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Palmer Amaranth: Now Is the Time to Scout

Updated August 06, 2021

Paul O. Johnson

SDSU Extension Weed Science Coordinator

Green, Plamer amaranth next to a green sunflower.
Figure 1. Small Palmer amaranth plant in sunflower field. Courtesy: Paul O. Johnson.

South Dakota State University (SDSU) Extension encourages farmers, ranchers and other landowners to start scouting for Palmer amaranth (Figure 1), a weed that has been rapidly spreading north into the United States and is likely to be glyphosate-resistant.

In most cases, Palmer amaranth is easiest to identify when it is nearly or fully grown. It’s been about three years since the first Palmer amaranth (Amaranthus palmen) was found in South Dakota. Since that time, the weed has been confirmed in 16 counties throughout South Dakota. So far in 2021, the weed has been found in central South Dakota within sunflower or soybean fields. It seems like we are more likely to find it in areas that have stress periods.

Palmer amaranth is in the pigweed family. The weed has a lot of close relatives which can easily be confused for Palmer amaranth, the most commonly confused being common waterhemp. Other species include spiny pigweed, tumble pigweed, smooth pigweed and redroot pigweed.

What to Look for When Scouting

Green plant with green, spiny structure at leaf base.
Figure 2. Palmer amaranth has these spiny growths between the stem and petiole; waterhemp does not. Courtesy: Paul O. Johnson.

There is no one thing to look for that is a sure sign that a plant is Palmer amaranth. However, when scouting for Palmer amaranth, there are some physical characteristics to look for as follows:

  • The area where the stem connects to the petioles will have spines on it (Figure 2).
  • Some of the petioles (the short stem from the main stem to the leaf) will be a lot longer than the leaf length (Figure 3).
  • The leaf is more cordate (heart-shaped) than waterhemp, which is more elliptic (oblong).
  • The Palmer amaranth head will be long, and, if it is female, the plant will also be spiny.
Green leaf with long, green petiole.
Figure 3. Palmer amaranth has a longer petiole than leaf. Waterhemp is the opposite. Courtesy: Paul O. Johnson.

If you suspect that you have Palmer amaranth, take photos of the entire plant, the leaf and petiole area and the seed head. Please send those photos via email to Paul O. Johnson. Send them in high resolution. Please include your best contact information so that identification results can be shared and more questions can be asked if necessary. For more information, feel free to call Paul at 605-688-4591.

Related Topics

Soybean Weeds, Sunflower, Oilseed