South Dakota Weeds: Identifying Palmer Pigweed

Palmer Amaranth in South Dakota

Annual weeds are threat to many cropping systems in South Dakota. Palmer amaranth is a newer threat in the state depending upon your geographical location. Confirmed sightings in a few counties in South Dakota include Potter, Sully, Hughes, Lyman, Bennett, Buffalo and Douglas.

These are confirmed sightings and there could be other counties as well that could have Palmer amaranth in the state. Most Palmer plants found in South Dakota originated from a contaminated source, such as contaminated machinery, seed or manure. Palmer is an invasive annual plant originally from the southwestern U.S. with male and female plants.

Females may produce over 100,000 seeds per plant. Palmer plants emerge later in the growing season like tall waterhemp but research suggests that Palmer amaranth may germinate at 50° F while tall waterhemp like other amaranth species germinate around 68° F. Control of Palmer is similar to other amaranth species, such as waterhemp.

Research suggests Palmer grows more aggressively than waterhemp. The most resistance discovered in one Palmer plant to this point has been three sites of action, compared to waterhemp where up to five sites of action has been discovered in one plant. With these resistance possibilities, identification is very important with chemical control.

Seedling Identification

Seedling identification is of most important, and can be the factor that helps control it vs not controlling a certain species. Any plant at this stage is very vulnerable to most herbicides unless tolerance to herbicides exists. Most herbicide labels recommend controlling these amaranth species at six inches above ground height or smaller.

Palmer Amaranth seedling (Left) vs Tall Waterhemp seedling (Right).

Leaf Shapes & Growth Patterns

Leaf shapes and growth patterns of Palmer Amaranth and Tall Waterhemp are pictured below. Note the rounded/diamond shape leaves and long petioles (stem that attaches leaf to vertical stem) of the Palmer amaranth plant with most petioles of the lower leaves extending beyond the blades of the upper leaves. Petioles on Palmer amaranth will be longer than the leaf itself.

Palmer Amaranth young plant.
Tall Waterhemp young plant.
Petioles on Palmer amaranth will be longer than the leaf itself.

Herbicide Resistance Info

Other Features

Tall waterhemp and Palmer amaranth are hairless on the leaf surface and stems. Seed head of Palmer amaranth 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 seed.

In addition, when Palmer amaranth plants reach flowering they will have flowering parts on the node (area where lateral stems attach to vertical stems) and up the stem. Palmer amaranth does have hairless stem and leaves, except a single hair is rarely found at the tip of adult leaves. Another characteristic that is rare for Palmer amaranth is a watermark shaped like a “V” on each leaf surface.

More Information

If you as a producer or an agronomy specialist have questions about weeds or weed identification, feel free to contact Gared Shaffer.

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