With spring weather arriving earlier, almond growers in California can expect an earlier influx of pests in their orchards — and to get started sooner on preventing them from harming their nut crops. According to UCCE Sacramento Valley Integrated Pest Management Supervisor Sudan Gyawaly, now is the time to watch out for things like navel orangeworm (NOW), peach twig borers, leaf-footed bugs and spider mites.
Navel Orangeworm (NOW)
Navel orangeworms (Amyelois transitella) are the most prominent pests in almond orchards. These moths lay their eggs on the shells after hull split, and the larvae burrow inside the shell, eating the nuts and sheltering inside. Signs of a NOW infestation in an orchard include:
·Small, flat, oval-shaped eggs outside of almond hulls
·Pinhole-sized holes in almond hulls
·Hulls that are oily in appearance
·Fungal infections of nuts
·Extensive webbing inside the hulls
·Partially eaten nuts
·Larvae that are white to pink in color, with crescent marks behind the head
Navel orangeworm damage to an almond. Photo by Matthew Malcolm
To manage NOW in your orchards, be sure to continuously monitor male flight activity by using pheromone traps. Also continue to monitor NOW egg traps to establish a spring biofix (the first spring flight, which acts as the starting date for calculating growing degree days), to help you predict the timing of their later flights. The first flight usually occurs from late March through April, but earlier times are possible, depending on the weather.
According to UC IPM, prevention is key to avoiding infestation. This includes removing all mummy nuts by the start of February. To eliminate existing NOW populations:
·Establish a biofix
·Hang pheromone dispensers in the treesto disrupt mating
·Assess trap activity and begin spraying at hull split.Apply pesticides at night and early morning for optimal results
·Harvest and clean up as early as possible
·Remove mummy nuts by March 15 of the next year.Mow or disk mummies on the ground.
Peach Twig Borer
While less of a concern in mature orchards, peach twig borer — also a moth — can still be a menace for almond growers in younger fields by causing primary scaffold damage. Begin monitoring pheromone traps in April to establish a biofix and decide if a spray is necessary in May.Consult with your pest control adviser if you are unsure.
Leaf-Footed Bugs
Stink bugs and leaf-footed bugs are especially troublesome after a warm winter. Leaf-footed bugs can be trickier to spot.UC IPM lists the tell-tale signs of an infestation in your orchard as:
·Gummy nuts — nuts withexudates on the outside of the hull —a signature warning
·Nuts that have dropped prematurely
·Black spots on kernels and wrinkled, misshapen nutmeat from adult feeding
·Withered and aborted shells from bugs feeding on premature nuts
Leaf-footed bug
Stink bugs cause similar damage, but this is usually around May or June. Leaf-footed bug management can vary from orchard to orchard, depending on history and severity. Gyawaly advises basing spray decisions on this history plus on damage risk associated with seasonal conditions. Consult with your pest control adviser if you are unsure.
Spider Mites
Typically found in the lower parts of the canopy, spider mites may also come out earlier and in greater numbers with warmer weather. Their presence should be monitored once every two weeks.For this pest, beneficial arthropods that prey on spider mites may be one of your orchard’s best friends. These species include:
·Western predatory mites
·Spider mite destroyers
·Sixspotted thrips
·Pirate bugs
By monitoring and addressing mite and insect pests in the spring, the pest populations can be kept under control to ensure a large, high-quality almond harvest.
The Varroa mite (Varroa destructor) is a parasite of the honey bee (Apis mellifera L.) and is considered one of the species’ most serious threats, inflicting more damage and higher economic costs than all other bee keeping diseases. Varroa mites harbor numerous viruses and feed on honey bee adults and pupae, causing weakened immune systems, decreased body weight, and a shortened lifespan. The external wounds caused by repeated feeding can become infected with bacteria, fungi, and viruses.
ARS researchers at the Bee Research Lab tested numerous pesticides for the chemical control of Varroa mites, and screened 40 compounds of samples in the lab (and three in the field). They documented how the viruses, carried by the mites, move within honey bee colonies and play key roles in bee-to-bee transmission. Through this research, two promising miticides were identified to manage the Varroa mite. This ARS research could lead to new management techniques that can minimize bee losses, reduce bee keeper and crop pollination costs, and ensure food security for all Americans. —By USDA Ag Research Service
The U.S. Department of Agriculture’s National Institute of Food and Agriculture (NIFA) announces the launch of a new competitive grants program aimed at rapidly addressing emerging and re-emerging pest and disease threats across the nation’s food and agricultural systems.
