Weed of the Month: Palmer amaranth

  • Palmer amaranthCommon name: Palmer pigweed
  • Scientific name: Amaranthus palmeri
  • Grass or broadleaf: Annual broadleaf
  • Is native to the southwestern United States, has become a devastating weed problem in the South and has recently spread to the upper Midwest.
  • Most competitive and aggressive pigweed species. Season-long competition by Palmer amaranth at 2.5 plants per foot of row can reduce soybean yield by as much as 79 percent.1
  • Emerges later than many summer-annual broadleaf weeds, continues to emerge throughout the growing season and can grow 2 to 5 inches in three days or less.
  • A single female Palmer amaranth plant can produce approximately 600,000 seeds.
  • Ranked the most troublesome weed in the United States by the Weed Science Society of America (WSSA) for 2016.

A fact that may surprise you …

 The leaves, stems and seeds of Palmer amaranth are edible and highly nutritious; and the plants were once widely cultivated and eaten by Native Americans across North America, both for the abundant seeds and as a cooked or dried green vegetable.

Fast facts from Jeff Ellis, Ph.D., field scientist, Dow AgroSciences

To correctly identify Palmer amaranth when scouting, look for the following plant-distinguishing features:

  • The petioles, especially on older leaves, will be as long or longer than the leaf blade itself.
  • Plants sometimes — but not always — have a white V-shaped watermark on leaves. If a watermark is present, this rules out other members of the pigweed family.
  • Leaf shape on Palmer amaranth plants are wider than other pigweed species and ovate to diamond shape.
  • Individual plants are either male or female, which forces outcrossing and genetic diversity. This gives Palmer amaranth the ability to adapt and quickly produce resistance genes to single-mode-of-action herbicides.

Resistance statistics*

  • According to WeedScience.org, herbicide-resistant Palmer amaranth has been documented in corn and soybean fields in 24 states: Alabama, Arkansas, California, Delaware, Florida, Georgia, Illinois, Kansas, Kentucky, Louisiana, Maryland, Michigan, Missouri, Nebraska, New Jersey, New Mexico, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Texas, Virginia and Wisconsin.
  • Since the late 1980s, Palmer amaranth has evolved resistance to six herbicide sites of action: ALS inhibitors (Group 2); microtubule inhibitors (Group 3); Photosystem II inhibitors (Group 5); EPSP synthase inhibitors (Group 9); HPPD inhibitors (Group 27); and, most recently documented in Arkansas and Mississippi, protoporphyrinogen oxidase (PPO) inhibitors (Group 14).

*Resistance confirmation does not necessarily include all weeds and may vary among different areas of each state.

Palmer amaranth control/management tips:

Ellis says:

  • In both corn and soybeans, aim to control Palmer amaranth before plants emerge by using a residual herbicide.
  • Tank-mix residual herbicides with postemergence herbicides to prevent Palmer amaranth from emerging later in the season.
  • Palmer amaranth germinates throughout the season, so it is important to “layer” residual herbicides.
  • Growers, especially for soybeans, should always use residual herbicides and avoid having to control Palmer amaranth after emergence because there are very few effective postemergence herbicide control options.

General tips to manage herbicide-resistant weeds

Growers in the Midwest and Midsouth face some of the toughest weed species, including Palmer amaranth, waterhemp, ragweed and marestail. Taking the following steps during the season can help them manage weed resistance issues:

  • Develop an integrated weed management plan that delivers multiple modes of action throughout the season. With resistance increasing, the Enlist weed control system may allow use of effective postemergence modes of action, including glufosinate in soybeans and a new 2,4-D in corn and soybeans.
  • Use full rates of the herbicides during applications. Do not use partial rates or trim back for any reason, including cost.
  • Spray when weeds are small. Although it can be challenging because of weather and other factors, this is the ideal application timing.
  • Scout fields regularly to identify weeds when they are small and easy to control.

Dow AgroSciences weed control solutions:

Corn
SureStart® II herbicide
Resicore® herbicide
Keystone® NXT herbicide
Keystone® LA NXT herbicide 

FulTime® NXT herbicide
Surpass® NXT herbicide 
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Soybeans
Sonic® herbicide
Surveil® herbicide
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Additional information:
More information can be found through these weed science resources:

Palmer Amaranth Biology, Identification and Management — Purdue University Extension

Guidelines for the Identification and Management of Palmer Amaranth in Illinois Agronomic Crops — University of Illinois Department of Crop Science

Palmer Amaranth Identified in Nine Iowa Counties — Iowa State University Extension and Outreach/Integrated Crop Management

1United Soybean Board. 2013. Palmer Amaranth Management in Soybeans. http://takeactiononweeds.com/wp-content/uploads/2014/01/palmer-amaranth-management-in-soybeans.pdf

®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. FulTime NXT, Keystone LA NXT and Keystone NXT are federally Restricted Use Pesticides. Duramax, Durango DMA, Enlist Duo, FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, Sonic, SureStart II, Surpass NXT and Surveil are not registered for sale or use in all states. FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, SureStart II and Surpass NXT are not available for sale, distribution or use in Nassau and Suffolk counties in the state of New York. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions. ©2016 Dow AgroSciences LLC