Palmer amaranth (Amaranthus palmeri) is an annual agricultural weed native to the dry areas of the southwest United States. Waterhemp (Amaranthus tuberculatus) is native to the United State’s Midwest but is now found from Texas to Maine.
Both are invasive pigweed species that pose a major threat to farmers across much of America. These weeds are aggressive and competitive and can cause significant crop loss to farmers.
The Threat Posed By Palmer Amaranth and Waterhemp
Palmer Amaranth
In the right conditions, palmer amaranth can grow as much as two to three inches daily. If the plant is left unchecked, it can grow from six to eight feet tall, stealing resources from crops.
In perfect conditions, this prolific plant emerges throughout the growing season in late May and extends all the way through July, and can produce up to one million seeds. This profuse seed production allows the plants to take over crop fields quickly.
In field trials, corn fields have seen yield loss of up to 91 percent and soybeans of up to 79 percent. New populations found in North Dakota show that palmer amaranth can adapt to the northern Great Plains. It was first identified in North Dakota in 2018.
Waterhemp
The Tall Waterhemp (Amaranthus tuberculatus) and Common Waterhemp (Amaranthus rudis) are often grouped together due to similarities in growth and habit. Waterhemp thrives in wetter conditions but can quickly adapt, especially where the soil has been tilled. Waterhemp can grow one to 1.25 inches daily and produce 500,000 seeds from a single female plant.
Both palmer amaranth and waterhemp have long emergence periods, from spring until frost. Both plants are very aggressive and competitive and are very difficult to eradicate once they have become established.
Palmer Amaranth Herbicide Resistance
Palmer amaranth and waterhemp are dioecious, meaning they have separate male and female plants. This reproductive system forces the plants to outcross, leading to increased genetic diversity and a better ability to adapt to changing environments. Research shows that resistance to herbicide traits can transfer with the pollen, and with their growth rate, herbicide resistant strains can quickly expand to many geographic locations.
Palmer amaranth and waterhemp show resistance to multiple herbicides, including glyphosate, ALS, and PPO herbicides. They are constantly evolving new resistances in different areas. Palmer Amaranth has been among the most difficult-to-control weeds in the southern United States for two decades.
Why Is Identification of Palmer Amaranth and Waterhemp Difficult?
Palmer amaranth is easy to misidentify because it looks similar to three other common amaranth species: redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus
hybridus), Powell amaranth (Amaranthus powellii), and waterhemp (Amaranthus tuberculatus). Identifying when they are seedlings is challenging, as they all resemble one another. However, hair on the stem is typically the most apparent identifier between Palmer amaranth and waterhemp from redroot and smooth pigweed.
Characteristics of these plants can be variable even in the same field, so it is essential to inspect multiple plants within the field to determine if there are multiple species of amaranth in the same patch. Weeds that have been sprayed and survived multiple herbicide applications (especially PPO inhibitors) may have a variety of leaf shapes that may not accurately represent the species as a whole.
Even for the most experienced weed scientists, visually identifying these weeds at emergence when they are most vulnerable to herbicides is challenging. Despite having well-documented descriptions and photos of Palmer amaranth and waterhemp, their ability to adapt and change as needed means their physical attributes are not always clean-cut.
Identification of Palmer Amaranth and Waterhemp
Correctly identifying palmer amaranth and waterhemp is the first step to managing these invasive weeds. Two commonly mistaken weeds for Palmer amaranth and waterhemp are redroot pigweed and Powell amaranth. Both redroot pigweed and Powell amaranth can be differentiated from Palmer amaranth and waterhemp by their short, dense hair.
For easier identification, common characteristics of Palmer amaranth and waterhemp typically include the following:
| Palmer Amaranth | Waterhemp | |
| Leaves | – Generally rounded, ovate leaves with hair-like protrusions at the leaf tip – Often egg-like or diamond-shaped in appearance – Leaves are symmetrical in arrangement | – Long, narrow, lanceolate leaves – Leaves appear to be waxy and shiny – Open canopy |
| Stems | Smooth, no hair | Smooth, no hair |
| Petioles | Petioles are as long or longer than the leaf | Short |
| Bracts | Spiny bracts are at leaf axils on the female plants | No spiny bracts |
| Flowering Structures | – Flowering structures are thick, unbranched, and one to two feet long – Males are smooth; females are prickly. | – Flowering structures are slender, unbranched, and usually only six inches long – Male and female are smooth |
| Reproductive | Dioecious (separate male and female plants) | Dioecious (separate male and female plants) |
For more information and images to help identify Palmer amaranth and waterhemp, read Identification, Biology, and Control of Palmer Amaranth and Waterhemp in North Dakota by Joe Ikley and Brian Jenks.
Let Us Test Your Samples
The best, and safest way to be sure if your field is afflicted by Palmer amaranth or waterhemp, is to send samples for DNA testing to the NAGC lab. Getting on top of these species quickly is important – preferably while they are still seedlings. Please contact us directly if you have any questions about the submission process.
Sources
Palmer Amaranth
North Dakota State University
No date
https://www.ag.ndsu.edu/palmeramaranth
Ikley, J. & Jenks, B. (2019). Identification, Biology, and Control of Palmer Amaranth and Waterhemp in North Dakota. [Online]. Available: https://www.ndsu.edu/agriculture/ag-hub/publications/identification-biology-and-control-palmer-amaranth-and-waterhemp-north-dakota
Legleiter, T. & Johnson, B. (2013). Palmer Amaranth Biology, Identification, and Management [Online]. Purdue University. Available: www.extension.purdue.edu/extmedia/ws/ws-51-w.pdf.
Libhart, T. n.d. Palmer Amaranth and Waterhemp PDF. [Online]. PA Invasive Species Reporting. Available: https://www.agriculture.pa.gov/Plants_Land_Water/PlantIndustry/NIPPP/Documents/Palmer2.pdf
Sprague, C. & Burns, E. (2021). Multiple herbicide-resistant Palmer amaranth & waterhemp in Michigan Keys to management in soybean, corn, and alfalfa. [Online]. Michigan State University Weed Science. Available: https://www.canr.msu.edu/weeds/extension/2023-Weed-Control-Guide/2023%20E-434%20Palmer%20&%20Waterhemp.pdf
