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Palmer Amaranth Biology, Identification, and Management in Indiana

https://ag.purdue.edu/btny/weedscience

Travis LegleiterWeed Science Program SpecialistBill JohnsonProfessor of Weed ScienceExtension Weed Science

Palmer amaranth has been confirmed in seven counties in Indiana since the fall of 2012, with five of these counties occurring in Northwestern Indiana. The confirmation of Palmer amaranth in Northwestern Indiana plus recent confirmations in Michigan, Ohio, and Illinois has raised concern that Palmer amaranth has the potential to become a major agronomic weed in Indiana within the next couple of years. The economic hardships that this weed has caused Southern United States cotton and soybean producers should be a warning to Indiana corn and soybean producers. The remainder of this publication will cover the current status and location of Palmer amaranth in Indiana, the biology of Palmer amaranth, proper identification, and management strategies for controlling Palmer amaranth.

Current Status of Palmer Amaranth in IndianaPalmer amaranth was first confirmed in Indiana in 2011 with populations occurring in the river

bottoms of Posey and Vanderburgh counties. Plants grown from seed collected from one of the river bottom fields were able to survive 20 lb ae/acre glyphosate (equivalent of 7 gallons/acre of generic glyphosate) in greenhouse screenings conducted at Purdue University. In the fall of 2012, 51 fields across 5 counties in Northwestern Indiana were confirmed to have Palmer amaranth plants that were not controlled by management tactics used during that growing season. The majority of fields (and the heaviest infestations) were confirmed in Jasper County. Many of the fields observed had received multiple applications of glyphosate and attempted rescue applications of PPO inhibiting herbicides (Flexstar, Cobra, Ultra Blazer, etc.).

The initial transport of Palmer amaranth to Northern Indiana is proposed to have been in manure spread on fields from dairy or beef operations that feed cotton seed hulls from the Southern U.S. that were contaminated with Palmer seed. The exact timing of the initial event is unknown, but is estimated to have happened at least two to three years ago due to the severity of infestation in multiple fields. Further spread of Palmer seed has and will occur through farm equipment, specifically combines. Wildlife can also spread the seed into new, previously uninfested fields. It is likely that more Palmer amaranth populations exist within Northwestern Indiana, as well as other regions, but these areas have yet to be properly identified.

Created: 1/7/2013

Information listed here is based on research and outreach extension programming at Purdue University and elsewhere.The use of trade names is for clarity and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current and update precautions and restrictions. Copies, reproductions, or transcriptions of this document or its information must bear the statement: “Produced and prepared by Purdue University Extension Weed Science” unless approval is given by the author.

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Palmer AmaranthJan 7, 2013

Biology and Impact Of Palmer AmaranthPalmer amaranth is an aggressive, weedy invader that is native to the desert regions of the

Southwestern United States and Northern Mexico. It has slowly infiltrated into the Southeastern United States and has become the most significant weed pest of cotton and soybean producers with the majority of populations having resistance to glyphosate and ALS herbicides. The adaptability and invasiveness of Palmer amaranth is further observed by the success of the populations in Michigan and Northern Indiana where average temperatures are below the preferred temperatures of native Palmer populations. The following biological distinctions highlight the success of Palmer as a weed.• Genetic Diversity: Dioecious reproduction in which individual plants are either male or females, which forces outcrossing and genetic

diversity. This gives Palmer amaranth the ability to adapt and quickly spread herbicide-resistance genes when selection pressure is applied, such as repetitive use of single mode of action herbicides.

• Seed Production: Palmer amaranth is a prolific seed producer with plants producing at least 100,000 seeds in competition with a crop, and upwards of a half million seeds in non-competitive scenarios.

• Seed Size: Palmer amaranth seeds are rather small and thrive in no-till or minimum tillage fields where seeds are allowed to stay in the ideal emergence zone in the top inch of soil. The small seeds are easily transported by humans either through grain, seed, or feed contamination; and they are also transported on equipment such as combines.