The grants program, Rapid Response to Emerging and Re-emerging Pest and Disease Events Across Food and Agricultural Systems, is designed to deploy timely, science-based solutions to protect agricultural productivity, ecosystem health, and food security.
“Invasive and emerging pests and diseases can devastate crops, livestock, and ecosystems, disrupt supply chains and threaten food security,” said NIFA Director Dr. Jaye Hamby. “This program empowers researchers and Extension professionals to act quickly, delivering practical solutions that safeguard our agricultural communities and the nation’s food supply.”
This program is part of NIFA’s Agriculture and Food Research Initiative (AFRI), designed to improve plant and animal production and sustainability, and human and environmental health. AFRI is the nation’s leading and largest competitive grants program for agricultural sciences. These grants are available to eligible colleges, universities, and other research organizations.
With a maximum award of $500,000 and a grant duration of 12 to 24 months, the program supports grants for research, Extension, or integrated projects. Applications are accepted on a continuous basis, within 180 days of a qualifying pest or disease event, ensuring swift mobilization of resources when threats arise.
Applications must directly address the effects associated with the emergence or re-emergence within the last 180 days of pests and/or disease in animal and/or plant production systems or within the food supply, and one or more of the following emphasis areas: One Health, plant and animal health, and ecosystem health.
Funded projects will focus on applied research and Extension and outreach activities that generate actionable knowledge and tools. These may include:
Rapid understanding of pest/pathogen ecology, epidemiology, and immunology.
Development and validation of diagnostics, vaccines, and control methods.
Communication strategies, training programs, and best practices for mitigation.
Technologies and protocols for detection, management, and prevention.
To ensure impact, projects must implement or develop at least one deliverable within six months of award receipt, with all activities completed within the grant period. Proposals are encouraged to integrate biological and social sciences, engage public-private partnerships, and support small- and medium-sized farms and ranches.
“This program reflects USDA’s commitment to proactive, science-driven responses to agricultural threats,” Hamby said. “By supporting rapid innovation and collaboration, we’re helping communities stay resilient in the face of evolving challenges.”
The program also welcomes proposals that address ecosystem health, including strategies to mitigate the environmental impacts of pests and diseases affecting plants, animals, and pollinators. Projects that incorporate community development, youth development, and 4-H are also eligible, provided they align with the program’s emphasis areas.
Applicants are encouraged to coordinate efforts across institutions and regions, and to contact program staff at afri-rapidresponse@usda.gov with questions regarding project fit. While this program is designed for short-term, rapid response efforts, longer-term projects may be better suited for other AFRI program area priorities such as Agricultural Biosecurity, Diseases of Agricultural Animals, or Food Safety and Defense.
Eastern filbert blight is an ongoing and costly problem for hazelnut growers, but researchers at the Oregon State Extension are researching newer and more efficient ways to combat it. At the Nut Growers Society’s Winter Meeting, Jay Pscheidt spoke with Matthew Malcolm at Malcolm Media Ag Publishing and shared their findings on drenching. Watch this quick video and read more in Pacific Nut Producer Magazine.
This work was partially funded by the Stockton East Water District — Crown rot and root rot caused by Phytophthora species are diseases of great concern for almond growers. Many people believe that irrigating with surface water puts your trees at risk of Phytophthora, and that irrigating with groundwater is the best way to prevent these diseases. However, a recent study of Phytophthora in irrigation water and orchard soils tells a different story.
The Bottom Line:
Irrigation management is Phytophthora management. This study showed that Phytophthora is common in orchard soils. Regardless of whether they are irrigated with surface water or groundwater, over 30% of tested orchard soils were positive for Phytophthora. Since water (either standing water or saturated soil) allows Phytophthora to cause disease, good irrigation management is crucial to prevention, no matter what your water source.
— Reduce ponding: irrigation application rate should not exceed soil infiltration rate.
—Reduce length of soil saturation: run shorter sets (24 hrs max) more frequently.
— Avoid water on the trunk of the tree and avoid soil saturation at the trunk: plant on berms, use stream splitters, choose microsprinkler wetting patterns that avoid the trunk, move drip emitters away from the trunk.
Using a resistant rootstock is an excellent tool to help limit Phytophthora disease if you are planting a new orchard (see this chart for rootstock info) or putting in replants. Even with a resistant rootstock, good irrigation management is crucial because no rootstock is immune.
