Herbicide drift, native plants, irrigation water, weed zappers

Weeds abound: On farmland, pastures, rangelands, in natural areas, in our yards and along our sidewalks. Managing those pesky plants costs plenty: California farmers alone spend more than $900 million annually on weed control, according to a University of California study. Rangeland managers reported spending as much as $5 million to manage weeds on their land, according to a survey.

“Weed management is a critical component of managing California lands,” said Brad Hanson, a professor of Cooperative Extension in the UC Davis Department of Plant Sciences. That’s why research on the impacts of weed control tactics on the targets, on crops and on other plants is one of the missions of weed researchers.

Some of the latest findings and updates on ongoing research were presented during the UC Davis Weed Day 2024, hosted by the department and the UC Weed Research and Information Center. Topics included the potential damage caused by herbicide drift, the benefits of using native plants to combat invasive weeds, impacts of using endothall in irrigation water, new weed detection and treatment technologies, and other issues related to weed management.

Herbicide drift explored

When  herbicides are applied on California rice fields, breezes can blow the chemicals into adjacent fields. Growers and others are concerned about the damage herbicide drift could cause to other crops nearby. In a field west of campus,  postdoctoral scientist Deniz Inci showed rows of corn, beans, squash, tomatoes and sunflowers that had been exposed to benzobicyclon, sold under the name Cliffhanger. “Due to California’s diverse cropping systems, off-target herbicide drift into nearby fields of annual crops can be a significant problem,” Inci said.

In mid-June, Inci simulated this problem in the field by exposing annual crops to different concentrations of benzobicyclon, including anticipated drift rates plus higher concentrations. Injury is still being evaluated weekly, and the results will be shared with California rice growers during the Winter Rice Grower Meetings in January, Inci said.

As part of the extension program, rice herbicide drift studies are presented annually at different locations across the Sacramento Valley. For updates and more information about drift, visit the UC Rice Research and Information Center and the Weed Research and Information Center.

Inci recently graduated with a Ph.D. from the lab of Kassim Al-Khatib, in the Department of Plant Sciences.

Native plants and soil interactions offer clues

The interaction of native plants and the soil they occupy could provide clues to both fighting weeds and restoring drought-resistant plants to landscapes. Justin Valliere, an assistant professor of Cooperative Extension in the department, explained work in native grasslands that shows promise.

The key may be using late-season summer annuals: In a pilot study, they appeared to help restore bunchgrasses and prevent re-invasion of non-native annual grasses, Valliere said. Feedbacks between plants and tiny things living in the soil may play a key role in facilitating restoration. When non-natives take over, they may drastically change soil microbial communities. That, in turn, can pose challenges for re-introducing natives.

“We are currently exploring how seeding different mixes of native annual plant species may help recover soil microbial communities and improve restoration success,” Valliere wrote.

Restoration of native California bunchgrasses is generating interest because they have deep and wide fibrous root systems that allow them to tolerate drought, reduce water run-off and resist erosion. They provide good forage for livestock, plus provide food and habitat for other creatures, store significant amounts of carbon and improve air quality.

Endothall in irrigation water

Aquatic weeds that grow in thousands of miles of irrigation canals in California are commonly managed using herbicides.  In one part of the state, growers reported crop damage in almonds, annual crops and alfalfa after using herbicide-treated irrigation water, and they worry the chemical could be the culprit. Stephen Chang, a master’s student in the lab of Brad Hanson, is working on a multi-year experiment to dig into these claims.

“We are testing applications of two formulations of the aquatic herbicide endothall in a young almond orchard and an older almond orchard to see how trees of differing ages will react,” Chang wrote. More work is coming: “Additionally, we are looking to investigate the effect of endothall in several annual crops commonly grown in California.”

Chang took first place in a poster contest where he outlined his research. The contest was held during a meeting of the California Weed Science Society in Santa Barbara in January.

Weed-zapper works in organic almonds

As part of his Ph.D. studies, Tong Zhen is looking at how well an electrical device can zap weeds growing in almond orchards. The device, which generates electricity using a power take-off generator, shoots electrical current through plants as the tractor slowly drives along. As the current passes through the plant stems and roots, resistance generates heat, which damages plant cells. 

Zhen and the research team found the electrical weed control system didn’t bother the growth of young almond trees, measured by height and trunk diameter, in an organic orchard at a UC Davis research site. The almond trees and a nearby blueberry planting where weeds were treated with the electrical weeder grew similarly to plots where weeds were managed with hand-weeding, hoeing and spot-spraying with an organically-approved herbicide. Plus, soil respiration also was about the same, suggesting little impact on soil microbial activity.

At Weed Day, Zhen demonstrated the device on an open field infested with lovely, but pernicious, field bindweed: Most of the plants were toast soon after treatment, and the smell of crispy greens 

More research

• Branched broomrape, or Orobanche ramosa, is a growing concern, especially in California tomatoes. Research on various aspects of this parasitic weed was presented, including remote sensing, equipment sanitation and candidates for chemical fumigation of soil.  Several areas of related research were discussed; read this article for details.
• Brake On! appears safe for pomegranates: Rohith Vulchi presented research showing no visible crop injury in pomegranate trees after 10 weeks of application of fluridone, sold commercially as Brake On! Bindweed control was 85 percent or better eight weeks after treatment. Vulchi is a postdoctoral researcher in Hanson’s lab.

Weed resources

• More information about weed research and herbicides is at the UC Davis Weed Research and Information Center.
• The University of California Statewide Integrated Pest Management Program offers more information here.
• For updates about rice herbicide drift research, visit the UC Rice Research and Information Center and the Weed Research and Information Center.