Palmer Amaranth Confirmed in Todd and Douglas Counties
Tuesday, November 7, 2017
Posted by: James Calkins, MNLA Regulatory Affairs Manager
In recent weeks, the Minnesota Department of Agriculture (MDA) has unfortunately confirmed two new infestations of Palmer amaranth (Amaranthus palmeri) in two additional Minnesota counties – Todd County and Douglas County, adjacent counties located in west-central Minnesota. The Todd County infestation was reported by the MDA in a news release on October 13 followed by the Douglas County infestation on October 27 which resulted from an ongoing investigation of the Todd County discovery (see reference list for links to the news releases). As a consequence of these new finds, small, isolated populations of Palmer amaranth have now been confirmed in four Minnesota counties, all located in west-central Minnesota – Douglas, Lyon, Todd, and Yellow Medicine Counties. Notably, all of the infestations discovered in Minnesota thus far have been associated with first-year conservation plantings and are believed to have resulted from seed mixes contaminated with Palmer amaranth seed. The MDA takes this highly-problematic weed very seriously and is currently working diligently to eradicate these existing infestations and is optimistic that all of the known infestations can be successfully eradicated. So far, no Palmer amaranth plants have been identified in crop production fields which is good news and the MDA continues its search for the source(s) of the seed mixes contaminated with Palmer amaranth seed in an effort to prevent additional introductions and identify other sites that should be investigated for the presence of Palmer amaranth. It is likely that none of these activities, including the possibility of eradicating the existing Palmer amaranth infestations, would be possible without the efforts of conscientious land owners whose attentive scouting activities, proactive reporting, and partnerships with the MDA have played a central role in these early detection and eradication efforts.
Palmer amaranth is a serious threat to Minnesota agriculture and nursery and landscape professionals should remain vigilant to help prevent new infestations by understanding the Palmer amaranth threat, becoming familiar with this new weed and its identifying characteristics, avoiding new introductions of Palmer amaranth through nursery and landscape activities, watching for Palmer amaranth on the lands they manage, and reporting any suspected finds to the Minnesota Department of Agriculture.
Additional Background on the Palmer Amaranth Threat and Status in Minnesota
Palmer or Palmer’s amaranth (Amaranthus palmeri S. Watson) is one of many species of amaranth found in North America, but one that had not been documented in the State of Minnesota until September, 2016, when the first infestations were unfortunately confirmed in conservation plantings in Yellow Medicine and Lyon Counties in the southwestern corner of the state. A swift and sustained response by the MDA to the infestations discovered in 2016 has resulted in only a small number of plants being found in these areas in 2017 and eradication efforts continue. A similar approach is being followed for the new infestations in Douglas and Todd Counties. Although the MDA is optimistic that the current infestations can be eliminated, these finds have highlighted the Palmer amaranth threat and the need for landowners and land managers to be on the lookout for Palmer amaranth in order to catch and manage new infestations quickly and proactively in an attempt to prevent this weed invader from becoming established in the state.
Palmer amaranth is a very serious weed whose distribution has been expanding northward and eastward and a weed the agricultural community in Minnesota should be concerned about. Palmer amaranth has been spreading across the country from what is thought to be its native range in Mexico and the southwestern United States and has also been recently found in neighboring and nearby states, including Illinois, Iowa, Nebraska, South Dakota, and Wisconsin. As an agricultural weed, Palmer amaranth is considered the most aggressive and competitive of the pigweeds, even more competitive than common/tall waterhemp (Amaranthus tuberculatus), and is likely to become a serious and significant weed problem in Minnesota if it becomes established in the state.