The Details:
Phytophthora is a genus of fungus-like organisms which contains over 200 different species. Many of these species are important pathogens of orchards, causing root rot or crown rot (for example Phytophthora cinnamomi, Phytophthora cactorum, and Phytophthora mediterranea) or pruning wound cankers (mainly Phytophthora syringae). Many previous studies have shown that Phytophthora species are common in surface sources of irrigation water, such as rivers, canals, and sloughs. Phytophthorahas not been found in groundwater from wells, unless that well has been contaminated with surface water. From this, people have assumed that irrigating with surface water puts an orchard at risk of Phytophthora root or crown rot, and that irrigating with groundwater is ”safe.” However, biology is rarely that simple: in my experience, orchards irrigated with groundwater can struggle with Phytophthora, and many orchards irrigated with surface water do not show symptoms of Phytophthora disease.
In 2021 and 2022, collaborators and I conducted a study to look at the effects of Phytophthora in irrigation water from a broader perspective. This study took place in the Stockton East Water District (SEWD), a local water agency that manages both groundwater and surface water use by agricultural producers east of Stockton, CA. The study had three objectives:
— Test SEWD surface water for Phytophthora during the irrigation season.
— Test for live Phytophthora coming through irrigation emitters.
— Test orchard soils for Phytophthora, comparing orchards irrigated with surface water vs groundwater.
Objective 1. Test SEWD surface water for Phytophthora during the irrigation season
During the 2021 irrigation season, we sampled water from surface water irrigation sources throughout the SEWD. Some locations (core sites) were sampled monthly from June through October, and some locations (additional sites) were sampled twice during the season, in July and October. Samples were taken back to the lab and tested for Phytophthora using DNA sequencing.
Overview of the Stockton East Water District (SEWD) with water sampling sites marked.
We found that Phytophthora species were common in SEWD waterways. Over the course of sampling, we found 39 Phytophthora species, 10 of which are known pathogens of orchards crops grown in the SEWD. Many of these species were found throughout the irrigation season. We did find that different waterways had different Phytophthora species, and some waterways had more orchard pathogens than others. However, every waterway, and nearly every site, had at least one Phytophthora species of concern to orchards. These results are consistent with many previous studies, which have determined that Phytophthora species are common in surface sources of irrigation water.
Phytophthora species detected in Stockton East Water District waterways throughout the irrigation season.
The Phytophthora species listed in this table are reported to cause disease on walnut (W), cherry (C), almond (A), or pistachio (P). If the crop is bold, the Phytophthora species is particularly aggressive on that crop. A plus (+) indicates that the Phytophthora species was detected at least once in that waterway. A period (.) indicates that the species was not detected in that waterway. Nineteen other Phytophthora species (not listed) were detected, but these are not known to cause disease on orchard crops.
Objective 2. Test for live Phytophthora coming through irrigation emitters
In objective 1 we determined that Phytophthora is common in the SEWD waterways. However, very few studies have looked at whether Phytophthora in a waterway can get into an orchard through the irrigation system. This is especially true of drip irrigation systems, which require substantial filtration of surface water to keep emitters from plugging.
During the 2021 and 2022 irrigation seasons, we collected water directly from irrigation emitters during a normal irrigation and tested this water for the presence of live Phytophthora. This was done in three surface-water-irrigated orchards, two with drip emitters and one with sprinklers, as well as two groundwater-irrigated orchards, one with drip emitters and one with sprinklers.
Orchard sampling stations used to detect live Phytophthora in water from A, sprinklers and B,
drip emitters.
We found that Phytophthora commonly survives the journey from surface water sources into the orchard, and that the irrigation system type did not seem to matter. Also, the presence of a sand media filter did not seem to affect how regularly we detected live Phytophthora coming through irrigation emitters.
Summary of Phytophthora detections in water collected from irrigation emitters
Objective 3. Test orchard soils for Phytophthora, compare orchards irrigated with surface water vs groundwater
In objectives 1 and 2 we determined that Phytophthora is common in SEWD waterways and that it can survive the trip through an irrigation system and into the orchard. However, in most cases Phytophthora must survive in the soil to infect the orchard when conditions are right. Even though we knew that irrigation with surface water is bringing Phytophthora into orchards, we didn’t know if this affects the incidence of Phytophthora in orchard soils.
