WESTMINSTER, Colorado – May 11, 2021 – The latest survey conducted by the Weed Science Society of America (WSSA) 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 U.S. 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 www.wssa.net/wssa/weed/surveys.
Most Troublesome, Hard to Control Weeds
- Corn: Palmer amaranth and waterhemp
- Pasture, rangeland and other hay: Canada thistle
- Rice: Sedge spp. (yellow nutsedge and rice flatsedge) and Echinochloa (barnyardgrass and coast cockspur)
- 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 (downy and Japanese brome, cheat, rescuegrass)
Most Commonly Found Weeds
- Corn: Common lambsquarters and foxtail spp. (green, yellow and giant foxtail)
- Pasture, rangeland and other hay: Bromus (downy, red and smooth brome, cheat) and Canada thistle
- Rice: Sedge spp. (yellow nutsedge and rice flatsedge) and barnyardgrass
- 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 (henbit and purple deadnettle)
About the Weed Science Society of America
The Weed Science Society of America, a nonprofit scientific society, was founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment. The Society promotes research, education and extension outreach activities related to weeds, provides science-based information to the public and policy makers, fosters awareness of weeds and their impact on managed and natural ecosystems, and promotes cooperation among weed science organizations across the nation and around the world. For more information, visit www.wssa.net.
EDITOR’S NOTE: Images of Palmer amaranth and annual bluegrass are available for download online.
Lee Van Wychen
Executive Director of Science Policy
National & Regional Weed Science Societies