Palmer amaranth emerging. Photo by OSU Extension.

Vigilance required this year for clean fields in the future

While my four-year-old son is a notorious dinner table food waster, there are some notable exceptions. He loves berries. Raspberries, blackberries, strawberries or blueberries — he loves them all. He especially loves to slather them in homemade whipped cream in a berry parfait. In a “no berry left behind” policy, he is consistently a berry parfait “clean plater.” With great tenacity he seeks out every last berry until they are all devoured.

A similar “clean fielder” approach appears to be a necessity this growing season for farmers dealing with the notorious palmer amaranth outbreaks that are springing up in crop fields around the state. When it comes to this problematic weed, they need to be as scarce as a berry parfait on our table after dinner to prevent years of future weed problems.

OSU Extension weed control specialist Mark Loux emphasized this point in this week’s CORN Newsletter after some troubling findings in Ohio fields already this season.

“We were scouting some fields in southern Madison County last week to see if we could find emerged Palmer amaranth plants this early,” Loux said. “We did find some, and if the concentration that we observed within a few square feet is any indication, it reinforces the principle that letting even one plant go to seed is a big mistake.”

Loux cited research from the Weed Science Society of America in Arkansas cotton fields where 20,000 glyphosate-resistant Palmer amaranth seeds were released within a one-square-mile area.

“Keep in mind that 20,000 seeds may represent only 2% of the possible seed from one plant, so this would appear to be an underestimation of future problems. However, glyphosate was the only herbicide used for several years after the introduction of seed, evidently to allow determination of the ‘worst case scenario,’” Loux said. “So here’s what happened. In the first growing season, a separate patch of Palmer amaranth emerged 375 feet from the original location. In the second year, resistant plants expanded to reach field boundaries and infested 20% of the entire area, resulting in decreased cotton yield and interference with harvest. By the third growing season, Palmer amaranth had completely colonized the fields, making cotton harvest impossible.”

Loux and the Weed Science Society of America agree that a zero tolerance threshold for even a single Palmer amaranth plant going to seed this year is vital for clean fields in the future.

“Hopefully your first thought upon reading this is, ‘well I am using residual herbicides in addition to glyphosate, which should reduce the rate of increase following an initial Palmer amaranth introduction.’  This is correct, and residual herbicides are a valuable tool in the management of Palmer amaranth,” Loux said. “But it’s difficult to achieve 100% control of an established Palmer amaranth infestation with even the most effective program. The best approach is to not let it get established in the first place. The benefits of mid- and late-season scouting to find and remove Palmer plants before they produce seed cannot be overestimated.”

For more photos from Loux’s early season scouting efforts visit

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