As members of the genus Amaranthus, Palmer amaranth and its relatives are representatives of the foundation genus of the Pigweed or Amaranth Family (Amaranthaceae) which, according to Wikipedia, includes about 180 genera and 2500 species. Specifically, the genus Amaranthus includes about 60 species; of these, about 33 species are found in the United States and seven in Minnesota. Members of the genus are commonly and often interchangeably called amaranths or pigweeds. The seven species found in Minnesota include redroot pigweed (also called rough pigweed and green amaranth; Amaranthus retrofexus), tumble pigweed (also called tumbleweed; A. albus), prostrate pigweed (A. blitoides), smooth pigweed (also called slender pigweed and spleen amaranth; A. hybridus), Powell or Powell’s amaranth (also called green pigweed; A. powellii), spiny, thorny, or prickly amaranth/pigweed (Amaranthus spinosus), and common/tall waterhemp (hereafter simply referred to as waterhemp; also called rough-fruited waterhemp; A. tuberculatus; synonyms A. rudis and A. tamariscinus; common waterhemp and tall waterhemp have historically been categorized as separate species, A. rudis and A. tuberculatus, respectively, but in response to genetic similarities and hybridization between the two species, they have more recently been treated as a single species using the name A. tuberculatus by many plant taxonomists). Of these, redroot pigweed, smooth pigweed, Powell amaranth, and waterhemp are fairly similar in appearance and can be difficult to distinguish from one another. All have a branched, upright habit and can reach heights in excess of six feet. The color of the taproot (red or not) and leaf characteristics (color and shape) can also be very similar. Unfortunately, Palmer amaranth also resembles these species so understanding the differences and being on the lookout for this new threat will be critical in keeping this species from being introduced to new areas, gaining a foothold, and becoming established and a problem in Minnesota. Weed managers should become familiar with all seven species and especially Palmer amaranth and the four species it may be confused with, and especially waterhemp which it is most likely to be confused with, as the swift identification of Palmer amaranth will be critical in preventing the spread and establishment of this harmful weed in Minnesota.
Researchers continue to study Palmer amaranth, including its life cycle, spread, and management, and identification is the critical first step in managing this important weed and preventing it from becoming endemic in the state. Individual plants and populations of all five species can be quite variable as a result of genetic variability, hybridization between species, and environmental conditions, but several characteristics appear to be useful for differentiating Palmer amaranth from the other species found in Minnesota. Whether plants are pubescent (hairy) or glabrous (without hairs, smooth) is a good place to start. Redroot pigweed, Powell amaranth, and smooth pigweed all tend to have hairs on their leaves and stems and especially on younger portions of the plant. Redroot pigweed tends to be the most hairy of the three. In contrast, waterhemp and Palmer amaranth are hairless and have smooth leaves and stems.
Leaf shape and petiole length are additional characteristics that can be useful in making an accurate identification. Waterhemp leaves tend to be the most distinctive; they have a tendency to be long, narrow, and waxy and are typically longer and narrower than the other pigweeds. The leaves of the other four species are more similar to each other and tend to be egg-shaped (ovate) to diamond-shaped. The leaf arrangement of Palmer amaranth is also unique as the plants typically have a distinct, poinsettia-like rosette leaf arrangement when viewed from above. The leaves of Palmer amaranth also have very prominent white veins when viewed from underneath. The leaf veins of redroot pigweed can also sometimes be white, but remember that the leaves of redroot pigweed are rough and hairy while those of Palmer amaranth are hairless and smooth. Palmer amaranth also has long petioles, at least as long, and usually longer, than the leaves, while waterhemp has short petioles. Petiole length for the other three species is intermediate. As a consequence of these differences, if a plant has hairless stems and leaves with petioles that are longer than the leaf blade, it is likely Palmer amaranth. Finally, young plants of Palmer amaranth will often have a distinct, white to pink, v-shaped, chevron marking on their leaves which is never present on redroot pigweed, Powell amaranth, or smooth pigweed.
