Palmer amaranth is a member of the pigweed family that is native to the southwest United States, but has slowly migrated into the Midwestern U.S. over the past decade or so. In the bootheel of Missouri, like western Tennessee, Arkansas, and a host of other southern states, Palmer amaranth has been the predominant pigweed species for several decades. It has only been in the past several years that we have begun finding Palmer amaranth in more northern geographies of Missouri outside of the bootheel. Over the years I have made an effort to track the spread of Palmer amaranth in these areas, and this information is shown in figure 1.
I can’t say for sure that these are the only counties where Palmer amaranth occurs in Missouri, but I can say for sure that these are the counties where I know it is present because I have identified it in that location myself. Each year it seems I add a few more counties to the list (figure 2), which is a big problem because Palmer amaranth is a much more competitive and aggressive species than waterhemp (our most common pigweed species throughout the rest of the state); one we don’t want to have to contend with in the future. If you think you have Palmer amaranth in a county that is not colored in on this map, I’d be glad to receive a sample and/or photos so that we can keep this information as up to date as possible.
Palmer amaranth is fairly easy to distinguish from waterhemp and the other pigweeds once it gets past the seedling stage of growth (figure 3). Both waterhemp and palmer amaranth are hairless and have no hairs on their leaves or stems. However, palmer amaranth has much wider and distinctively diamond-shaped leaves when compared to waterhemp. Also, the leaves of palmer amaranth occur on petioles that are usually as long or longer then the leaves themselves. The leaves of Palmer amaranth have a poinsettia-like leaf arrangement when viewed from above and an occasional V-shaped variegation or watermark on the upper surface of the leaf. Mature palmer amaranth can often grow to more than seven feet in height. But perhaps the most distinguishing characteristic is the seedhead; Palmer amaranth will have seedheads very different from those of waterhemp (figure 4). The female seedheads have large spiny bracts that extend beyond all other flower parts and will be prickly to the touch unlike waterhemp. For more information on the identification of Palmer amaranth, see this publication: http://weedscience.missouri.edu/publications/50737_FINAL_FactSheet_PalmerAmaranth_poster_v2.pdf.
Many farmers ask me how this weed has found its way into other areas of the state in recent years. There are a lot of possibilities, and I won’t try to tackle them all here but I will mention a few of the ways that we know Palmer amaranth has spread in Missouri. First, Palmer amaranth has been introduced into fields through the purchase of used equipment (usually combines) and/or through custom harvesting crews that have come from other regions where Palmer amaranth is more prevalent.
Second, we have seen at least one case in Missouri where Palmer amaranth was introduced into an area as a result of contaminated hay that was purchased from Arkansas where Palmer amaranth is the predominant weed species in just about every cropping system, and can occur in hay fields as well. Similarly, Palmer amaranth can be introduced in an area through the purchase of contaminated animal feeds like cottonseed meal. While I haven’t personally encountered this situation in Missouri, my colleagues in other states with bigger dairy industries like Indiana, Michigan, and Wisconsin have tracked the introduction of Palmer amaranth into their states in this manner.
Third, we know that waterfowl can transport and distribute viable Palmer amaranth seed over fairly long distances, and that Palmer amaranth can be introduced into fields in this manner. In fact, the initial distribution of Palmer amaranth along the river bottoms in Missouri (figure 1) led us to this hypothesis, and after several years of research we have found this method of dispersal to be a very real possibility. For more information on waterfowl and Palmer amaranth distribution, you can view a slideshow of our research results here: http://weedscience.missouri.edu/extension/pdf/waterfowl%20and%20weed%20seed.pdf.
Fourth, Palmer amaranth can be introduced into an area through contaminated seed. While I have not encountered this situation in Missouri yet, it is important to note that Iowa, Illinois, and Ohio have recently documented new infestations of Palmer amaranth in newly seeded CRP “Pollinator Habitat” fields and suspect that the problem is contaminated seed sources of native seed mixes. For more information about these cases you can visit the following links: http://bulletin.ipm.illinois.edu/?p=3700 , http://agcrops.osu.edu/newsletter/corn-newsletter/status-palmer-amaranth-ohio, and http://crops.extension.iastate.edu/cropnews/2016/08/new-palmer-amaranth-findings-iowa.
In short, any seed, feed, or equipment coming onto your farm should be thoroughly examined for the presence or even the possibility of Palmer amaranth seed. This is not a species that Midwest farmers will want to contend with in the future. If you find newly introduced Palmer amaranth plants in your fields, rogue them out immediately so that the population does not establish itself and become a much bigger problem for you in the future.
REVISED: August 17, 2016