So, this weekend we had some hot weather and the damage from that heat is apparent in all kinds of plants. Sycamores, cottonwoods and willow in the Santa Clara River bottom look torched. Redwoods in the landscape look like a new disease has hit them.
Even old coast live oak in Ojai have been toasted. Orchards have been hit also with been hit without exception. This has been a widespread weather phenomenon like a major freeze. And the trees should be treated as if they have been freeze damaged.
So, what to do with the avocados and citrus that have been hit? Well, if it's just a slight toasting, nothing. They will grow out of it. It's a setback. The growing points, the terminal buds, have been damaged and in the case of avocados those may not flower next spring. If the damage is not extensive, the whole canopy has not been damaged, then flowering should be sufficient for a good crop next year. If the whole canopy has been hit, it's likely that flowering will be minimal next year.
If the trees have lost significant portions of the canopy, though, the heat damage is not the problem, it's the sunburn damage that is going to happen that is the problem. It's the loss of the leaves that transpire and cool the tree that lead to this kind of damage that can kill small trees and lead to significant branch loss in older trees.
The leaves act like the radiator in a car. They move water through the tree and that water movement carries off the heat that accumulates in the branches and stems. When water flow stops, the bark heats up and tissue is damaged. The worst-case scenario occurs when a “renovated” tree that has been brought down to 6 feet in January and since then there has been new growth all over the tree. The heat fries that new growth and now the whole tree structure is exposed to sunburn damage.
The branches exposed to the sun need to be protected with whitewash. The whitewash needs to be WHITE, not grey. It needs to be able to reflect the sun and prevent the surface from heating. The tops of branches and the west and south sides need to be the most protected, so it often involved hand work. And it needs to be done soon after the canopy loss. That wood heats up fast and damage occurs soon after it heats up.
So what else needs to be done? No canopy, no water loss, so it's necessary to manage the water differently. With no leaves, there is no water moving from soil through the tree, so it just sits there, and the ground stays wet. Perfect conditions for root rot.
Growers who were watering their trees knowing that a heat spell was coming, did the right thing. It probably reduced the severity of the damage, but even growers who had water on before the heat and it was running during the heat have had damage. With canopy damage and loss, applied water needs to be restricted to just enough to get tree recovery without creating a wet, soggy condition. And with tree recovery, it's going to need a continually changing irrigation schedule as new growth occurs.
So now more than ever, water to the tree's growing needs. And the normal fertilizer program needs to be adjusted. There's probably sufficient nutrients in the soil from prior fertilization that nothing new needs to be applied.
And don't' prune the trees. Leave the hanging leaves there. They will help protect the tree from sunburn, but the extent of the damage is not clear. Let the tree push new growth and that will tell you sometime in the future 3-6 months, even a year from this event, when to do significant pruning.
Phlood, Phyre, Phrost, Fytophthora and Phahrenheit continue to plague our industry. It seems like we are always coping with some natural and some unnatural issues affecting agriculture. Oh, yeah and pH.
Photo: Heat singed new avocado growth.
- Author: Sonia Rios, Julie Pedraza and Ben Faber
Firefighters from all over the country worked around the clock to put out fires throughout the state of California. Fires could be devastating to growers and, in some ways, they could be beneficial by reducing populations of weeds and unwanted vegetation. However, after the loss of vegetation after a fire, growers have to prepare for the next possible disaster- mudslides, debris flow and flashfloods. Vegetation that once secured soil and gravel, preventing erosion on mountain and hill slopes is no longer there. Instead the waxy residue from burnt plant debris has formed into a baked waxy layer that prevents water from infiltrating more than a few inches into the soil, creating a water-proof surface layer. When a significant amount of rainfall occurs after a fire, it becomes an environment for a mudslide.
According to Randy Brooks, author of the article “After the Fires: Hydrophobic Soils,” during a fire, burning plants release gases from waxy plant substances that permeate through the soil pore space, coating soil particles with a hydrophobic substance, thus repelling water. Over time, the wax-like, hydrophobic layer that has formed a few inches below the soil could persist in repelling water causing damage years later. Orchard trees with shallow roots can be destroyed and/or develop weakened root systems if a mudslide occurs post-fire. As rain continues to fall, large chunks of topsoil can break loose and slide down sloped landscapes. In some cases, mud and debris can exceed 35 mph, causing massive damage and major mudslides.
Rapid moving mudslides can enter into infiltration basins, irrigation canals, and reservoirs moving silty-clay sand suspension sediment that could clog pumps and irrigation lines creating an expensive problem for growers.
Erosion in Orchards Post-Fire
Post-fire rains result in the transport of fertile soil particles into downstream waterways. These sediments can carry unwanted pesticides and nutrients that adhere to them. Erosion problems can include water pollution, loss of soil quality, increased flooding, impairment of stream ecosystems, decreased groundwater storage, release of carbon, slope failures, degradation of habitat and loss of species, damage to downstream lands and properties. Not to mention the amount of time and costs associated with addressing these issues.
Preventable Management Practices
Orchard floor management can include anything from the addition of soil amendments to changes in tillage practices. One way to minimize soil erosion is to implement management practices that improve soil structure. Soil structure is the arrangement of mineral particles into aggregates. A well-structured soil having stable aggregates can easily accommodate infiltrating water that decreases runoff and reduces erosion. In addition, stable aggregates resist particle detachment, prevent the formation of crusts, and are less susceptible to compaction. Light tillage where possible can break up the hydrophobic topsoil layer post-fire, followed by planting a cover crop, such as a grass or a forb can prevent soil erosion and be a moderate barrier in the event of a mudslide.