As for all plants, the inflorescence and individual flowers can be very helpful in identification, but pigweed flowers are very small and this makes their use in identification difficult for most casual observers and requires magnification in order for the key identifying characteristics to be seen clearly. Waterhemp tends to have a more delicate habit than the other three. Although it is often found in moister environments adjacent to lakes, stream, and wetlands, waterhemp is also commonly found in drier environments. Unlike redroot pigweed, smooth pigweed, and Powell amaranth, which are monoecious (perfect flowers or male and female flowers produced on the same plant; the latter is the case for these pigweeds), Palmer amaranth is dioecious (individual plants male or female; male and female flowers produced on separate plants). This is also a characteristic which makes the species more problematic as a weed species. It is important to note, however, that the number of male flowers produced on the monoecious pigweeds is often very limited so making this determination can be difficult. Waterhemp is also dioecious. Redroot pigweed has a short, stout, terminal inflorescence compared to the other four species and the inflorescence is quite prickly. The flower spikes/seed heads of waterhemp tend to be smooth, long and slender while those of smooth pigweed and Powel amaranth are long, thick, and prickly. The flower spikes/seed heads of Palmer amaranth tend to be the longest (typically ½ to 1½ feet long with terminal inflorescences/seed heads sometimes up to 3 feet long) and are very prickly in response to sharply-pointed bracts subtending the flowers. In all cases, more detailed identification resources should be used to ensure proper identification. As mentioned earlier, it is also important to remember that pigweeds can sometimes hybridize with each other which can make it more difficult to identify individual plants as a specific species. As a result of their similarity and familiarity to weed managers, and its ability to hybridize with Palmer amaranth, watching for waterhemp plants that seem unusual is especially advisable.
The remaining three species of Amaranthus that are commonly found in Minnesota are much easier to identify. Prostrate pigweed is easily identified by its low, mat-forming habit and, secondarily, by the presence of sharp sines at the bases of the petioles where they are attached to the stem at the nodes. Similarly, spiny amaranth also has these types of spines, but has an upright habit and is taller like the other species of Amaranthus found in Minnesota. Tumble pigweed is much branched and has a rounded form, greenish-white stems, and relatively small leaves; plants also tend to abscise at ground level when mature allowing them to be tumbled by the wind to disperse their seeds. The flowers of prostrate pigweed and tumble pigweed are also unique, being found only in small, relatively inconspicuous clusters in the leaf axils.
Many of the weedy species of Amaranthus are variously listed as being native or introduced in North America which is likely the result of historical misidentifications. Most are believed to have originated from tropical America, but some are considered native to northern Mexico and the southwestern United States and the Great Plains. Palmer amaranth is a good example; although it has been reported as being native in Wisconsin and neighboring states, it is listed as an excluded species by the University of Wisconsin - Stevens Point, Robert W. Freckmann Herbarium based on the belief that the name was misapplied or there was a misidentification, as there are no records that it has been present in the state until recently. While knowing their original, native distributions and whether specific species are native to the upper Midwest or have simply become naturalized is of interest and important from a scientific and ecological perspective, it has little bearing on the importance of these species as weeds in agricultural systems. Botanists, taxonomists, and weed scientists have studied the distribution patterns and impacts of pigweeds for years and research designed to better understand and manage these species continues today.
Palmer amaranth is native to northern Mexico (Chihuahua) and the far southwestern United States (New Mexico, Arizona, and southern California), but has subsequently spread east to Georgia and Florida and north to Massachusetts, New York, Ontario (Canada), Pennsylvania, Ohio, Indiana, Illinois, Nebraska, and Colorado and most recently lower Michigan, Wisconsin, South Dakota, and Iowa – and now Minnesota. Outside North America, it has also spread to Europe, Asia, and Australia. Palmer amaranth seeds have likely been spread around North America and the world by agricultural activities and commerce including contaminated agricultural products like hay, cattle feed, and manure. Most recently, research has suggested that Palmer amaranth has been spread through native seed mixes contaminated with Palmer amaranth seeds that have been used for conservation and restoration purposes including Conservation Reserve Program (CRP; USDA Farm Service Agency/FSA) and Reinvest in Minnesota (RIM) Reserve Program (Board of Water and Soil Resources/BWSR) plantings and other wildlife, pollinator, and cover crop plantings. Although Palmer amaranth may not persist in these habitats over the long term, even ephemeral populations may serve as a seed source for infestations on nearby disturbed lands including agricultural lands. This is unfortunate and ironic given the goal of restoration efforts and awareness and due diligence will be required to prevent this type of contamination and to avoid the introduction of Palmer amaranth to new areas by such means. To reduce the risk of introducing this Palmer amaranth and other weed species, use local seed sources, purchase certified seed, and avoid seed sources from states where Palmer amaranth is known to be present. Contaminated farm equipment and wildlife can also facilitate the movement of Palmer amaranth seeds into new areas.
