July 19, 2022

Palmer Amaranth Management

Palmer amaranth (Amaranthus palmeri S. Watson) is an aggressive, invasive weed originating from the United States Sonoran Desert and northern Mexico. Other common names for Palmer amaranth include Palmer pigweed and careless weed. 

The weed spread slowly through the early 1900’s through the southern United States where it adapted easily to the droughty environment; however, it did not become a problematic weed in cotton until 1995 in North and South Carolina.1 In recent years, it has spread rapidly throughout the Midwest in part through contaminated Conservation Reserve Program (CRP) and other conservation seed mixes.2 In 2021 Palmer amaranth was verified in Manitoba, Canada. Yield losses of up to 91% in corn and 79% in soybean have been documented.2,3 

 

Description and Identification 

 

Palmer amaranth is an annual weed in the pigweed family. It has a reddish, smooth central stem that is relatively void of hairs and has numerous lateral branches. The leaves are hairless, alternate, lance to diamond-shaped, two to eight inches long, 0.5 to 2.5 inches wide, have a whitish vein on the leaf underside, and grow symmetrically around the stem (Figures 1 and 2). Some plant leaves have a whitish V-shaped mark on the top surface and a single spine in the leaf-tip notch. An identification characteristic to distinguish Palmer amaranth from common waterhemp (Amaranthus rudis) is the leaf petiole is longer than the leaf itself (Figure 3). The seed heads of female plants have stiff, sharp bracts, and may be three feet long (Figure 4). Each plant can produce thousands of seeds (100,000 to 1,000,000) that are viable for three to five years.2 With each reproductive year, the time length of viable seed is extended.  

Having dioecious (male and female plants) reproduction, it can easily adapt to different growing conditions and develop herbicide resistance. Identified resistance includes ALS inhibitors, triazines, HPPD inhibitors, dinitroanilines, glyphosate, PPO inhibitors, growth regulator herbicides, glutamine synthesis inhibitors, and long-chain fatty acid inhibitors.2,6 Livestock producers should exercise caution as Palmer amaranth can be toxic because of oxalates and nitrates.2 

 

Figure 1. Small Palmer amaranth Figure 1. Small Palmer amaranth
Figure 2. Palmer amaranth leaf structure. Figure 2. Palmer amaranth leaf structure.
Figure 3. Distinguishing characteristic of Palmer amaranth is the petiole is longer than the leaf. Figure 3. Distinguishing characteristic of Palmer amaranth is the petiole is longer than the leaf.
Figure 4. Seed head of female Palmer amaranth plants are long and sharp to touch. Courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org. Figure 4. Seed head of female Palmer amaranth plants are long and sharp to touch. Courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org.

Management 


Palmer amaranth management can be a difficult task. The seeds are very small and can be distributed by wind, wildlife, manure, machinery, and flowing water. After emergence, plants can quickly become too large for labeled herbicide management as they can grow two to three inches per day. Flowering and seed set can occur from early spring through the first frost and with thousands of genetically diverse seeds being produced, herbicide resistance can easily develop. 

Compared to annual weeds that germinate under cool soil temperatures, Palmer amaranth seeds generally germinate under warmer soil temperatures. This characteristic often leads to germination after corn and soybean crops have been planted. Information indicates a minimum threshold for Palmer amaranth germination occurs under field conditions when temperatures near the soil surface reach 64° F with germination best achieved at temperatures of 86° to 99° F. Germination can occur during daytime and nighttime temperatures as hot as 113° F and 104° F, respectively.4 However, research data have shown that 8% of Palmer amaranth seed can germinate with a minimum average soil temperature of 41° F; therefore, though the highest percentage of germination occurs with warmer temperatures, diligent early-season scouting and verification is required.5

Integrated pest management (IPM) methods must be used to successfully manage Palmer amaranth. Awareness that the weed is present in fields by scouting and identification is a key for management. Without awareness, Palmer amaranth populations can rapidly explode and become difficult to manage with the potential development of herbicide resistant populations. Palmer amaranth can be easily misidentified with other pigweed species making proper identification extremely important.

Methods to help manage Palmer amaranth can include pre- and post-emergence herbicides and herbicide tank mixes of multiple mode-of-action (MOA) herbicides, tillage, early crop planting, crop seeding rates, crop rotation, cover crops, and weed management in waste areas (roadsides, fence rows, waterways). 

 

Palmer amaranth Management for Corn

 

  1. Scout early to determine presence, density, size, and species of early season weeds. 
  2. Start with a clean field. Use a burndown tank mix of different MOA labeled herbicides such as Corvus® herbicide (Groups 2 and 27) tank mixed with Harness® Xtra herbicide (Groups 5 and 15) for management in no-till operations and aggressive tillage in conventional fields.
  3. Scout planted fields 10 to 12 days after corn emergence for new weed growth. Waste areas should also be scouted.
  4. Apply postemergence herbicides such as DiFlexx® DUO herbicide (Groups 4 and 27) tank mixed with a Roundup® brand agricultural herbicide (Group 9) before Palmer amaranth and other weeds are three to four inches in height. A residual herbicide should be included in the application to help manage late emerging weeds.
  5. Consider walking infested Palmer amaranth fields to remove escapes. Plants should be bagged and removed from the field as immature seeds have the potential to germinate. 
  6. If escapes are not removed, harvest infested fields last to reduce the potential of infesting another field via the combine. The combine should be thoroughly cleaned in the infested field before moving to another field.