In 2021, we collected soil from 20 SEWD orchards exclusively irrigated with groundwater for at least 60 years and from 20 SEWD orchards mainly or exclusively irrigated with surface water over the same timeframe. This soil was tested for the presence of Phytophthora species using both the DNA sequencing from objective 1 and the live detection methods from objective 2.
We found that Phytophthora is common in orchard soils, with 32.5% of sampled orchards (13 out of 40) testing positive. We also found that the source of irrigation water did not affect the chances of finding Phytophthora in the soil: groundwater irrigated orchards were as likely to have Phytophthora as orchards irrigated with surface water. This indicates that irrigation with surface water was not the main factor determining whether Phytophthora was present in orchard soils and that irrigation with surface water may not increase risk of Phytophthora disease in orchards.
Hytophthora species in bold are particularly aggressive on orchard crops
The results from this study confirm that Phytophthora is common in surface water but show that irrigating with surface water was not the main factor determining whether Phytophthora was present in an orchard. Where is the Phytophthora coming from? It is hard to know, but historical flooding may play a role. We also know that Phytophthora can be moved into an orchard on planting material and in soil on equipment. Where does this leave us in terms of management? This study shows that Phytophthora is very common in orchard soils, regardless of the source of irrigation water. Since over 30% of tested orchards were positive for Phytophthora, it would be prudent to assume you have Phytophthora in your orchard. Saturated soil allows Phytophthora to infect and cause disease, so good irrigation management is crucial to prevention. By “good irrigation management”, I mean that you want to reduce the length of time that orchard soils are fully saturated and avoid having standing water. This can be done by using irrigation emitters with output volumes suited for your soil infiltration rate and by irrigating more frequently for a shorter duration (24 hrs maximum). In addition, you want to apply irrigation water in the root zone but away from the trunk to reduce the opportunity for infection of the trunk or major roots. This can be done using stream splitters with sprinklers to protect the trunk, choosing a microsprinker wetting pattern that avoids the tree trunk, or moving drip emitters away from the trunk of the tree. At planting, consider using a resistant rootstock, and plant on berms. Note that even if you are using a resistant rootstock, good irrigation management is crucial because no rootstock is immune.
I want to leave you with this thought: irrigating well is much more important for preventing Phytophthora than the source of your irrigation water. I have been to many surface-water-irrigated orchards with no symptoms of Phytophthora. Some of the worst orchards I have seen, in terms of Phytophthora disease, were irrigated with groundwater with the driplines right against the trunk on 3rdleaf trees. Irrigation management is Phytophthora management.
Thank you to my collaborators on this project, Greg Browne (USDA-ARS) and Mohamed Nouri (UCCE San Joaquin County). Special thanks to Justin Hopkins with the SEWD for his help planning and executing this project. Thank you also to the SEWD growers who welcomed me into their orchards for sampling. This work was partially funded by the Stockton East Water District. — By Jaime Ott, UC Extension Tehama, Shasta, Glenn, and Butte Counties
Though our weather outlook for the next couple of months is leaning toward warmer weather than usual, a cold snap during bloom and leaf-out could cause significant bacterial blast damage. Make a plan now in case of bad weather.
The Bottom Line:
Bacterial blast can be a problem when cold, wet weather coincides with bloom or leaf-out.
Copper resistance is common: spraying copper is ineffective for preventing blast.
Frost protection is the most economical prevention option and will help prevent damage from both frost and bacterial blast.
The antibiotic kasugamycin (Kasumin®) is effective for preventing blast when applied up to 6 days before cold, wet weather. It is available this year under Section 18 exemption (Feb 1- April 15), consult with your PCA regarding rates and timings.
The Details:
Pseudomonas syringae can infect all aboveground parts of an almond tree, causing bacterial blast of the leaves and flowers. Pseudomonas is everywhere, so weather conditions are the factor driving this disease. Freezing cold, wet weather favors this disease. Pseudomonas is spread by water hitting the tree. If this occurs during frost, Pseudomonas can enter the tree through cells broken by frost damage. Trees are especially susceptible to frost damage during bloom and leaf-out, when tender new growth is exposed to cold weather.