Factors that make Palmer amaranth a serious weed include extended germination during the growing season, a rapid growth rate (as much as two or three inches per day), large size (6-10 feet) and significant biomass production, prolific seed production (up to 600,000 seeds per female plant), a high level of genetic diversity, environmental adaptability (including drought tolerance), superior competitiveness, and herbicide resistance (multiple modes of action) including glyphosate resistance (Roundup® and related products). Herbicide resistance has also been observed in other species of Amaranthus including waterhemp, redroot pigweed, Powell amaranth, and smooth pigweed and the development of herbicide resistance by these, as well as other weed species, is a significant and growing concern. Interestingly, with the exception of herbicide resistance, many of the same characteristics that have enabled the pigweeds to become serious weeds have been viewed positively for selections of Amaranthus grown for food purposes.
As every weed manager should know, reliance on a single mode of action has the potential to be problematic when it comes to the development of herbicide resistance and herbicide resistant weeds and weed populations. The widespread and growing reliance on glyphosate resistant, “Roundup Ready” crops, mainly agronomic crops including corn, soybeans, and cotton, but also Kentucky bluegrass, ignores this basic principle and we are now facing the consequences as an increasing number of weed species, including Palmer amaranth, are becoming resistant to glyphosate and other herbicides with various modes of action. In the case of Palmer amaranth, and waterhemp, enhanced outcrossing as a consequence of the species being dioecious results in the rapid spread of inherited characteristics like herbicide resistance. As a result, Palmer amaranth has become a leading example of herbicide resistance and its negative and costly consequences. Pesticide applicators must recognize, understand, and respect the threat posed by pesticide resistance – the ability of pests, including weeds, to evolve and develop resistance to pesticides, including herbicides, as a result of exposure to pesticides under field conditions – and must select and apply pesticides properly to help prevent the development of pesticide resistance.
Understanding the life history of weeds is also important in developing effective management strategies. All of the pigweeds are alternate-leaved, tap-rooted annuals. Flowering typically begins in June or July and continues through October. The inflorescence is typically represented by dense, terminal and/or axillary clusters of small flowers subtended by bristly, green bracts (a modified, specialized leaf associated with reproductive structures like a flower or inflorescence). Depending on species, the flower clusters vary in size from small to large, are spike or catkin-like, and are upright and stiff or variously drooping. The fruit is most often described as an urticle and sometimes a capsule or an achene (or akene). An urticle is a small bladder-like, one-seeded, indehiscent (doesn’t open to release seeds when mature), but occasionally dehiscent (opens to release seeds when mature), fruit with a papery, inflated covering (pericarp) derived from a single carpel; similar to an achene, but with a loose pericarp (the pericarp not in close contact with the seed as it is for an achene). An achene is, however, always indehiscent. The dehiscent nature of the fruits of some species is likely the reason why the fruit is sometimes described as a capsule and specifically a pyxis (a type of capsule that opens by a lid); a capsule is, however, typically derived from two or more carpels and contains several to many seeds. Mature seeds are small, rounded (sometimes slightly oval or flattened), shiny, and dark reddish-brown or black. They are eaten by upland birds and rodents, and waterfowl, and were commonly collected and eaten and processed into flour by the indigenous peoples of North America. The young leaves and stems, taproots, and seeds of several species of Amaranthus are also eaten by people and selections have been made for seed production as a food crop and for flour production. Several species are also important horticultural plants and are planted in gardens and containers. The pigweeds can sometimes accumulate nitrates and become toxic to cattle (bloating) and other livestock.