• Aggressive and Competitive: Palmer exhibits aggressive growth and competitiveness with plants growing 2-3” per day in ideal conditions and creating yield losses of 11-91% in corn and 17-79% in soybean when allowed to compete throughout the growing season.

• Herbicide Resistance: Palmer populations have conferred resistance to multiple herbicide modes of action including: ALS inhibitors, triazines, HPPD- inhibitors, dinitroanilines, and glyphosate, with a large majority of southern populations being glyphosate- and ALS-inhibitor-resistant.

Amaranth IdentificationThe first and often critical key to managing Palmer amaranth, or any

weed for that matter, is to scout and identify the species that exist in each agronomic field. Palmer amaranth can easily be misidentified as redroot pigweed (Amaranthus retroflexus) or common waterhemp (Amaranthus rudis), especially in the seedling stages of growth. All too often, the amaranths are all called “pigweed” and not identified properly by species. The populations in Northwestern Indiana were misidentified as waterhemp for at least the last couple of years and were therefore not managed as aggressively as needed. There are several key characteristics that differentiate the three amaranth species. The ability to use these characteristics to correctly identify amaranth species will assist producers in determining the management strategies needed to control the weed in question.

Key Characteristics for Differentiating the Amaranths of Indiana:• Presence of Hair (Pic 1): Only redroot pigweed will have pubescence

present on stems and leaf surfaces; the fine hairs will be most noticeable on the stems towards the newest growth. Palmer amaranth and common waterhemp will not have hair present on any surface. Looking for pubescence is a quick and easy way to differentiate redroot pigweed from the other two amaranths

• Leaf Shape (Pic 2 & Pic 4): Leaf shapes of amaranths can vary within species, although in general leaf blades of common waterhemp will be long, linear, and lanceolate. In comparison to Palmer amaranth leaves, which will be wider and ovate to diamond shaped. Plants that have been sprayed and survived multiple applications of herbicides, especially PPO-inhibitors, can exhibit variable leaf shapes that may not correctly represent the species.

• Petiole Length (Pic 2): The petiole is the stem-like structure that connects the leaf blade to the main stem. In Palmer amaranth the petioles, especially on older leaves, will be as long, if not longer than the leaf blade itself. The petioles of waterhemp, on the other hand, will be shorter than the long lance shaped leaves. A quick way to determine the petiole length is to simply pull a leaf and petiole off of the plant and bend the petiole back over the leaf blade (as demonstrated in Pic 2) to compare the petiole and leaf blade lengths.

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Pic 1. Redroot pigweed plant exhibiting the presence of hairs on stem and leaf surfaces that distinguish it from common waterhemp and Palmer amaranth


• GrowthPatternofApicalMeristem(Pic4): The leaf shape and petiole lengths of Palmer amaranth are a result of the overall growth pattern of the apical meristem to capture as much light as possible, which results in a rosette-like appearance as you look directly down from the top of the plant. This growth pattern and rosette appearance is often compared to that of a poinsettia. The short petioles and long linear leaves of common waterhemp form a less patterned rosette appearance when observed from above.

• Seed Head Structures (Pic 3): Although identification of weeds should be accomplished prior to reproductive stages, it can be useful to look at seed head structures of escaped weeds to assist in identifying the species. Palmer amaranth females will have a long main terminal seed head that can reach upwards of 3 ft in length. Palmer amaranth female seed heads also have stiff sharp bracts that give the seed heads a prickly feeling when touched. Common waterhemp will have multiple branched seed heads that are similar in length and will lack the stiff, prickly bracts.

• Watermark on Leaf Blades (Pic 5): The leaves of some, but not all Palmer amaranth plants, will have a watermark in the shape of a chevron or V. This characteristic will not be present on common waterhemp, but should only be used as a verification tool for identifying Palmer amaranth as not all Palmer plants will express this characteristic.