 

 

Palmer amaranth management for Soybean

 

  1. Scout early to determine presence, density, size, and species of early season weeds. 
  2. Start with a clean field. Use a burndown tank mix of different MOA labeled herbicides in no-till operations. Warrant® Herbicide (Group 15) can be used preplant and at planting in a tank mix with the active ingredients chlorimuron-ethyl (Group 2), cloransulam-methyl (Group 2), fomesafen (Group 14), a Roundup® brand agricultural herbicide (Group 9), imazethapyr (Group 2), metribuzin (Group 5), paraquat (Group 22), and pendimethalin (Group 3). 
  3. Where labeled, XtendiMax® herbicide with VaporGrip® Technology (a restricted use pesticide*) (Group 4) plus Warrant® Herbicide (Group 15) may be a preemergent management tool for use with Roundup Ready 2 Xtend® Soybeans and XtendFlex® Technology Soybeans in no-till operations. Aggressive tillage in conventional fields is recommended and can be complimented with a residual herbicide such as Warrant® Herbicide. 
  4. *XtendiMax® herbicide with VaporGrip® Technology must be used with VaporGrip® Xtra Agent or an equivalent Volatility Reduction Adjuvant (VRA).
  5. Scout planted fields 10 to 12 days after soybean emergence for new weed growth. Waste areas should also be observed.
  6. Post emergence: For Roundup Ready® Soybeans, Roundup Ready® 2 Yield Soybeans, Roundup Ready 2 Xtend® Soybeans, and XtendFlex® Technology soybeans, apply postemergence herbicides such as a Roundup® brand agricultural herbicide (Group 9) tank mixed with Warrant® Herbicide (Group 15) before Palmer amaranth and other weeds are three to four inches in height. A residual herbicide should be included in the application to help manage late emerging weeds. Liberty® Herbicide (Group 10) plus Warrant® Herbicide (Group 15) is an option in XtendFlex® Technology soybeans. 
  7. For conventional soybean seed: Apply a residual herbicide such as Warrant® Herbicide (Group 15) before weeds emerge. If weeds have emerged, a labeled post emergence herbicide product for emerged weeds should be used. Warrant® Herbicide can be tank mixed with acifluorfen (Group 14), bentazon (Group 6), chlorimuron-ethyl (Group 2), clethodim (Group 1), cloransulam-methyl (Group 2), fenoxaprop-P-ethyl (Group 1), fluazifop-P-butyl (Group 1), fomesafen (Group 3), imazamox, imazethapyr (Group 2), lactofen (Group 14), quizalofop-P-ethyl (Group 1). 
  8. Consider walking infested Palmer amaranth fields to remove escapes. Plants should be bagged and removed from the field as immature seeds have the potential to germinate. 
  9. If escapes are not removed, harvest infested fields last to reduce the potential of infesting another field via the combine. The combine should be thoroughly cleaned in the infested field before moving to another field.

 

For additional Bayer Herbicide information, please see https://www.cropscience.bayer.us/products/herbicides/.

 

Sources:

1Ward, S.M., Webster, T.M., and Steckel, L.E. 2013. Palmer Amaranth (Amaranthus palmeri): A review. Weed Technology, 2013 27:12-27. Weed Society of America.

2Palmer Amaranth (Amaranthus palmeri S. Watson). 2017. Montana Natural Resources Conservation Service. MT-2017. United States Department of Agriculture. https://www.nrcs.usda.gov.

3Palmer amaranth. Minnesota Department of Agriculture. https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/palmeramaranth/.

4Mohler, C.L., Teasdale, J.R., and DiTommaso, A. 2021. Palmer amaranth. Manage weeds on your farm. SARE Outreach. Sustainable Agriculture Research and Education. United States Department of Agriculture. University of Maryland. https://www.sare.org/publications/manage-weeds-on-your-farm/palmer-amaranth/#Weed-Characteristics-Summary-Table/.

5Steckel, L. 2021. Palmer amaranth has started to emerge. UTcrops News Blog. Institute of Agriculture. University of Tennessee. https://news.utcrops.com/2021/04/palmer-amaranth-has-started-to-emerge/.

6Heap, l. 2022. The International Herbicide-Resistant Weed Database. Online. WeedScience.org. https://weedscience.org/Pages/Species.aspx/.

Additional Source:

Enroth, C., Johnson, K., Parker, K. 2021. Invasive Palmer amaranth threatens Illinois crops. Illinois Extension. https://extension.illinois.edu/blogs/good-growing/2021-05-20-invasive-palmer-amaranth-threatens-illinois-crops/.

Web sites verified 4/27/22

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