Palmer Amaranth

Overview

Palmer Amaranth, also referred to as Amaranthus palmeri, ranks among the most problematic invasive weeds in the southern United States, and its reach has recently extended to the Midwest and Northeast. Its rapid growth, high seed production, and herbicide resistance make it a formidable foe.

(Image Credit: Howard F. Schwartz, Colorado State University, Bugwood.org)

palmer-amaranth-corn

How to Identify Palmer Amaranth

Palmer Amaranth is a tall, erect, branching plant that can reach 6–8 feet. Its long, slender leaves, arranged alternately along the stem, are typically diamond or lance-shaped. They have a broad base and taper to a point at the tip. The leaves are typically smooth and shiny, with a prominent white or pinkish vein on the underside. 

A key sign of mature Palmer Amaranth is its long, terminal inflorescence (the cluster of flowers that are on a branch), which can extend up to 3 feet in length. The female plants exhibit a spiky appearance due to the presence of sharp, pointed bracts surrounding the seeds, while the male plants have softer, less spiky inflorescences. A distinctive feature is a small, sharp spine at the leaf axil, a trait not shared by other similar-looking weeds like its close relative, Waterhemp.

Geographic Impact of Palmer Amaranth

Originally from the southwestern United States and northern Mexico, Palmer Amaranth has spread across many Midwestern and Southern states. Although it is not common to the Northeast, it has been seen there on rare occasions. It thrives in warm climates and frequently appears in disturbed areas such as roadsides, fields, and waste areas.

Timing of Palmer Amaranth Emergence 

Palmer Amaranth can emerge at any point during the growing season, but it typically appears in early spring. However, its emergence can continue until late summer or early fall, making it a persistent issue for farmers.

How Palmer Amaranth Impacts Corn

Palmer Amaranth's competitive nature poses a significant threat to corn yields. With its ability to produce up to 600,000 seeds per plant and grow up to 2–3 inches per day under optimal conditions, it can quickly overrun corn crops.

As it competes for nutrients, water, and sunlight, corn plants may experience stunted growth and reduced yield. In severe cases, Palmer Amaranth infestations can reduce corn yields by up to 91%.

Palmer Amaranth root systems outcompete the corn plant's ability to absorb essential nutrients and water. It can also alter the corn plant's photosynthetic efficiency by shading the corn plants with its large leaves. This shading reduces the amount of sunlight the corn plants can absorb, leading to reduced energy production and slower growth.

(Image Credit: Howard F. Schwartz, Colorado State University, Bugwood.org)

palmer-amaranth-corn

How to Manage Palmer Amaranth in Corn

For managing Palmer Amaranth in corn, consider a layered approach with pre-emergence residual herbicides and effective post-emergence herbicides. Recommendations include:

Pre-emergence

Post-emergence  

Always follow label instructions and guidelines for proper application rates to ensure effective control of Palmer Amaranth while minimizing potential risks.

How Palmer Amaranth Impacts Soybeans

Palmer Amaranth can inhibit soybean yield by up to 79% if not effectively managed. Known for its swift growth rate, high seed production, and resistance to multiple herbicides, Palmer Amaranth poses a significant threat to soybean production.

Competing with soybean for light, water, and nutrients, Palmer Amaranth can stunt growth and reduce yield. Its rapid growth rate enables it to quickly overtop soybean plants, blocking light and causing shading. This can lead to reduced photosynthesis, resulting in smaller, less productive soybean plants.

The competition for resources can also disrupt the soybean plant's physiology. It can lead to reduced root growth and nutrient uptake, affecting the plant's ability to produce seeds. Additionally, Palmer Amaranth can induce stress in soybean plants, leading to reduced pod and seed number and, ultimately, lower yield.

(Image Credit: Rebekah D. Wallace, University of Georgia, Bugwood.org)

palmer-amaranth-soybeans

How to Manage Palmer Amaranth in Soybeans

The best method to control Palmer Amaranth is to start out with a clean field through tillage or a burndown application, followed by a good pre-emergence application and a subsequent post-emergence application.

For effective Palmer Amaranth management in soybeans, these products are recommended: 

Pre-emergence

Pre-emergence and burndown 

Post-emergence 

Where there are significant Palmer Amaranth issues, growers often use Enlist® or XtendFlex® soybean traits. FBN offers adjuvants labeled for those herbicides. 

Remember, these rates are general recommendations. Always read and follow the specific instructions on the product label for accurate application rates and guidelines. 

Disclaimer

© 2014 - 2024 Farmer's Business Network, Inc. All rights Reserved. The sprout logo, “Farmers Business Network”, “FBN”, “FBN Direct” are trademarks or registered trademarks of Farmer's Business Network, Inc. This content was written by Norm℠, reviewed and edited by Mikaela Tierney.

This content was generated with the assistance of Norm℠, FBN’s artificial intelligence (AI) Ag Advisor, based on a dataset of information containing general industry best practices and research. The AI model did not use specific external sources to generate this content. Our process involves using AI to aid human subject matter experts with the initial drafting and/or refinement of content. 

Every product that labels PALMER AMARANTH as a weed, pest, or disease the product can be applied against is shown below. Click any product for label and more information.
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