- 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.
- Author: Ben Faber
The USDA - Natural Resources Conservation Service has a wealth of information on fire recovery and erosion management. They can also do site visits to make recommendations about fire recovery:
Their publications include directions for physical barriers, such as sand bagging and wattles, but also erosion managment practices which are critical going into the rainy season.
- Author: Jim Downer
All the media are full of stories about the impending El Nino effects and possible flooding rains in California. As I write this, it is the day before our first significant storm (Jan. 4) with a 100% chance of rain throughout Ventura county for tomorrow. Still long range forecasts for Jan and Feb show near normal or less than normal rainfall possibilities for the Avocado growing areas of California. The “real rain”, they say, will come in March. Although nobody can accurately predict the occurrence of rain, we are certainly due for an increased rain year based on statistical models and estimates of the El Nino effect by NOAA, JPL and NASA. If heavy rains are in the near future, nurserymen and growers will need to take measures to protect their operations from the many and disastrous effects of downpours. Physical displacement of soil as erosion and loss of soil are common during heavy rains. While runoff water should not leave agricultural properties, it has to move somewhere when rains come, so ditches should be cleared of weeds and other obstructions to permit efficient flow of water. Most operations will have already done this by now. So what can be expected when the rains come and after they leave? It will likely be a banner year for diseases both biotic and abiotic.
Many pathogens are splash inoculated from plant to plant or soil to plant, so it is imperative to prevent the development of flooded or puddled ground near growing areas. Now or between storms would be a great time to lay down additional gravel under container beds or other outside nursery areas. Keeping containers off soil, either with a gravel or fiber mat and gravel system is imperative when trying to control Phytophthora in nurseries. Compacted walkways and beds may become saturated this winter and create ideal sporulation conditions for Oomycetes or water molds which may then move in water flows to new areas of the nursery causing infections where never seen before. Consider boardwalks or additional gravel in known low spots and walkways so that workers don't move infested mud from one part of a nursery to another. For citrus and avocado growers it is vital to give water a place to go on flat or low lying areas. When we get the expected deluges, trees can suffocate from extended soil saturation and defoliate rapidly due to anoxic conditions.
For Phytophthora sensitive crops, it may be wise to increase the calcium by adding additional gypsum now to reduce sporulation and potential spread of disease. It is also wise to use preventative fungicides such as mefenoxam, and phosphorous acid to increase plant readiness for Phytophthora increases following wet weather.
This is also a great time for woody plant growers to prune any diseased or dead materials from plants ahead of winter rains because many Ascomycete canker fungi that cause disease in woody plants will have inoculum in dead twigs. This has been a banner year for Botryosphaeira fungi which have caused canker diseases in citrus, avocado and ornamentals at record levels due to drought stress. When rain comes, spores are splashed to new plants and can cause new infections. Since this is an El Nino year, it is warm, and warm rains are best for disease promotion. Remove inoculum now, cull and remove weak, diseased or dead plants ahead of the rains to cut down on disease spread. Even though it may be inappropriate to plant some crops now, it is always a good time to remove weak trees, plow diseased row crops and chip up the waste to be used as mulch.
With rains often come strong winds. Greenhouse and tunnel growers should consider the effects of wind this winter on their operations and possible crop loss from this damage. Tunnel houses used in berry and other production are at risk but other greenhouse materials such as polycarbonate sheeting can be detached by wind. Now is a great time to inspect and repair these structures or apply new sheeting as necessary. Wind can also move woody plants to rub against each other, causing injury to the main stem or fruit if tightly spaced. Trees that are blown over due to high winds can be damaged and devalued. Spend time now inspecting trellis systems and staking of woody plants to minimize damage that may be coming.
Outdoor nurseries that have planting media storage piles should start now to downsize these piles or provide new tarps in advance of wet weather. Greenhouse operations with media stored outside should ensure that bales are properly covered with new tarps to prevent saturation of the media. Media bales should be stored off the ground on raised pallets to avoid contamination with soil or mud flows.
The challenge of a wet and potentially stormy winter is to envision what excess water can do in your operation and then try to prepare. Flooding conditions create a time of potential pathogen movement and the best protection for plants is to keep them elevated above the mud and keep workers from spreading it with the movement of machinery or foot traffic. It is also useful to imagine invasion of soil from adjacent land owners who may have diseases or weeds not on your own property. Money spent now on infrastructure will prevent disease loss later this spring or summer.
- Author: Ben Faber
Soil likes to be covered at all times. It doesn't “want” to be exposed to the elements, so you either cover it (plants, asphalt, paper etc.) or it will cover it for you with plants (weeds). If it can't be covered fast enough, it disappears – erodes. This can be from wind or rain or just natural movement down slope. Plants that are managed for other than their agricultural return are called cover crops, although they can also have a crop that is saleable. Often weeds can be managed to be a cover crop, as well.
We are looking at a possibly wet winter and many tree crops grown on hillsides and sloping ground are prone to soil erosion. Covers can be grown year round, but that usually means they require water all year round. That means they need an irrigation system dedicated to their needs. It also means having extra water which may be limiting.
A winter cover crop that grows out in the winter, does its thing (although that “thing” can include lots of other things, e.g. insectary, nutrients, water retention, etc.), and then dies or goes dormant, can be ideal. It also requires less water than a permanent cover.
But there is a big problem here. Establishment of an introduced cover still requires water. Rainfall in Southern California is erratic and there may be early rains to germinate seed, but it may not be consistent enough to get the plants established. In fact, they may die for lack of further rain or be delayed.
Delayed germination means that soil is cooler and there is less growth. The real growth may occur after there has been sufficient rainfall by January and February when the soils are cooler and there is even less chance for growth
So when rainfall is doing its worst, there's no effective cover. Or what cover there is, is what has germinated from “native” seed. It may not have the characteristics you want for management: low stature, low entanglement with the trees, low water use, holds the soil without holding up harvest, etc.
So what do you do? There are several approaches. You can move the sprinklers out into the middles and irrigate up the seed. If you are in a limited water situation, you can do alternate middles, not cover cropping the whole area, or every third middle. Whatever it takes to break the surface flow of water. Or you can turn to mulching. Put down sufficient mulch in a middle or every other middle to break overland water flow.
Cover cropping is easier than mulching, but it takes water and timing.
Below are two websites with descriptions of cover crops and how to distinguish them from “weeds”. Often a good cover can be the residential weeds. A low–lying cover allows pickers in to get lemons without mess and fear of snakes. It also means that it can be more easily treated (mowed, weed whipped) at the end of the rainy season to reduce fire hazard.
1) Characteristics of different cover crops
2) Weed identification from the UC IPM