Pigweeds are commonly found in waste places, agricultural and fallow fields, pastures, railroad right-of-ways, roadsides, shorelands, and other disturbed areas and habitats and can be problematic as weeds in agronomic (including corn, soybeans, and cotton) and horticultural crops (including vegetables, field and container nurseries, orchards, and vineyards), prairie/savanna restorations, and landscapes including turf (rarely; mainly during establishment from seed). In general, sunny, fertile conditions are preferred by most species, but plants will tolerate a variety of soils and moisture conditions.
Early detection and eradication will be the keys to preventing the spread and establishment of Palmer amaranth in Minnesota. The hope of keeping Palmer amaranth from becoming a serious weed problem in Minnesota will depend on the awareness and vigilance of agricultural producers and land managers, including nursery and landscape professionals, and a commitment to do everything possible to prevent the introduction of this species into new areas across the state. This is true for all non-native species that have the potential to harm agricultural production and native ecosystems. Agricultural production and distribution systems that include best management practices designed to prevent the introduction of new weed species, regular scouting, proper identification, and a quick response to eradicate pioneer populations and prevent seed production will be required to prevent the establishment and spread of Palmer amaranth in the state. If pioneer weeds and populations are not identified and seed production is not stopped, Palmer amaranth, including glyphosate resistant populations, will become a serious threat across the state. As of this writing, the source of the Palmer amaranth infestation found in Yellow Medicine County remains unknown and it is hoped this find represents a singular infestation that can be eradicated. From a management perspective, new infestations should be eradicated quickly to prevent seed production and dispersal followed by pre-emergence herbicides known to be effective on Palmer amaranth and timely (plants less than 3” tall) post-emergence herbicide applications (where feasible as needed and remembering to rotate the modes of action of the herbicides used; using herbicide combinations with unrelated modes of action can also be effective in preventing herbicide resistance) and other methods like mowing and crop rotation.
The fear that Palmer amaranth would find its way to Minnesota has unfortunately been realized. Still, controlling existing populations and preventing new infestations remains an important goal and the Minnesota Department of Agriculture is fully engaged and believes eradication of the existing infestations is still possible. To this end, Palmer amaranth has been listed as a Prohibited-Eradicate Noxious Weed and a Prohibited Weed Seed in Minnesota. This means all of the above and below ground parts of any Palmer amaranth plants must be destroyed (http://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist.aspx) and no Palmer amaranth seed is allowed in any seed offered for sale in the state including all agricultural, vegetable, flower, tree, shrub, native grass, and forb seed sold in Minnesota (http://www.mda.state.mn.us/licensing/licensetypes/seed.aspx; http://www.mda.state.mn.us/licensing/licensetypes/~/media/Files/licensing/seed/proresweedseeds.pdf). In addition the Minnesota Department of Agriculture, with legislative support, has a plan that will hopefully result in the eradication of Palmer amaranth from the state and prevent future introductions.
If plants suspected as being Palmer amaranth are found in Minnesota, more technical identification aids or a weed specialist should be consulted to confirm the identification and the findings reported to your county agricultural inspector, University of Minnesota Extension personnel, or the Minnesota Department of Agriculture (firstname.lastname@example.org; phone/1-888-545-6684). Effective weed control is an important activity for growers and landscape managers who should keep themselves up-to-date concerning potential, new weed threats and management practices. Be on the lookout for Palmer amaranth in your fields (including conservation and prairie plantings) and other production areas, and in managed landscapes and lesser managed areas adjacent to such areas, and be proactive in helping to prevent new introductions of Palmer amaranth to help keep this weed from becoming established in Minnesota.
