Palmer Amaranth: Now Is the Time to Scout

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.

Figure 1. Small Palmer amaranth plant in sunflower field. Courtesy: Paul O. Johnson.

What to Look for When Scouting

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.