Look out for Palmer amaranth

An aggressive pigweed, Palmer amaranth is known for its quick spread, resistance to herbicide and crop-damaging impact.

Farmers and residents in Richland County, North Dakota, are advised to watch for Palmer amaranth.

An aggressive pigweed, Palmer amaranth is known for its quick spread, resistance to herbicide and crop-damaging impact. Richland County was one of five Red River Valley counties where Palmer amaranth was positively identified in October 2018.

“No additional counties have been added,” said Chandra Langseth, agriculture and natural resource agent for the NDSU Extension of Richland County. “Now is a great time (to search), because the weeds are bigger and identification is easier.”

Palmer amaranth is often mistaken for waterhemp, redroot pigweed and Powell amaranth. Each weed has distinct characteristics, Langseth explained.

The stem of a Palmer amaranth plant is smooth and hairless. Its petiole, or leaf stem, is long and often longer than the leaf blade. The seed head is open, unbranched and especially long.

Palmer amaranth is dioecious, capable of separate male and female plants. Male plants have smooth seed heads. Female plants have prickly seed heads. Spiny bracts are also found on female plants.

Waterhemp has a smooth, hairless stem. Its petiole is short and its seed head is open, unbranched and smooth. Although dioecious, waterhemp does not have spiny bracts on any of its plants.

Redroot pigweed has short, dense hairs on its stems. Its petioles are typically short. The seed head is compact, branched and has a length of about 1-2 inches. The seed head can be smooth or rough, but is not prickly. Redroot pigweed is monoecious, having male and female flowers on the same plant. Neither flower has spiny bracts.

Powell amaranth has short hairs on its stems, ranging from dense to sparse. Its petioles are typically short, but can be long on mature plants. The seed head is compact, branched and measures about 4-8 inches. The seed head can be smooth or rough, but is not prickly. Powell amaranth is also monoecious, with bract-less flowers.

Once Palmer amaranth is identified, Langseth said, concern shifts to site management.

“The site in Richland County (where it was found in 2018) was managed really well this year,” she said. “It wasn’t perfect, but I have to give kudos to the grower. They kept on top of that particular plant, knowing it was going to be a tough one to manage.”

Farmers are advised to follow effective practices, closely monitoring components like feed and equipment. If Palmer amaranth is detected, it is best to reduce the possibility of seed travel. Transporting pigweed in open truck beds is never recommended.

“You want to keep seeds contained to that field,” Langseth said.

For additional information, contact the NDSU Extension’s Richland County office. It’s open from 8-5 a.m. at the Richland County Courthouse, 418 Second Ave. N. in Wahpeton. Langseth can also be reached at 701-642-7793.

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