Until about a decade ago, Palmer amaranth was relatively unheard of in the Midwest. Since then, this pigweed has made a name for itself as one of the most competitive weeds in corn, soybean and cotton fields across the country.
However, because Palmer amaranth, redroot pigweed and waterhemp all fall into the pigweed category, telling Palmer amaranth apart from other pigweeds can be a challenge. Correctly identifying the weed in question is critical in implementing adequate control measures.
Identifying Palmer amaranth
Here are 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 Midwest fields and can reduce soybean yield by nearly 80% and corn yield by more than 90%.1
- Palmer amaranth can grow by as much as 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 greater than 600,000 seeds.
- Populations of Palmer amaranth have shown resistance to many 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, 29 states have reported populations of herbicide-resistant Palmer amaranth in crops.
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 a weed control program to fit their needs. Learn about key corn and soybean herbicides available at the following links:
In addition to a strong herbicide program, customers can implement several cultural practices to control Palmer amaranth. Some of those 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.
1Hager, A. 2018. Remain Vigilant for Palmer Amaranth