Managing Weeds in California Groves

Josh McGill California Corner, weeds

By Danielle Leal

Over the years, new weeds have surfaced, and old weeds have developed resistance and become tougher to control. Crop scientists are currently researching ways to successfully manage weeds in citrus.

Italian ryegrass

Sonia Rios, technical development representative for Bayer Crop Science, said growers are heavily dependent on post-herbicides to help mitigate weeds. “In order to use a pre-herbicide, you need to incorporate water or rain, and we don’t get very much of that, especially in California,” Rios said. “Unfortunately, there are always more pre-emergents than there are post-emergents.”

Rios spoke during the 2022 California Citrus Conference saying, “citrus is very lucky,” because it’s one specialty crop that many different herbicides can be effectively applied to. According to recent data provided by the Weed Research and Information Center at University of California, Davis, there are currently 19 pre-emergent herbicides and 13 post-emergent herbicides for citrus groves. There are also five herbicides registered for organic use in California.

Rios said California is leading the world in herbicide-resistant weeds and that resistance can happen in multiple ways. A weed can be resistant to a single herbicide. Weeds can also be cross-herbicide resistant, which means they are resistant to two or more herbicide families that have the same attacking mechanism. There is also multiple resistance where a weed is resistant to two or more herbicides with different modes of action. Italian ryegrass, found in California citrus groves, is the only weed that is triple weed resistant.

Switching up the modes of action is a possible solution to managing tough weeds. Scientists have found that tank mixes, sequences and rotations of herbicides remain important steps. Rios’ most recent field trials mixed both pre- and post-emergent herbicides. Also important is catching weed problems early by field scouting and identifying the type of weeds in the orchards. Lastly, when using weed control chemicals, apply the right products at the right rate at the right time.

For California growers who experience long, hot and dry summers, Rios gave two tips for when to apply herbicides: “You don’t want to do it when it is a hundred degrees outside. Remember most of the herbicides get systemically through the plant through the stomata … If it’s too hot during the day, the stomata are going to close up and nothing can get in,” she said. Her second application tip is “…don’t ever try to mow and then apply herbicides. It has to go in systemically in an actively growing plant. It won’t circulate well if it’s already chopped.”

Danielle Leal is a multimedia journalist for AgNet West.