• MisidentificationwithSpinyAmaranth(Amaranthusspinosus): The presence of stiff, pointy bracts in the female seed head and leaf axil’s can lead to the misidentification of Palmer amaranth with spiny amaranth or spiny pigweed. Spiny amaranth is a predominate weed of pastures, livestock holding pens, and feeding areas; and is rarely encountered in agronomic fields. Spiny amaranth has a bushy growth pattern and will exhibit the spiny bracts throughout its life cycle where as Palmer will only have the bracts present at maturity and reproductive stages.

Pic 2. a) Palmer amaranth leaf blade with extended petiole b) Palmer amaranth petiole bent back over leaf blade, demonstrating petiole length c) Linear lance shaped leaf blade and short petiole of common waterhemp d) common waterhemp petiole bent back over leaf blade, demonstrating petiole length

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Pic 3. Palmer amaranth plant growing in soybean with multiple terminal seed heads (top). Palmer seed head measuring close to 30 inches in length (bottom).



PIc 4. Photos demonstrating leaf shapes and apical meristem growth patterns of a) Palmer amaranth and b) common waterhemp.

Palmer Amaranth ManagementOnce a producer has taken the critical steps of identifying Palmer amaranth in a field, the next

step is to make a proactive plan of attack that should focus on controlling Palmer amaranth, preventing any further addition of seed to the seedbank, and avoiding further spread of seed to other production acres. Indiana producers encountering Palmer amaranth should treat the population as if it is ALS- and glyphosate-resistant, as the original transplanted seed likely came from the Southern U. S., where the majority of Palmer populations are ALS- and glyphosate-resistant. Again, the most crucial step in this process is scouting your fields and properly identifying Palmer amaranth when it first appears on your acres.

Palmer amaranth in Indiana has only recently been identified. Therefore, the weed science

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Pic 5. White chevron or V shaped watermarks that appear on some Palmer amaranth leaves.

program at Purdue University has yet to establish research and collect data on management of Palmer within the state. The following recommendations are based on research conducted and recommendations given by our colleagues in the Southern U. S. and Michigan. Producers should consider combining cultural practices and herbicide programs for the most effective management of Palmer amaranth. Palmer is a very aggressive and adaptive weed, and management programs which rely on a single action (i.e. glyphosate alone as the only post herbicide) will typically be ineffective at completely controlling the weed.

Cultural practices:• Crop Rotation: Although it’s not a new concept to rotate corn and soybean, the rotation of fields to corn offers additional modes of action that

will control Palmer amaranth. Rotation also helps with preventing further resistance issues and preserving current herbicide tools. In highly infested fields, producers may want to grow corn for at least two years to maximize Palmer control. Although, as noted below, caution should be used in order to prevent resistance to corn herbicides.

• Tillage: Deep tillage (moldboard plow) will bury the small Palmer amaranth seed below the 1” emergence depth. Deep tillage will not provide complete control, but will reduce the number of seeds that emerge from the top 1” of soil. This should be used in extreme cases, as one pass of a moldboard plow will essentially delete the benefits of years of no-till/minimum tillage practices.

• Crimped Cereal Rye Cover Crop: A properly managed and crimped cereal rye cover crop can provide a mulch that will suppress Palmer amaranth emergence. The majority of research conducted on crimped cereal rye cover crop suppression on Palmer amaranth has been combined with deep tillage; hence the cover crop is used for its weed suppression capabilities and not the soil health benefits often promoted by cover crop advocates. No other cover crops have been extensively studied for use as a Palmer amaranth suppressor.

• Hand Weeding: In severe infestations in Southern U.S. cotton fields, producers have gone to the lengths of hiring hand weeding crews to remove Palmer amaranth. The hope is that Indiana producers will be able to gain control of Palmer amaranth through other practices before going to these extremes. Although, producers should be willing to scout for late season escapes and pull these plants to avoid any further spread of seed or pollen.

• DrainageDitchesandFieldBorders: Take care to control Palmer amaranth plants that may exist in ditches and field edges. This past fall, Purdue weed scientists were able to identify Palmer amaranth scattered along multiple roadside ditches in Northwestern Indiana when observing fields. Although these plants do not compete with field crops, they still contribute to the spread of the population through pollen and seed.