Preventing infection by Pseudomonas is the only way to control bacterial blast. Frost protection is the most inexpensive strategy: if the trees are not damaged by frost, Pseudomonas will not have a way in to cause disease. As a second line of defense, kasugamycin (Kasumin®) is effective to prevent bacterial blast when applied no more than 6 days before cold, wet weather. For this spray, complete coverage is crucial: all the tender new growth must be covered in a protective layer of the antibiotic for it to be effective. Any tissue left uncovered will be unprotected. Kasugamycin recently received a Section 18 for use in almond between February 1 and April 15, 2026. If the weather warrants treatment, kasugamycin can be used as a preventative spray up to two times during the temporary use window. Current work by UC researchers shows that copper-resistance is common in Pseudomonasthroughout the state. In some cases, mixing mancozeb with the copper may provide some level of control, but research shows that this mixt ure is not as effective as kasugamycin and can cause phytotoxicity.
As almond growers prepare for pollinating their orchards — one of the most important tasks for establishing the new crop — here are insights from the Almond Conference to consider.A “Honey Bees & Pollination” panel was moderated by Josette Lewis of the Almond Board of California (ABC).
Hives Per Acre for Self-Fertile Cultivars: Elina Niño
Following a string of bee losses over the past twenty years, 2025 was especially severe, with a 62% loss.According to Elina Niño, apiculture scientist at UC Davis, the losses are becoming unsustainable.
Niño pointed out that, in spite of having only 0.6 million of the 2.5-2.7 million colonies in the US, California ranks third in honey production, after North and South Dakota.
Niño conducted a study of the pollination needs of self-fertile cultivars.She excluded bees from some trees and for others used one or two hives per acre.She counted the number of bees per flower per hour and after harvest she measured the kernel mass of 100 nuts per tree.
The results showed that the number of visits per flower was similar for one hive as for two hives per acre.Although the trees set more nuts and yielded more weight when one or two hives were present, kernel mass was higher where bees were excluded, presumably due to spreading the trees’ resources over fewer nuts.
State of the US Commercial Beekeeping Industry: Matt Beekman
The biological state of the beekeeping industry is precarious, according to Matt Beekman, a beekeeper based in Hughson, California who spends summers in North Dakota.He presented a biological and financial State of the Beekeeping Industry report.
Parasites & Pathogens: Varroa Mites & Viruses
The varroa mite parasite continues to afflict beehives.Reproducing quickly, the mites vector viruses that then spread from bee to bee in the densely populated hive.The viruses can also spread in semen.Area-wide applications of miticide help manage the mites.
Only one chemical had been approved to suppress varroa mites since 2005.That chemical, Amitraz, was registered for beehive use in 2013 in California, the last state to do so.Some softer chemistries are available, but they are temperature dependent.
California approved two new products in 2025, one oxalic acid-based and the other RNA-based.They are expensive, though, costing $74 per colony per year.
Nutrition: Where Have All the Flowers Gone?
Each hive needs about 50 pounds of pollen and 100 pounds of nectar year.With fewer acres enrolled in the federal Conservation Reserve Program, pollen and nectar may be scarce.
Alternatively, almond growers can plant cover crops in the orchards to meet the need.Seeds for Bees supplies free, appropriate cover crop seeds the first year and a discounted price the second year.Many growers continue buying the seeds annually after seeing how effective they are.
Pesticides: No to Organosilicon Adjuvant
Used to amplify the effectiveness of pesticides, organosilicon surfactants harm bees.Growers should ask their PCA to use one of the available alternatives instead.
Beekman said that fungicides need to be sprayed during bloom, but advised using no tank mixes, adjuvants or insecticides during bloom, and applying fungicides in the early evening when bees are not flying.
Financial Snapshot
The net profit for beekeepers in 2024 was $11 per hive.Overtime wages adversely impacted beekeepers, who need workers to do nighttime maintenance and overnight travel.US honey production is declining due to fewer locations where bees can be placed to collect nectar.The bees need more access to public land.
Meanwhile, honey imports are increasing.Imported honey is sometimes cut with synthetic sugar syrups.Since honey is one of the few commodities for which the US does not have a standard definition, ambiguity and adulteration occur.
India is now the largest exporter of honey into the US, with prices of $0.99-1.08/lb. The US price is $2.69/lb., and the yield is declining.
Endangered Pollinator Species: Alexi Rodriguez
Some insect species have been proposed for endangered species status for the first time, according to Alexi Rodriguez of the Almond Alliance.One such insect is the monarch butterfly, which has little overlap with agriculture except for coastal sites in winter.
A 2018 petition proposed adding four bumblebee species.
Rodriguez said that if growers take reasonable, research-based steps to support pollinators, they will be immune to prosecution if incidental harm occurs.The steps include using IPM (Integrated Pest Management) and planting pollinator habitat.
