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, 5.1 to 20.3 cm (2 to 8 inches) long, 1.5 to 6.3 cm (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 plants may have leaves with a whitish V-shaped mark on the top surface and a single spine in the leaf-tip notch. An identification characteristic to help distinguish Palmer amaranth from common waterhemp (Amaranthus rudis) is the leaf petiole which is longer than the leaf itself (Figure 3). The seed heads of female plants have stiff, sharp bracts, and may be 91.4 cm (3 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, and glyphosate.2 Livestock producers should exercise caution as Palmer amaranth can be toxic because of oxalates and nitrates.2
Figure 1. Small Palmer amaranth.
Figure 2. Palmer amaranth leaf structure.
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.
Management
Palmer amaranth management can be difficult. 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 5.1 to 7.6 cm (2 to 3 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 17.7° C (64° F) with germination best achieved at temperatures of 30° to 37.2° C (86° to 99° F). Germination can occur during daytime and nighttime temperatures as hot as 45° to 40° C (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 5° C (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 Palmer amaranth 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
- Scout early to determine presence, density, size, and species of early season weeds.
- 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 atrazine and a residual herbicide such as Zidua® SC herbicide (Group 15) or Dual II Magnum® herbicide (Group 15) for management in no-till operations and aggressive tillage in conventional fields.
- Scout planted fields 10 to 12 days after corn emergence for new weed growth. Waste areas should also be scouted.
- Apply postemergence herbicides such as XtendiMax® herbicide with VaporGrip® Technology (Group 4) tank mixed with Laudis® herbicide (Group 27) or a tank mix of Roundup Xtend® 2 with VaporGrip® Technology (Group 4) and Laudis® herbicide 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.
- 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.
- 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
- Scout early to determine presence, density, size, and species of early season weeds.
- 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), glyphosate (Group 9), imazethapyr (Group 2), metribuzin (Group 5), paraquat (Group 22), and pendimethalin (Group 3).
Where labeled, XtendiMax® herbicide with VaporGrip® Technology (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.
- Scout planted fields 10 to 12 days after soybean emergence for new weed growth. Waste areas should also be observed.
- 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.
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).
- 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.
- 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.
Canadian farmers should be diligent in scouting fields for the presence of Palmer amaranth plants, preemergence and postemergence. The weed was discovered and verified in Manitoba in 2021. Palmer amaranth plants should be considered resistant to some herbicides until proven otherwise.
Labeled preemergent herbicides for broadleaf control in row crops should be used along with labeled residual herbicides in no-till operations. Tillage along with labeled residual herbicides should be used in conventional tillage operations.
Fall planted and spring planted cereal crops are likely to mature prior to Palmer amaranth producing seeds. After cereal crops are harvested, fields should be scouted and managed accordingly for the presence of broadleaf weeds.
Consideration should be given to manual removal of Palmer amaranth escapes prior to seed set. Harvesting operations can spread thousands of seeds throughout the field.
XtendiMax® herbicide with VaporGrip® Technology used with labeled tank mixes can help manage pigweed species, preplant and preemergence. Additionally, XtendiMax® herbicide with VaporGrip® Technology is labeled for pre and early postemergence on Roundup Ready 2 Xtend® Soybeans and XtendFlex® Soybeans. An R1 soybean growth stage application of Liberty® Herbicide is encouraged in XtendFlex® Soybeans.
XtendiMax® herbicide with VaporGrip® Technology is labeled for postemergence use on spring wheat, spring barley, winter wheat, oats, and spring rye.
For additional Bayer herbicide information, please see https://www.cropscience.bayer.ca/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/Internet/FSE_PLANTMATERIALS/publications/mtpmcfs13130.pdf.
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/.
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
Legal Statements
Bayer is a member of Excellence Through Stewardship® (ETS). Bayer products are commercialized in accordance with ETS Product Launch Stewardship Guidance, and in compliance with Bayer’s Policy for Commercialization of Biotechnology-Derived Plant Products in Commodity Crops. These products have been approved for import into key export markets with functioning regulatory systems. Any crop or material produced from these products can only be exported to, or used, processed or sold in countries where all necessary regulatory approvals have been granted. It is a violation of national and international law to move material containing biotech traits across boundaries into nations where import is not permitted. Growers should talk to their grain handler or product purchaser to confirm their buying position for these products. Excellence Through Stewardship® is a registered trademark of Excellence Through Stewardship.
ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
It is a violation of violation of federal law to use any pesticide product other than in accordance with its labeling. NOT ALL formulations of dicamba or glyphosate are approved for in-crop use with products with Roundup Ready 2 Xtend® soybeans. NOT ALL formulations of dicamba, glyphosate or glufosinate are approved for in-crop use with products with XtendFlex® Technology. ONLY USE FORMULATIONS THAT ARE SPECIFICALLY LABELED AND APPROVED FOR SUCH USES. Contact the Pest Management Regulatory Agency with any questions about the approval status of dicamba herbicide products for in-crop use with Roundup Ready 2 Xtend® soybeans or products with XtendFlex® Technology.
Roundup Ready® 2 Technology contains genes that confer tolerance to glyphosate. Products with XtendFlex® Technology contains genes that confer tolerance to glyphosate, glufosinate and dicamba. Roundup Ready 2 Xtend® soybeans contains genes that confer tolerance to glyphosate and dicamba. Glyphosate will kill crops that are not tolerant to glyphosate. Dicamba will kill crops that are not tolerant to dicamba. Glufosinate will kill crops that are not tolerant to glufosinate. Contact your Bayer retailer, refer to the Bayer Technology Use Guide, or call the technical support line at 1-888-283-6847 for recommended Roundup Ready® Xtend Crop System weed control programs.
Tank mixtures: The applicable labeling for each product must be in the possession of the user at the time of application. Follow applicable use instructions, including application rates, precautions and restrictions of each product used in the tank mixture. Bayer has not tested all tank mix product formulations for compatibility or performance other than specifically listed by brand name. Always predetermine the compatibility of tank mixtures by mixing small proportional quantities in advance. Bayer, Bayer Cross, Roundup Ready 2 Xtend®, Roundup Ready 2 Yield®, Roundup Ready®, Roundup®, VaporGrip®, XtendFlex® and XtendiMax® are registered trademarks of Bayer Group. Used under license. LibertyLink and the Water Droplet Design are trademarks of BASF. Used under license. All other trademarks are the property of their respective owners. Bayer CropScience Inc. is a member of CropLife Canada. ©2022 Bayer Group. All rights reserved. 1123_27650_CA