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Posted 2 April 2017. PMN Crop News.

Palmer Amaranth Threat to S.D. Agriculture

Source: South Dakota State University Press Release.

Brookings, South Dakota (March 29, 2017)--Palmer amaranth is a newer weed threat in South Dakota.


Originally, from the southwestern U.S., this invasive annual has been found in the following counties: Bennett, Buffalo, Dewey, Douglas, Hughes, Lyman, Potter and Sully.

"There could be other counties as well that are host to Palmer amaranth," said Gared Shaffer, SDSU Extension Weeds Field Specialist.

Shaffer explained that most Palmer amaranth plants found in South Dakota originated from a contaminated source - contaminated machinery, seed or manure.

Get to know Mr. & Mrs. Palmer amaranth

Palmer amaranth boasts both male and female plants. Females may produce more than 100,000 seeds per plant. Male Palmer plants are the pollinators and without them, there will be no seed on the female plants. However, if you have a female plant you will most likely have a male plant as well in the same vicinity.

Palmer plants emerge later in the growing season, like tall waterhemp, Shaffer explained. Research suggests however that Palmer amaranth may germinate at 50 degrees Fahrenheit while tall waterhemp, like other amaranth species, germinates around 68 degrees Fahrenheit.

Controlling this weed

Control of Palmer is similar to other amaranth species, such as waterhemp.

"Research suggests Palmer grows more aggressively than waterhemp," said Shaffer.

He explained that the most resistance discovered in one Palmer plant, to this point, has been three sites of action. Compare this to waterhemp, which has up to five sites of action resistance in one plant. A site of action is the area in the plant the herbicide is attacking.

"With these resistance possibilities, identification is very important with chemical control," Shaffer said.

Seedling identification

Seedling identification is most important and can be the factor that helps control Palmer, said Shaffer.

"Any plant at seedling stage is very vulnerable to most herbicides, unless tolerance to herbicides exists," he said. "Most herbicide labels recommend controlling these amaranth species at 6-inches above ground height or smaller."

Leaf shapes & growth patterns

When comparing the leaf shapes and growth patterns of Palmer amaranth to Tall Waterhemp, Palmer amaranth has rounded/diamond shape leaves and long petioles (stem that attaches leaf to vertical stem) with most petioles of the lower leaves extending beyond the blades of the upper leaves. Tall waterhemp will have narrow leaves and short petioles.

Petioles on Palmer amaranth will be longer than the leaf itself. Whereas, Tall Waterhemp petioles in general, will not be longer than the leaf itself.

Other features

Tall waterhemp and Palmer amaranth are hairless on the leaf surface and stems, and each species have female and male plants.

Male Palmer plants are soft to the touch and contain pollen. Female Palmer heads are prickly to the touch and contain seeds. In addition, when Palmer amaranth plants reach flowering Shaffer said they will have flowering parts on the node (area where lateral stems attach to vertical stems) and up the stem, tall waterhemp will not have this characteristic.

"Palmer amaranth does have hairless stem and leaves, except a single hair is rarely found at the tip of adult leaves, tall waterhemp does not" Shaffer said. "Another characteristic that is rare for Palmer amaranth is a watermark shaped like a "V" on each leaf surface, tall waterhemp does not."

Need ID help?

South Dakota producers and agronomy specialists who have questions are encouraged to contact Shaffer at