Field Facts: Palmer Amaranth
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Originating in the Southwest United States, Palmer amaranth is an invasive weed that continues to evolve. Over the last decade, Palmer amaranth has infiltrated the Midwest and made a name for itself as one of the most competitive weeds in corn, soybean and cotton fields across the country. This weed is a serious challenge due to its long germination period and rapid growth.
However, because Palmer amaranth, redroot pigweed and waterhemp all fall into the pigweed category, telling Palmer amaranth apart from other pigweeds can be difficult. The best way to minimize the threat of Palmer amaranth is to correctly identify new infestations and quickly initiate control measures.
- Common name: Palmer amaranth
- Scientific name: Amaranthus palmeri
- Leaf: Ovate to diamond-shaped
- Stem: Smooth (hairless) and can be green, red, or red-and-green striped in color
- Flowers: Male flowers can appear yellow due to pollen presence, while female flowers have large, green bracts.1
- Reproduction: Seeds
Identifying Palmer amaranth
Here are the characteristics that can help you identify Palmer amaranth.
- The terminal seed heads on female Palmer amaranth plants can grow up to 3 feet long and will feel prickly.
- The petioles will be as long or longer than the leaf blades themselves.
- The leaves tend to be wider and ovate to diamond-shaped. Leaves may have a sharp spine at their tips.
- The stems are smooth and hairless.
- Some leaves have a white chevron-shaped watermark.
Fast Facts on Palmer Amaranth:
- Palmer amaranth is now considered the most competitive weed in Midwestern fields. This weed can reduce soybean yield by nearly 80% and corn yield by more than 90%.2
- Palmer amaranth can grow up to 2½ inches per day.
- Palmer amaranth is dioecious, meaning male and female flowers grow on different plants. This trait increases the weed’s genetic diversity, allowing it to develop herbicide resistance more easily.
- Female flowers have large bracts (up to ¼ inch long) that can become sharp, making the seed heads painful to handle. One female Palmer amaranth weed can produce more than 600,000 seeds.
- Palmer amaranth seeds are small and thrive in no-till or minimal tillage fields, where they can remain in their ideal emergence zone: the top inch of soil. Humans can also easily transport the small seeds through grain, seed or feed contamination; or on harvesting equipment.3
- Palmer amaranth’s growing period extends well into the typical crop season and can even occur after harvest. This wide growth period forces growers to manage the weed throughout the year, unlike most other summer annual weeds that are typically managed only through early summer.
- Populations of Palmer amaranth have shown resistance to several herbicide groups, such as Group 2 (ALS inhibitors), Group 9 (EPSP synthase inhibitors) and Group 14 (PPO inhibitors). According to the International Herbicide-resistant Weed Database, populations of herbicide-resistant Palmer amaranth have been reported in 31 states, which is an increase of two states since 2023.
Control Tips:
Encourage customers to use a herbicide program approach with multiple modes of action and residual activity to control Palmer amaranth. This means including residuals in both the preemergence and postemergence applications. Additionally, timely applications (when weeds are small: 4 inches or shorter) are critical in reducing Palmer amaranth populations.
Corteva Agriscience offers several herbicide solutions so customers can tailor their weed control programs to fit their unique agronomic needs. Learn about top crop protection products to help your customers fight the toughest weeds found in corn and soybean fields this year.
In addition to a strong herbicide program, customers can implement several cultural practices to control Palmer amaranth. Some of these include:
- Rotating crops to allow for different herbicide modes of action to be used.
- Harvesting any fields with heavy infestations last to prevent spread.
- Deep tillage to reduce the amount of weed seeds that germinate by burying them at unfavorable depths.
- Planting in narrow rows to help soybeans, for example, outcompete weeds for sunlight and nutrients.
- Planting fall-seeded cover crops like cereal rye.
Work with your customers and your Corteva Agriscience representative to identify which products and practices make the most sense to control Palmer amaranth in your area.
1Vollmer, K., and B. Beale. 2024. A Guide for Identifying Pigweed Species Commonly Found in Maryland.
https://extension.umd.edu/resource/guide-identifying-pigweed-species-commonly-found-maryland-eb-2023-0654/
2Hager, A. 2018. Remain Vigilant for Palmer Amaranth.
https://farmdocdaily.illinois.edu/2018/07/remain-vigilant-for-palmer-amaranth.html
3Legleiter, T., and B. Johnson. 2013. Palmer Amaranth Biology, Identification, and Management.
https://www.extension.purdue.edu/extmedia/ws/ws-51-w.pdf