Herbicide Control in Corn:• MultipleOptions: There are a number of herbicides available in corn that effectively control Palmer amaranth and offer alternate modes

of action to incorporate into your herbicide rotations. In highly infested fields, growing corn for multiple years can be effective in reducing Palmer populations. However, care should be taken when growing multiple years of corn to not heavily rely on single modes of action as atrazine- and HPPD-resistant Palmer amaranth populations have previously been confirmed.

• Table 1and 2: These tables outline the products available for use in corn pre-emergence and post-emergence along with the active ingredients and site of action. Use these tables along with knowledge of other weeds present to formulate an effective herbicide program that maximizes the number of sites of action to delay resistance. Also be aware that a maximum of 2 lb/acre active ingredient atrazine can be applied per application and the total of all applications cannot exceed 2.5 lb active/acre.

Herbicide Control in Soybean:• LimitedOptions:The number of effective herbicides available to replace glyphosate for control of Palmer amaranth is limited and the

herbicides must be applied at appropriate weed sizes for consistent control. The major herbicide limitation occurs with post-emergence products and thus producers must take advantage of the burndown and residual products available.

• StartWithaCleanField(Table3): Starting with a clean field (either with tillage or a herbicide burndown) is the first key to successful management of Palmer amaranth in soybean. Burndown treatments of glyphosate + 2,4-D or glyphosate + dicamba can be inconsistent on Palmer larger than four inches. Researchers in Tennessee have found that gramoxone + metribuzin to be the most consistent burndown, even on large Palmer plants. Producers may have to consider a two-pass burndown, as gramoxone + metribuzin can be weak on winter annuals that may also exist in the field.

• ResidualHerbicides(Table4): The use of residual herbicides should be the foundation of all Palmer amaranth herbicide control programs in soybean. There are a variety of residual soybean herbicides that will provide control of Palmer amaranth at its weakest point (emergence) and substantially reduce the number of plants requiring a post-emergence application. Using residual herbicides to manage Palmer will reduce the selection pressure of the few post-emergence herbicide options.

• Post-emergenceHerbicideTiming(Table5): Post-emergence herbicides in soybean are limited to PPO-inhibiting herbicides and Liberty (Liberty Link beans only). The timing of these products is key for effective Palmer amaranth control. The consistency and overall control of the PPO-inhibitors and Liberty is dramatically decreased once Palmer amaranth plants exceed four inches. Soybean producers in the Southern U. S. often plow under and replant soybean once Palmer exceeds four inches without an effective post-emerge herbicide application.

• Liberty Link Considerations: In fields with heavy infestations producers may consider planting Liberty Link soybean as Liberty offers consistent control of small Palmer amaranth as well as adding post-emergence options and another site of action to your rotation thus reducing pressures on the PPO-inhibiting herbicides. A pre-emergence residual should remain the foundation of all Palmer amaranth herbicide programs, even in Liberty Link soybean.

• Post-emergenceResiduals(Table6):Although pre-emergence applied residuals are the foundation of Palmer amaranth herbicide programs, these products will not last the entire season while Palmer plants will continue to emerge. Adding a residual herbicide such as Dual II Magnum, Warrant, or Outlook to your post-emerge application will add residual activity into the latter parts of the season. These products do not offer control of emerged plants and must be tank mixed with one of the post-emerge options mentioned above.

• MultipleApplicationsandSitesofAction(SOA): The days of one and two pass single mode of action soybean programs no longer exist, and producers should prepare to accept this when managing a Palmer amaranth infested field. Producers will have to make multiple applications prior to and after planting. Producers should also avoid applying any herbicide or site of action more than twice in a growing season in order to avoid resistance to the few remaining products left to control Palmer amaranth in soybean.

• Tables3,4,5,&6:These tables outline products available for effective control of Palmer amaranth at burndown, pre-emerge, and post-emerge and the sites of action of each product. Use these tables along with the information above to formulate a multiple-pass herbicide program for control of Palmer amaranth that maximizes the number of herbicide sites of action to prolong future resistance.