Questions from the Audience
Q: Is it worth getting mason bees to complement honeybees?
A: Yes — blue orchard bees (a type of mason bee) — but you have to contract it separately.They fly in colder temperatures and in rain.
Q: Was there a bee shortage in 2025?
A: That was not studied, but growers were saying that they did not get the number of frames they contracted for.It’s possible that insufficient bees caused the lower yield last year, because everything else was good in 2025.
Q: What about indoor cold storage of bees in empty warehouses to break the varroa mite cycle?
A: It did not go well when tried in Kern County.
Q: Does supplementing the almond orchard with pollen help with pollination?
A: There is no research on that, but it is unlikely to help based on flower anatomy and the need for insects to get into the blooms to move pollen.
Q: Does it help to move hives around during pollination?
A: Yes, but it is a tradeoff economically due to the cost of labor to move the hives.The most important variable is having morning sun on the hive to warm the bees enough to fly.The bees will fly over the entire acre once they are warm.
Q: What is a fair contract between grower and beekeeper?
A: Eight frames average per hive; four to five frames minimum, 60% coverage.
Q: Do herbicides impact bees?
A: There is no research on that, but herbicides are not needed during bloom anyway.
For more information about preparing for pollination, see Honeybee ABC.– By Nancy Power, PhD, Assistant Editor, Pacific Nut Producer
Squirrels can be a serious headache for nut growers, who often have to come up with their own ways of catching and getting rid of them as rodenticides are not always an option. Oregon hazelnut grower Ken Baker talks to Matthew Malcolm with Pacific Nut Producer about the way he has learned to trap squirrels at his farm. Watch this quick video and learn more in Pacific Nut Producer Magazine. Please thank this video’s sponsor George Packing Company for their industry support.
Crop protection tools are becoming more and more regulated and limited in California. Biostimulants, biopesticides and biofertilizers offer more options for growers. How they fit into conventional and organic farming systems is still being investigated and how the California Department of Pesticide Regulation should treat and classify them is still in debate. The California Association of Pest Control Advisers (CAPCA) has been actively involved in the conversation though, advocating for tools for sustainable pest management. At their annual Conference, Malcolm Media Editor-in-Chief Matthew Malcolm sat down with CAPCA State Board Vice President Adam Tavares from AMVAC Chemical as he shared his perspective on where their at and priorities for the future.
The 2025 UCCE walnut pre-emergent herbicide demonstration trial is completed, and the results show strengths and weaknesses of 19 different herbicide treatments, applied either in February or March.
Takeaways:
1. All treatments were applied with a strong post-emergent mix, but hairy fleabane and white clover were not adequately controlled with post-emergent treatments alone. Control was dependent on the addition of an ALS inhibitor herbicide (products like Craze, Matrix, or Mission).
2. Summer annual weeds were best controlled by products with long residuals (products like Alion, Chateau, Brake On! (not registered in CA), and Prowl.
3. Yellow nutsedge was controlled by treatments with Zeus or Craze.
4. Conclusion: Pre-emergent treatments in late winter must account for winter annual weeds that have already emerged and perennial weeds while also maintaining good residual control further into the summer.
Introduction
Winter pre-emergent herbicide applications are critical to get right if yearly weed management operations are going to be successful in walnut orchards. Poor selection of herbicides or missed timing could make the difference between good control and battling an overwhelming infestation of weeds. In walnut orchards I have regularly heard two comments concerning pre-emergent herbicide applications in walnut orchards:
1. Those who apply pre-emergent herbicides in the fall, just after harvest, tend to have the cleanest orchards.
2. February or March are the most common times to apply pre-emergent herbicides in walnut orchards.
Maybe my perception is incorrect, but these weed control strategies seem to be at odds. When it comes to winter annual weeds, it is likely safest to apply pre-emergent herbicides before germination due to several common weed species with herbicide resistance issues (hairy fleabane, Italian ryegrass, annual bluegrass). Applications later in the winter will have to deal with emerged weeds, potentially resulting in more escapes and misses.
