Invasive weed takes hold in Kansas

Palmer amaranth, otherwise known as “Palmer pigweed,” has become a problem to many farmers in central and western parts of Kansas.

Palmer amaranth is an aggressive and invasive weed that used to be controlled by a popular herbicide called glyphosate.

However, the weed has started to resist the herbicide. Pigweed was spurred by late spring and early summer rainfall. It grows rapidly in hot conditions and is known for its prolific seeding.

One pigweed plant can produce several hundred thousand seeds.

Dallas Peterson, and agronomist and weed specialist at Kansas State University, has taken many phone calls regarding the issue.

“Glyphosate-resistant Palmer amaranth was first confirmed in Kansas three years ago but seems to be exploding across central Kansas this year,” he said.

If herbicide treatment provides good control of some pigweed plants and fails to control others, it is an indication of resistance.

“Producers need to use an integrated approach to weed control that utilizes a variety of cultural practices and herbicide modes of action to help control weeds and minimize herbicide resistance,” Peterson said. “The use of effective pre-emergence residual herbicides is probably going to be very important to help manage Palmer amaranth in the future.”

If a farmer notices just a few scattered pigweed plants that have escaped herbicide treatment, it might be worth it for him to remove those plants by hand to prevent seed production.

“If not, the resistant biotypes will increase and spread across the field and to other fields by way of the combine,” Peterson said. “If poor control was achieved with glyphosate, it is probably safe to assume that it is resistant and plan accordingly, both this year and in the future.”

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