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Table 1. Herbicides that will provide pre-emergence control of Palmer amaranth in corn Trade Name(s) Active Ingredient Site of Action Site of Action Group #a

Aatrex and othersb atrazine PSII-Inhibitor 5Balance Flexx isoxaflutole HPPD-Inhibitor 27Bicep II Magnum, Brawl II ATZ, and Cinch ATZ

atrazine PSII-Inhibitor 5S-metolachlor Long chain fatty acid-Inhibitor 15

Bullet and Lariatatrazine PSII-Inhibitor 5alachlor Long chain fatty acid-Inhibitor 15

Corvusisoxaflutole HPPD-Inhibitor 27

thiencarbazone ALS-Inhibitor 2Dual II Magnum and Cinch S-metolachlor Long chain fatty acid-Inhibitor 15

Fiercepyroxasulfone Long chain fatty acid-Inhibitor 15

flumioxazin PPO-Inhibitor 14

Guardsman Max and G-Max Liteatrazine PSII-Inhibitor 5

dimethenamid-P Long chain fatty acid-Inhibitor 15Outlook dimethenamid-P Long chain fatty acid-Inhibitor 15

Lexar and Lumaxatrazine PSII-Inhibitor 5

S-metolachlor Long chain fatty acid-Inhibitor 15mesotrione HPPD-Inhibitor 27

Verdictsaflufenacil PPO-Inhibitor 14

dimethenamid-P Long chain fatty acid-Inhibitor 15Zidua pyroxasulfone Long chain fatty acid-Inhibitor 15

a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Maximum of 2lb ai/A per application and 2.5 lb ai/A total for all applications per season

Table 2. Corn herbicides that provide post-emergence control of Palmer amaranthTrade Name(s) Active Ingredient Site of Action Site of Action Group #a

Various 2,4-D Growth Regulator 4Aatrex and othersb atrazine PSII-Inhibitor 5Callisto mesotrione HPPD-Inhibitor 27Banvel, Clarity, Sterling Blue, and others dicamba Growth Regulator 4

Callisto Xtraatrazine PSII-Inhibitor 5

mesotrione HPPD-Inhibitor 27

Caprenotembotrione HPPD-Inhibitor 27

thiencarbazone-methyl ALS-Inhibitor 2

Expert*glyphosate EPSPS-Inhibitor 9

S-metolachlor Long chain fatty acid-Inhibitor 15atrazine PSII-Inhibitor 5

Halex GT*glyphosate EPSPS-Inhibitor 9

S-metolachlor Long chain fatty acid-Inhibitor 15mesotrione HPPD-Inhibitor 27

Impact and Armezon topramezone HPPD-Inhibitor 27Laudis tembotrione HPPD-Inhibitor 27Liberty** glufosinate Glutamine Synthesis Inhibitor 10

Statusdicamba Growth Regulator 4

diflufenzopyr Auxin Transport 19a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Maximum of 2lb ai/A per application and 2.5 lb ai/A total for all applications per season* Roundup Ready corn hybrids only ** Liberty Link corn hybrids only

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Table 3. Herbicide products and tank mixes for burndown of Palmer amaranth prior to soybean plantingTrade Name(s) or Tank Mix Active Ingredient Site of Action Site of Action Group # a

glyphosate + 2,4-Dbcdglyphosate EPSPS-Inhibitor 9

2,4-D Growth Regulator 4

glyphosate + Claritybcdglyphosate EPSPS-Inhibitor 9dicamba Growth Regulator 4

Gramoxone + Dimetricbparaquat PSI-Electron Diverter 22

metribuzin PSII-Inhibitor 5

Libertyd glufosinate Glutamine Synthesis Inhibitor 10

Sharpen/OpTill/OpTill PRO/Veridct + saflufenacil PPO-Inhibitor 14glyphosate or Liberty or Gramaxoned glyphosate or glufosinate or

paraquatEPSPS-Inhibitor or Glutamine Synthesis

Inhibitor or PSI-Electron Diverter9 or 10 or 22

a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Specific trades names used only for clarification, multiple glyphosate, 2,4-D, dicamba, and metribuzin products are available for use and have equivalent efficacy on Palmer amaranthc Burndown applications containing 2,4-D or Dicamba must be applied 7-28 days prior to planting depending on product and rate. Refer to label for specific pre-plant intervalsd Must be applied to Palmer amaranth that is 4 inches or less in height to achieve maximum consistent control