However, herbicide treatments in February or March are common, and many growers have been successful with these treatments. Why is this, and what is the key to achieving good control later in the winter? Success at later timings depends on how much post-emergent activity you are getting out of your tank mix. Some pre-emergent herbicides can contribute to your post-emergent activity. Common pre-emergent herbicides like Goal, Chateau, Matrix, Pindar, and others add some extra post-emergent activity to tank mixes when used in winter months. Dr. Brad Hanson (UCCE Weed Specialist at UC Davis) discussed this topic when he addressed tank mixes of Alion and Matrix in his UC Weed Science Blog article (linked here). My recent work with rimsulfuron on established johnsongrass also explored this subject (linked here). These prior discussions focused on rimsulfuron products as tank mix partners for pre-emergent applications, but I wanted test a broader range of products applied in February and March. This year’s walnut preemergent herbicide demonstration made some progress toward that goal.
I will discuss my findings next, and at the end of this article I have included results tables. All products except one are currently labeled for use in walnut orchards, though with some restrictions on orchard age. The one unlabeled product in this study was Brake On!, manufactured by SePRO, currently pending registration in CA. This is not an exhaustive list, and the data provided here does not constitute a recommendation of any product.
Trial design
In 2025, I arranged my pre-emergent demonstration trial to evaluate individual products and tank mixes for weed control efficacy when applied in February and March. This trial was installed in the Loybas Hill region of Tehama County, in a 4-year-old walnut orchard with Tehama silt loam soil. A post-emergent mix of 2 qt/A Roundup Powermax 3 and 2 qt/A Rely 280 was added to every plot in February, and pre-emergent herbicides were applied either in the mix, or one month later in March. The tables at the end of this article show the full list of treatments and when they were applied. The first table represents weeds that were present at application, hairy fleabane and white clover. The second represents weeds that grew to be prominent in May and June, so their control depended on residual activity from my herbicide treatments lasting long into the summer.
Results overview
White Clover and Hairy Fleabane (Table 1): Both weeds were controlled by all three group 2 herbicides tested: Craze, Revolt, and Mission. Revolt was weaker on fleabane while Craze was weaker on clover and these differences also showed up in the tank mix treatments with both products applied with Prowl. February and March treatments with these three herbicides produced very similar results. The 12 fl oz/A rate of Chateau also effectively suppressed both species.
Summer annual weeds (Table 2): The primary weeds in the categories recorded here were grass weeds (jungle rice and large crabgrass) and broadleaf weeds (prostrate knotweed, spurge, and pigweed). Many herbicides provided good control until June (3-4 months after treatment) but only Alion, Brake on! (not currently labeled for CA), and March-applied Mission maintained “good” control of broadleaves through July. Only Alion and Prowl maintained “good” control of grasses through July.
Yellow Nutsedge (YNS; Table 2): Zeus was the best tested product for nutsedge control, but Craze also showed some effects in July by keeping nutsedge coverage low. Rimsulfuron products like Matrix and Revolt are also labeled for YNS control, but labels suggest sequential applications, and the application timings used in this trial may not have been ideal to target YNS with this product.
Summary and cautions:
This is a single trial only representing one weed population and one soil type, so results may vary by location. However, hopefully this demonstrates the importance of tank mixing to control emerged weeds as well as future emergence. Group 2 herbicides like Revolt, Mission, and Craze can provide good post-emergent control in addition to their pre-emergent activity. But be cautious, this herbicide group is notorious for herbicide resistance. Do not overuse this class of herbicides or you will likely start seeing weeds escaping your treatments.
Look through the following tables if you are interested in seeing more information on efficacy in these trials. To see weed coverage data, view the report on the UCCE Tehama website here. Thanks to the California Walnut Commission for supporting this work. For any questions or additional information, contact Ryan Hill at 530-527-3101 or rjahill@ucanr.edu. — By Ryan Hill, UCCE Agronomy and Weed Science Advisor, Tehama County
Weed control tables:
Control was determined by calculating % reduction in weed coverage, relative to the plot with the worst infestation. Control is summarized by four categories. These categories may not represent each grower’s threshold of tolerance for weed control, but they can still provide relative comparisons between the products tested.
Poor control is indicated by a “p”, representing control between 0 and 50%.
Moderate control is indicted by an “m”, representing control between 50 and 70%.
Good control is indicated by a “g” and highlighted light green, representing control between 70 and 90%.
Excellent control is indicated by an “e” and highlighted dark green, representing control between 90 and 100%.
Table 1: The effect of different pre-emergent herbicides on two weeds that were present at application, hairy fleabane and white clover.
Table 2: The effect of different pre-emergent herbicides on weeds that germinated or sprouted in summer. Summer annual broadleaves included prostrate spurge, knotweed, and pigweed and summer annual grasses were primarily crabgrass and jungle rice.