Todd and Douglas County news releases:
Minnesota Department of Agriculture. 2017. Palmer Amaranth Found in Todd County. News Release; October 13, 2017. http://www.mda.state.mn.us/news/releases/2017/nr20171013amaranth.aspx
Minnesota Department of Agriculture. 2017. Palmer Amaranth Confirmed in Douglas County. News Release; October 27, 2017. http://www.mda.state.mn.us/news/releases/2017/nr20171027palmer.aspx
For additional information about Palmer amaranth, pigweeds in general, and the identification and management of Palmer amaranth, consult the following selected references:
Minnesota Department of Agriculture. 2017. Factsheet: Palmer Amaranth in Minnesota. https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/palmeramaranth/palmeramaranthfs.aspx
Minnesota Department of Agriculture. 2016. Palmer Amaranth Detected in Minnesota. News Release; September 22, 2016. http://www.mda.state.mn.us/news/releases/2016/nr20160922palmeram.aspx
Behnken, L., F. Breitenbach, J. Gunsolus, P. Bongard, and L. Stahl. 2016. Palmer Amaranth: A New Weed Threat to Watch Out for. University of Minnesota Extension, Minnesota Crop News; August 23, 2016. http://blog-crop-news.extension.umn.edu/2016/08/palmer-amaranth-new-weed-threat-to.html
Hartzler, B. 2016. Palmer Amaranth in Iowa: What We Know. Iowa State University, Integrated Crop Management News; August 19, 2016. http://crops.extension.iastate.edu/cropnews/2016/08/palmer-amaranth-iowa-what-we-know
College of Agriculture Life Sciences and Iowa State University Extension and Outreach. 2016. Palmer Amaranth Now Identified in at Least Nine Iowa Counties. Iowa State University, Integrated Crop News; August 15, 2016. http://crops.extension.iastate.edu/cropnews/2016/08/palmer-amaranth-now-identified-least-nine-iowa-counties (Incudes a video describing Palmer amaranth and its identifying characteristics)
Loux, M. 2016. New Palmer Amaranth Finds: We're Mowing Down Soybean Fields, Folks. Ohio State University, OSU Weed Management; August 18, 2016. http://u.osu.edu/osuweeds/2016/08/18/new-palmer-amaranth-finds-were-mowing-down-soybean-fields-folks/
University of Minnesota Extension. 2013. Palmer Amaranth Confirmed in Iowa What Does it Mean for Minnesota. http://blog.lib.umn.edu/umnext/news/2013/09/palmer-amaranth-confirmed-in-iowa-what-does-it-mean-for-minnesota-1.php
Davis, V.M. 2011. Palmer Amaranth is in Wisconsin Crop Production Fields. University of Wisconsin Integrated Pest and Crop Management (IPCM) Program, Wisconsin Crop Manager Newsletter; November 15, 2011. http://ipcm.wisc.edu/blog/2011/11/palmer-amaranth-is-in-wisconsin-crop-production-fields
United States Department of Agriculture (USDA) Plants Database. Plant Profile – Amaranthus L. http://plants.usda.gov/java/profile?symbol=AMARA
Mosyakin, S.L. and K.R. Robertson. 2004. Amaranthus L. Flora of North America 4:405-406. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=101257
Mosyakin, S.L. and K.R. Robertson. 2004. Amaranthus palmeri S. Watson. Flora of North America 4:418. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242415653
Wikipedia: Amaranthaceae. http://en.wikipedia.org/wiki/Amaranthaceae (accessed 9/15/2016).
