published July 19, 2018 in the journal Frontiers in Environmental Science, and is available here: https:/
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Care of Fire or Frost Damaged Trees
Recent and recurring fires are a problem where both citrus and avocado are grown in California.The winter forecast is out from NOAA and it looks like there might be frost in our future. What do you do if it hits your trees? https://www.noaa.gov/news-release/us-winter-outlook-warmer-and-drier-south-wetter-north Treatment of damage to both trees by fire and frost is often the same. Wait. Wait for the tree to tell you the extent of the damage. If it's a young tree with mainly green tissue, it becomes apparent pretty soon after the event, within a week to 10 days. With older trees, though it takes longer. With citrus if there is significant damage to the trunk, it's not likely to recover, or recover to commercial status. if it's just damage to the canopy, there's a good chance both trees will recover. If there is damage to the trunk of an avocado, though, it is amazing in its ability to recover. And waiting for up to a year may be necessary to truly evaluate recovery.
Care of Frost/Fire Damaged Trees
Determining the amount of damage is often a difficult job and cannot be done accurately for some months following the freeze. It is usually better to let the tree recover by itself.
Sunburn prevention
Exposed limbs can be badly damaged by sunburning. Whenever defoliated trees have not grown enough new leaves to protect the limbs before hot weather occurs, you should provide protection.
Protection is best provided by spraying or painting all exposed limbs with either a cold-water white 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 damaged 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. Root Rot occurs in soils with excessive moisture when the Phytophthora fungus is present, and growers must guard against this disease following frost damage. After fire and wind, if soils are dry and there is still green leaves on the canopy, irrigation after the event may be necessary. The key is to check the soil moisture, and if dry, water.
Pruning
Do no prune 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/fire event. At the same time, suckers should be thinned out to select the new limbs to replace those lost.
Care of young trees
Badly damaged 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 damaged to 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. Trees less than two years of age with only green tissue are usually a lost cause.
Severely damaged mature trees
The handling of severely damaged 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 can topwork badly frozen trees to a more resistant or productive variety. The hardest thing is to figure out how to treat the range of damage to trees in a single irrigation block - some will need a regular irrigation, some less, and some none at all. Balancing a checker board of trees is difficult.
Two Years After the Fire
avocado burned 2 years later 3
Fire Information for Tree Crop Growers
The following is a compendium of blogs for growers to help with the recent fire damage in the Ventura area:
Tree and Vine Loss Calculators
Spread sheets to help you calculate loss
Calculate Cost of Fire Damage to Avocado and Citrus Trees
Information from Ben Faber, Soils and Water, Avocado and Minor Subtropicals Advisor
Information on how to care for fire damaged trees from Ben Faber, Soils and Water, Avocado and Minor Subtropicals Advisor
Information on fire ecology and fire safe landscaping for homeowners, developed by Sabrina Drill, Natural Resources Advisor
UC Center for Fire Research and Outreach
Information on fire science from UC experts. Includes information on how to make homes and other structures more fire re
Publications Available From University of California ANR Catalog
You can find the publications listed below at the University of California DANR Catalog site (In the ANR Search type fire) and order more than one publication at a time or you may click on one of the links below. if you purchase a priced publication enter the promotion code PRVEN56 at check-out. You'll receive a 10% discount on your order, and a portion of the sales will benefit local programs.
A Property Owner's Guide to Reducing Wildfire Threat - describes ways homeowners can reduce the threat of fire to their property. Cost $1.50
Home Landscaping for Fire - Incorporating fire safe concepts into your landscape is one of the most important ways you can help your home survive a wildfire. FREE
Landscaping Tips to Help Defend Your Home from Wildfire - You can have both a beautiful landscape and a defensible fire-safe zone. FREE
Recovering from Wildfire - discusses issues that family forest landowners should consider following a wildfire. Cost $5.00
WildFire: How Can We Live With It? (DVD) - This program contains general information about wildland fire in California. Cost $20.00
Companion Set: How Can We Live with Wildland Fire? (Publication and DVD) - What role does fire play in the natural cycle and what choices can we make about coping with wildland fire? Cost $27.50
How Can We Live with Wildland Fire? - What role does fire play in the natural cycle and what choices can we make about coping with wildland fire? Cost $10.00
Don't hesitate to contact me, at
805-645-1462
bafaber@ucanr.edu
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USDA Brings Back the Goods - Fruit
You may not have heard of USDA plant explorers David Fairchild and Palemon Howard Dorsett, but they are among those who have had the greatest impact on what we eat in the United States. Now a silent film of their 1925–26 collecting trip to Ceylon (Sri Lanka), Sumatra, and Java has been found by the National Agricultural Library (NAL).
One hundred years ago, the national cuisine was rather plain—heavy on meat, potatoes, and cheeses, and with a very limited palette of fruits and vegetables. But Fairchild, Dorsett, and other USDA plant explorers trekked across the world to find new crops and ornamental plants for the country.
Fairchild alone is credited with the introduction of more than 200,000 exotic plants and crop varieties, including avocado, flowering cherry trees, hops, horseradish, kale, nectarines, papaya, and pistachios.
Dorsett is best known for collecting germplasm that has helped to build soybeans into a $40 billion-a-year U.S. crop.
In 1925, Fairchild and Dorsett led a USDA expedition to Ceylon, Sumatra, and Western Java, during which they scoured markets, botanical gardens, farms, roadsides, and even beaches to collect seeds and plant specimens. Accompanying the explorers was Dorsett's son Jim, whose job was to document the trip photographically.
Jim Dorsett was equipped with a then state-of-the art Sept camera that could take sharp still photos, fast-action sequence photos, and movies. Fairchild was the first to acknowledge that Jim Dorsett's equipment and techniques were far superior to his own. But he was not the last, as National Geographic hired Jim Dorsett soon after he returned from the expedition.
Recently, NAL's Special Collections staff began an inventory of their 16-mm films. They discovered a poorly labeled film canister. Once they began watching, it didn't take long into the 21:06-minute silent film to realize what they had.
“It is an amazing film,” says botanist Karen Williams with the Agricultural Research Service's National Germplasm Resources Laboratory. “It offers rare insights into how people were using plants at that time. Were they using the same plants in different ways than is now common in the region? In addition, they included natural habitats in the film. Botanists may be able to make comparisons between some of the plants that are identifiable in the film and plants known to occur in the same region today—or they may find that some are rare or even extinct today,” she adds.
A collaboration between NAL and the Fairchild Tropical Botanic Gardenin Coral Gables, Florida, has begun to interpret the botanical and historical context of the film.
“A fascinating movie to watch and a real jewel in ethnobotany, particularly when you consider it was made in the early 20th century,” says Fairchild Tropical Botanic Garden adjunct faculty and Florida International University professor Javier Francisco-Ortega. “The film covers so many things: new crops, plants as avenues to teach, ethnography, plants as tools and building materials. There is one sequence where you see them collecting a certain bamboo and then a man building with it.”
You can watch a digitized version of the entire film on YouTube Those were the days of really getting into travel.
Paula Sewell has biographies of three of the USDA explorers - David Fairchild, Frank Meyer (Meyer Lemon) and Wilson Popenoe (Pop Enoe). Turns out Popenoe retired to Antigua Guatemala and Tony Brown of Carpinteria went to the guru to learn more about the cherimoya and how best to nurture it here and from there the crop flourished and the whole family was involved - Johnny, Peter and Emily.
Wilson Popenoe in the field on his horse Starlight using a McClellan saddle