Table 4. Herbicides that will provide pre-emergence control of Palmer amaranth in soybeanTrade Name(s) Active Ingredient Site of Action Site of Action Group # a

Authority Assist/XL/First and Sonicsulfentrazone PPO-Inhibitor 14

Various ALS-Inhibitor 2

Authority MTZsulfentrazone PPO-Inhibitor 14


BoundaryS-metolachlor Long chain fatty acid-Inhibitor 15


Canopychlorimuron ALS-Inhibitor 2metribuzin PSII-Inhibitor 5

Dual II Magnum and Cinch S-metolachlor Long chain fatty acid-Inhibitor 15

Dimetric, TriCor, and others metribuzin PSII-Inhibitor 5

Intrro, Lasso, and Micro-Tech Alachlor Long chain fatty acid-Inhibitor 15

Outlook dimethenamid-P Long chain fatty acid-Inhibitor 15

Optill PROdimethenamid-P Long chain fatty acid-Inhibitor 15

saflufenacil PPO-Inhibitor 14imazethapyr ALS-Inhibitor 2

PrefixS-metolachlor Long chain fatty acid-Inhibitor 15

fomesafenb PPO-Inhibitor 14

Prowl pendimethalin Microtubule Inhibitor 3

Valor flumioxazin PPO-Inhibitor 14

Valor XLT, Gangster, and Enviveflumioxazin PPO-Inhibitor 14

Various ALS-Inhibitor 2

Ziduac pyroxasulfone Long chain fatty acid-Inhibitor 15a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Total of 0.313 lb/Acre (North of I-70) and 0.375 lb/Acre (South of I-70) fomesafen are allowed to be applied per season.c Pending label approval for use in soybean expected early 2013. Check label to confirm status. 7


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a b

Table 5. Soybean herbicides that provide post-emergence control of four inch tall or less Palmer amaranthTrade Name(s) Active Ingredient Site of Action Site of Action Group # a

Prefixb S-metolachlor Long chain fatty acid-Inhibitor 15fomesafenc PPO-Inhibitor 14

Reflex/Dawn and Flexstar/Rhythm fomesafenc PPO-Inhibitor 14

Flexstar GT*fomesafenc PPO-Inhibitor 14glyphosate EPSPS-Inhibitor 9

Cobra and Phoenix lactofen PPO-Inhibitor 14

Liberty** glufosinate Glutamine Synthesis Inhibitor 10a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Prefix has both post-emergence control as well as residual activityc Total of 0.313 lb/Acre (North of I-70) and 0.375 lb/Acre (South of I-70) fomesafen are allowed to be applied per season.* Roundup Ready soybean varieties only** Liberty Link soybean varieties only

Table 6. Soybean herbicides that can be tank mixed with post-emerge products listed in Table 5 for additional residual activity in the cropTrade Name(s) Active Ingredient Site of Action Site of Action Group # a

Dual II Magnum S-metolachlor Long chain fatty acid-Inhibitor 15

Outlook dimethenamid-P Long chain fatty acid-Inhibitor 15

Warrant acetochlor Long chain fatty acid-Inhibitor 15

Sequence*S-metolachlor Long chain fatty acid-Inhibitor 15

glyphosate EPSPS-Inhibitor 9

Zidua b pyroxasulfone Long chain fatty acid-Inhibitor 15a Classification system using numbers for each specific site of action developed by the Weed Science Society of America.b Pending label approval for use in soybean expected early 2013. Check label to confirm status.* Roundup Ready soybean varieties only

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