Mature avocado groves have high soil organic matter (SOM) due to leaf mulch and fine rootlets that die and decompose in the shallow soils. Soil organic matter promotes good soil aggregation and stable aggregates. The form of SOM that binds soil particles together into aggregates is called humus, which consists of highly decomposed organic material. Humus results from the breakdown of mulches, roots and any amended organic materials like compost or other supplemental mulches.
Periodic application of organic materials is a proven method for improving the water-infiltration capacity of certain soils: those that suffer from weak structure due to low organic matter content.
In many situations it is neither practical nor feasible to add soil amendments as an erosion control practice. Cover crops are an excellent alternative to reduce soil erosion. They protect the soil from raindrop impact, prevent the formation of surface crusts, increase infiltration rates, and intercept sediment-rich runoff. Cover crops are also a great source of SOM. Critical aspects to consider are nutrient and water competition with crops, cost of additional water for irrigation, shade tolerance, crop height, and maintenance practices such as mowing.
Like most management practices, cover cropping has disadvantages, too. All cover crops use water, some are invasive, some serve as habitat for pests, some can increase the potential for frost damage, and they may be costly to establish.
Management practices are ever changing for prevention and protection of orchards every year especially against fire and mudslides. Being informed and assessing the situation post-fire adds value to how we can evaluate the cost of protecting orchards and economically prepare fields from mudslides damages.
Mandarins, also known as “zipper skins” and “easy peelers” can have very fragile peels/skins/rinds/exocarp that make them easily subject to more damage than most oranges and lemons. Some are a bit tougher skinned than others, but some are so fragile that any rough handling often prevents them from going through conventional packing operations.
These skins were recently put to the test in the recent fires in Ojai. There was a mix of different varieties - ‘Pixie', ‘Gold Nugget', ‘W. Murcott', ‘Yosemite Gold', ‘Tahoe Gold' and others. Some of them were more sensitive than others, some were closer to the fire, all were affected by smoke to some degree. In Matilija Canyon where smoke was present for many more days than in the east of the Ojai Valley and possibly more ash, the trees have started flowering sooner. That might be temperature difference, either cooler or warmer, so it is hard to say how much effect the smoke has had versus, the ash and/or heat. Smoke has many different gasses in it, one of which is ethylene which is a naturally occurring ripening agent. Smoke not only has gasses, but it occludes the sun so less or more or altered light might have an effect on these fruit. It's not a controlled experiment, so some little scientist is going to have to come along and wriggle out these different effects. Whatever. Fire and smoke have an effect on mandarins as we have seen in other crops, such as cherimoya, avocados and other citrus.
Heat damage. Fruit facing the fire.
Ash effects on fruit coloring. Fruit was covered with ash for several days until rain washed it off. Might be a pH effect (ash is alkaline), temperature effect, uneven light radiation, or other…….
Same sort of uneven coloring, that actually looks like an ashy color, but the ash has washed off the cluster by rain
And here's something interesting where fruit facing the fire is much lighter colored than fruit facing away from the fire. Here are two pieces of fruit, one from the side directly facing the fire, and the other from the other side of the tree. The side of that fruit facing the fire was also lighter colored. So, it had an effect through the canopy (small tree). The canopy was otherwise intact, unaffected heat or flames.
Oh yeah, and there is the characteristic fruit drop from either the heat, smoke gases, water stress or ….
And then there's the fruit that looks like it had actual embers on the skin.
If the tree survives and keeps its green leaves, sometimes the fruit is affected in ways that don't appear for a while. The peel may be affected, but in many cases the fruit is just as sweet as it could be. It just looks terrible. That might even be a selling point. "Here have a wonderous piece of history that braved the horror of the Ojai fires."
The Ventura County Resource Conservation District (VCRCD) was awarded approximately $152,000 in grant funding through the State Water Resources Control Board to purchase materials for erosion control purposes on fire-impacted lands within Ventura County. Landowners impacted by the Thomas Fire are able to apply for the materials they need to help mitigate fire-based erosion on their properties. Materials include, but are not limited to:
- Silt fencing
- Jute netting
- Filtrexx Siltsoxx
- Livestock fencing
- Native plants
- Straw wattles
- Alternative water supplies (livestock)
The last day to apply for erosion control materials is March 19, 2018. For more information, or to apply for the program, click here.
When avocados are converted from one variety to another or in the case of severe frost or fire damage and there is still a healthy root system, sucker grafting should be considered. This can lead to more rapid production and is one of the easiest, most successful grafting procedures. It does take considerable attention to the graft and protection from damage by animals and overgrowth from competing suckers.
A detailed presentation of the process is found in a UC publication – Propagating Avocados, authored by Bob Whitsell, Grey Martin, BOB Bergh, Alvin Lypps and Hank Brokaw. This pamphlet was published in 1989 and is still the bible of avocado propagation. It mentions the use of parafilm as a wrapping material. It is now the standard for grafting and budding. Parafilm has pretty much replaced polyvinyl tape and asphalt emulsion for sealing and attaching the scions. It is a strong material, yet readily stretches to allow for bud growth. Where the grafts are exposed to the sun, this tape should also be painted white to protect the growing point from sunburn.
And an older publication by E.O. Stromberg