Watch for this weed Palmer amaranth poses serious risks to broadleaf crops, is glyphosate resistant, spreads easily

By Trista Crossley


Ian Burke, weed scientist at Washington State University, has seen a lot of weeds in his career, but few worry him more than Palmer amaranth.

“In every metric possible, it’s capable of outcompeting other broadleaf crops. It’s not even a fair fight. It also appears to have methods of adaptive capacity that most other weeds do not,” he explained. 

Palmer amaranth, also called Palmer’s pigweed, was one of the first weeds to evolve glyphosate resistance not long after glyphosate-resistant soybeans and cotton were introduced. Scientists working on the problem discovered that the genomes of resistant plants were dramatically changed, sometimes duplicating the glyphosate target gene over 100 times. Since then, the plant has developed resistance to eight different herbicidal modes of action.

“When glyphosate resistance happened, scientists in the Southeast worked really hard to try to find herbicides that would continue to work,” Burke said. “They realized that, yes, they could find them, but Palmer amaranth grows so fast in the spring, that the window to spray it after it germinates is two and a half days. This is a weed that would be very well at home in the irrigated Columbia Basin.”

A 2020 weed risk assessment by the U.S. Department of Agriculture estimates that about 81% of the U.S. is suitable habitat for Palmer amaranth. It is native to the Southwest, but has spread to at least 27 states; so far, the weed hasn’t gained a toehold in Washington. Palmer amaranth has an extended germination and emergence window, rapid growth rates, and high water-use efficiency. It can produce up to 250,000 seeds from one plant, and seeds can remain viable in the soil for up to five years. Palmer amaranth has a fast growth rate of approximately two to three inches per day and commonly reaches heights of six to eight feet. Yield losses have been reported up to 91% in corn and 79% in soybeans. Palmer amaranth can also be toxic to livestock due to nitrates in the leaves and is considered a noxious weed in Washington state.

Burke said there isn’t enough data to estimate the initial impact Palmer amaranth might have on wheat. Winter wheat is usually well established by the time the weed germinates, although spring wheat, especially during warmer springs, might fare worse. Burke points to a closely related species, redroot pigweed, as a guide to how much of a problem Palmer amaranth might be in a wheat crop. He said redroot pigweed is annoying and can be problematic in some areas, but in wheat, he doesn’t see nearly the kind of complaints that other crops have with it.

In 2023, two Palmer amaranth infestations were found in the Evergreen State. In Spokane County, Palmer amaranth was introduced via cover crop seed, resulting in hundreds of plants. In the Benton County instance, only two plants were discovered, and the source hasn’t been confirmed. In both cases, the plants were eradicated. Small infestations have also been found in Idaho and eastern Oregon.

In an effort to improve soil health, farmers may be unintentionally spreading a noxious weed through cover crops.

“Because there’s not enough cover crop seed in the country, the U.S. is importing cover crop seed for use. So, there’s a lot of seed moving around the country, and the cover crop seed itself is not as clean as some of our certified seed can be,” Burke said.

Palmer amaranth is easily spread through contaminated seed or hay, as a hitchhiker on vehicles or equipment, and by water or wildlife. Burke said that in Southern Idaho, it was spread by trains carrying cottonseed. Other ways he said the seed could be easily spread in Washington are from uncommon oil seeds imported from Montana and the Dakotas and vehicles or other farm equipment, especially combines, purchased from the Midwest that haven’t been meticulously cleaned. 

To help identify Palmer amaranth, look for:

  • Smooth stems with no hairs.
  • A petiole (leaf stem) that is longer than the leaf.
  • Nonwavy, diamond-shaped leaves with a small spine at the tip.
  • Elongated seed heads, up to 24 inches long.

Seeds and seedlings require genetic testing for identification.

If a farmer finds a suspicious plant, they should report it to their county weed board as soon as possible. According to the Washington State Noxious Weed Control Board, small infestations of Palmer amaranth can be hand pulled or dug out, but larger infestations will need repeated cultivation. Mowing alone will not control Palmer amaranth, as plants will survive and just set seed closer to the ground. Prescribed fire will also kill Palmer amaranth.

“A lot of the weed identification apps you can put on your phone are pretty good,” Burke said. “I encourage farmers to use that sort of technology in the moment. It only takes a few seconds to check.”

Click here to download a weed guide.