For the most troublesome weed in corn, the winner is …

large palmer amaranth in corn
Photo courtesy University of Tennessee

The latest survey conducted by the Weed Science Society of America shows that Palmer amaranth is the hardest to control weed in grass crops, while foxtail species are the most prevalent.

More than 315 weed scientists across the United States and Canada participated in the survey, which was conducted during 2020. Seven categories of grass crops were included, covering weeds found in corn; rice; sorghum; turf; spring cereal grains; winter cereal grains; and pastureland, rangeland and other hay.

In addition to leading the overall survey results as the most troublesome weed, Palmer amaranth was identified as the most troublesome weed in corn, followed closely by its cousin waterhemp. Palmer amaranth also ranked as both the most troublesome and the most common weed in sorghum.

Why is Palmer amaranth such a problem? Stanley Culpepper, professor and Extension weed scientist at the University of Georgia, says there are multiple factors at play.

“Palmer amaranth grows rapidly, has an extensive root structure and produces massive amounts of seeds that are easily transported and spread,” he says. “Even more impressive are its genetic capabilities. Palmer amaranth can quickly evolve resistance to many important herbicides and has the potential to transfer that resistance to new plants through pollen movement.”

Culpepper advises growers to have a healthy respect for Palmer amaranth’s ability to dominate the landscape. “Removing it before it produces seed is key,” he says. “You won’t win with herbicides alone. You need timely intervention using a holistic approach.”

Bluegrass on the rise

The survey also confirms the rising importance of bluegrass weed species, mainly annual bluegrass. Bluegrass species ranked second only to Palmer amaranth as the “most troublesome” weeds overall. They also were ranked as the most troublesome weeds in turf.

Jim Brosnan, head of turfgrass weed science research and Extension at the University of Tennessee, says bluegrass has much in common with Palmer amaranth. For example, it is a prolific seed producer and is highly adaptable.

One proof point: Less than a decade ago, Brosnan confirmed the first instance of herbicide resistance in annual bluegrass when the weed escaped treatment by glyphosate on a Tennessee golf course. Today annual bluegrass has adapted and evolved resistance to multiple herbicides globally, targeting 10 different sites of action.

“Annual bluegrass is especially problematic in urban ecosystems, including golf courses, lawns, sports fields, public parks and greenways,” Brosnan says. “Long-term, sustainable control will require a diversified approach that doesn’t rely solely on herbicides.”

Breakdown by crop/habitat

Below are results for weeds in each of the grass crops from the survey. To take a deeper dive into the results and to see other top-ranked weeds, visit

Most troublesome, hard-to-control weeds

• Corn: Palmer amaranth and waterhemp

• Rice: Sedge spp. (yellow nutsedge and rice flatsedge) and Echinochloa spp. (barnyardgrass and coast cockspur)

• Pasture, rangeland and other hay: Canada thistle

• Spring cereal grains: Foxtail spp. (green and yellow foxtail)

• Sorghum: Palmer amaranth and johnsongrass

• Turf: Bluegrass spp. (annual and roughstalk bluegrass)

• Winter cereal grains: Italian ryegrass and Bromus spp. (downy and Japanese brome, cheat, rescuegrass)

Most commonly found weeds

• Corn: Common lambsquarters and foxtail spp. (green, yellow and giant foxtail)

• Rice: Sedge spp. (yellow nutsedge and rice flatsedge) and barnyardgrass

• Pasture, rangeland and other hay: Bromus spp. (downy, red and smooth brome, cheat) and Canada thistle

• Spring cereal grains: Common lambsquarters and foxtail spp. (green, yellow and giant foxtail)

• Sorghum: Palmer amaranth

• Turf: Crabgrass spp. (large, smooth and southern crabgrass)

• Winter cereal grains: Lamium spp. (henbit and purple deadnettle)

The Weed Science Society of America contributed this article.

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