New research into Palmer amaranth and farmers’ increased familiarity with the yield-slashing pigweed are positives heading into the 2025 growing season even as the threat in southern Idaho remains high.

“More people know how to identify it, and whom to call,” Albert Adjesiwor, weed scientist based at the University of Idaho Kimberly Research and Extension Center, told Capital Press. Researchers can advise farmers about crop rotations and herbicide programs they can use to help reduce risk.

“It’s not a very cold-tolerant species,” he said. If spring is warmer than average, “it will come up earlier than it is supposed to.”

Palmer amaranth can grow up to 10 feet tall and threaten various crops. Since the weed resists glyphosate, it can be a major challenge for growers of sugar beets, alfalfa or corn varieties developed to withstand applications of Roundup herbicide that contains glyphosate, according to a UI news release. Some Bruneau-area farmers who planted Roundup-ready corn following Roundup-ready beets were hit especially hard.

Potato farmers also face risk because few good herbicide options are available for use after the weed emerges.

The weed is native to the desert Southwest. Adjesiwor discovered it in Idaho in 2022, in Glenns Ferry. It likely entered the state in bird feed, in cottonseed meal fed to cattle or on farm equipment originating from other states, according to UI.

Adjesiwor and colleagues estimated 3,000 acres of southern Idaho cropland were infested in 2023. Based on extensive scouting, they estimated infested acreage more than doubled in 2024. The weed has been found from Paul in the state’s south-central region to Parma in the southwest.

“It was clear that everywhere we had it in 2023, we had it in 2024,” he said in the release. “The farmers use the same equipment to harvest and till, and they move it around in their fields. The seeds are very small, and it doesn’t take much to have a problem on your hands.”

A Glenns Ferry farmer allowed Adjesiwor and researchers from Amalgamated Sugar and Oregon State University to conduct test digs at 17 sites in a beet field with varying levels of Palmer amaranth infestation. The researchers confirmed yield loss up to 47% due to the weed.

Growers should report Palmer amaranth and rotate to small grains during the following growing season if possible, Adjesiwor said. Most herbicides labeled for small grains will kill the pigweed, which often emerges after the grains are established.

The weed began appearing in Washington and Oregon fields in 2023. UI, OSU and Washington State University researchers are working on a Northwest Potato Research Consortium grant-funded project to scout Washington, Oregon and Idaho. On the Pacific Northwest Herbicide Resistance Initiative website, they document counties where the weeds are found.

The grant funding has also helped researchers collect weed seeds and grow them in a greenhouse to test for herbicide resistance. One group of Palmer amaranth plants withstood applications of 16 times the usual dosage of glyphosate, according to UI. Testing also confirmed at least 70% resistance to Group 2 herbicides commonly used in potato, dry bean and small grains production.

Adjesiwor submitted for grant funding to evaluate post-emergence options for controlling the weed in potatoes.

This year, he plans to conduct additional herbicide-resistance screening using funding from a USDA National Institute of Food and Agriculture grant that UI Extension entomologist Erik Wenninger received for integrated pest management.

“I’m concerned by the number of herbicides Palmer amaranth is resistant to and how fast it can develop resistance,” Adjesiwor said.

He discovered water hemp, another pigweed that developed resistance to glyphosate and other herbicides, in southern Idaho in 2022. Water hemp has not spread as quickly as Palmer amaranth, according to UI.