Culpepper, A.S., T.L. Grey, W.K. Vencill, J.M. Kichler, T.M. Webster, S.M. Brown, A.C. York, J.W. Davis, and W.W. Hanna. 2006. Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Confirmed in Georgia. Weed Science 54(4):620-626. http://wssajournals.org/doi/abs/10.1614/WS-06-001R.1 (abstract only)
Sprague, C. 2011. Palmer Amaranth: A New Invasive Weed to Watch for in Michigan. http://www.michigansoybean.org/Media/MichiganSoybean/Downloads/MSPC/Palmer-Amaranth/Palmer-Amaranth-A-New-Invasive-Weed-To-Watch-For-In-MI.pdf
Pratt, D.B., M.D.K. Owen, and L.G. Clark. 1999. Identification of the Weedy Pigweeds and Waterhemps of Iowa. http://www.extension.iastate.edu/publications/pm1786.pdf (an excellent comparative identification reference for weedy amaranth species)
DuPont Pioneer Agronomy Sciences. 2013. Palmer Amaranth in the North Central U.S. https://www.pioneer.com/CMRoot/Pioneer/US/Non_Searchable/agronomy/cropfocus_pdf/palmer_amaranth.pdf
Sprague, C. 2011. Glyphosate-Resistant Palmer Amaranth in Southwest Michigan. http://anrcom.msu.edu/uploads/files/122/4-28Palmer%20Glyphosate%20ConfirmationCHRISTY.pdf
Johnson, B. and T. Legleiter. 2013. Identifying Palmer Amaranth in the Field. Purdue Extension (video; July 17, 2013). http://www.youtube.com/watch?v=aVbgPGg0GO0&feature=youtu.be
Virginia Tech Weed Identification Guide. Palmer Amaranth: Amaranthus palmeri. http://www.ppws.vt.edu/scott/weed_id/amapa.htm
Briton, N.L. and A. Brown. 1970 (Second Edition). An Illustrated Flora of the Northern United States and Canada. Volume II. Dover Publications, New York, NY.
United States Department of Agriculture (USDA), Agricultural Research Service (ARS). 1971. Common Weeds of the United States. Dover Publications, New York, NY.
Uva, R.H., J.C. Neal, and J.M. DiTomaso. 1997. Weeds of the Northeast. Cornell University Press, Ithaca, NY.
Stubbendieck, J., G.Y. Friisoe, and M.R. Bolick. 1995 (Second Edition). Weeds of Nebraska and the Great Plains. Nebraska Department of Agriculture, Lincoln, NE.
Armstrong, J. 2011. Look-A-Like Weeds: Pigweeds. Oklahoma State University Extension; Department of Plant and Soil Sciences Extension News (June 9, 2011). http://cotton.okstate.edu/weed-resistance-management/look-a-like-weeds.pdf
Legleiter, T. and B. Johnson. 2013. Scout and Identify Pigweed Species Now. Purdue Extension/Weed Science (May 30, 2013). https://ag.purdue.edu/btny/weedscience/Documents/Palmer_ID.pdf
Legleiter, T. and B. Johnson. 2013. Palmer Amaranth Biology, Identification, and Management. Purdue Extension (April, 2013). http://www.extension.purdue.edu/extmedia/WS/WS-51-W.pdf
Legleiter, T. and B. Johnson. 2013. Palmer Amaranth Identification. Purdue Extension (video; May 21, 2013). http://www.youtube.com/watch?v=wNgRvvnPQJ8
Schonbeck, M. 2013. Weed Profile: Pigweeds (Amaranthus spp.). eXtension (August 6, 2013). http://www.extension.org/pages/65208/weed-profile:-pigweeds-amaranthus-spp
Bensch, C.N., M.J. Horak, and D. Peterson. 2003. Interference of Redroot Pigweed (Amaranthus retroflexus), Palmer Amaranth (A. palmeri), and Common Waterhemp (A. rudis) in Soybean. Weed Science 51(1):37-43. http://www.jstor.org/discover/10.2307/4046623?uid=3739736&uid=2&uid=4&uid=3739256&sid=21102749864953
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