This month’s article discusses one of the most problematic weeds in Kansas, Palmer amaranth, also called carelessweed.
Ecology of Palmer amaranth
Palmer amaranth (Amaranthus palmeri) is native to the American southwest. In recent years, it has spread across much of the United States and become an aggressive weed. Palmer amaranth is an annual plant that grows up to six feet tall or more. It has alternate, hairless leaves (the first true leaves are opposite; Figure 1). Mature leaves are distinctly egg-shaped, but younger leaves may be more oblong (similar to waterhemp). Palmer amaranth petioles are usually longer than the leaf (Figure 2). Palmer amaranth is a very diverse species. Some biotypes have a dark watermark on the upper leaf surface, some have a tiny hair in the notch at the tip of the leaf.
Figure 1. Examples of Palmer amaranth seedlings. Top photo taken by Dallas Peterson, K-State Research and Extension. Bottom photo credit: Bruce Ackley.
Figure 2. Leaf demonstrating petiole length of Palmer amaranth. Photo credit: Ross Recker
Palmer amaranth features are very diverse, due in part that it is a dioecious species, meaning male and female flowers grow on separate plants (Figure 3). Female flowers must be pollinated by wind-blown pollen, which can come from up to 1000 feet away. In addition, Palmer amaranth can hybridize with waterhemp and smooth pigweed, albeit at low rates.
Figure 3. Example of the diversity of Palmer amaranth within the same field. Photo by Dallas Peterson, K-State Research and Extension.
A single Palmer amaranth plant can produce over 500,000 seeds if it emerges early (Figure 4). If it emerges later, after July, a single plant can produce up to 80,000 seeds. Palmer amaranth seeds are small, dark brown, and shiny (Figure 5). Palmer amaranth seed has relatively low rates of dormancy. Eighty percent or more of seeds in the upper ½ of the soil will germinate quickly once moisture and temperature are favorable.
Figure 4. Palmer amaranth seed head. Photo credit: Howard Schwartz
Figure 5. Palmer amaranth seeds. Photo credit: Bruce Ackley
If uncontrolled, Palmer amaranth can reduce yield by up to 90% in corn, 50% in sorghum and 79% in soybean%. Palmer amaranth populations in Kansas have developed confirmed resistance to: Pursuit, Glean, and Harmony (Group 2), 2,4-D (Group 4), atrazine (Group 5), glyphosate (Group 9), and Armezon, Callisto, and Laudis (Group 27).
Resistance to key post-emergence herbicides, coupled with an extended period of emergence, makes residual herbicides critical for Palmer amaranth control. Herbicides with residual activity should be used in combination with other effective herbicides at planting and in-crop. Examples of herbicides that provide excellent control before crop emergence are products that contain metribuzin, flumioxazin (Valor), or sulfentrazone (Authority/Spartan). Examples of herbicides to consider using after crop emergence include acetochlor (Warrant), pyroxasulfone (Zidua), or S-metolachlor (Dual Magnum). Herbicides such as atrazine and mesotrione (Callisto) could be used for Palmer amaranth control either before or after emergence of corn or sorghum. Always read and follow label directions.
Ward, et al. 2013. Palmer Amaranth: A review.
Did you miss these World of Weeds articles?
Kochia – http://bit.ly/2udtQK1
Common sunflower – http://bit.ly/2wSy8be
Stinging nettle – http://bit.ly/37nOqFC
Mistletoe - http://bit.ly/2QbmXQO
Stay tuned for the next World of Weeds article coming out soon! Feel free to send Dr. Lancaster or Kathy Gehl (firstname.lastname@example.org) an email if you have a special request for a future article.
Sarah Lancaster, Extension Weed Science Specialist