A single Palmer amaranth is way too many, say Arkansas scientists

How much damage can one little Palmer amaranth seed cause?

A lot, according to research done by weed scientists at the University of Arkansas, who set out to document both the distribution patterns of Palmer amaranth and its impact on cotton yield. Their findings led them to recommend a zero tolerance threshold for the weed.

Palmer amaranth can produce as many as 1,800,000 seeds from one plant, and because of their small size, they can float in water, allowing them potentially to cover large areas. That, with its resistance status, makes it a formidable enemy to a cotton producer. Despite its reputation as a significant weed pest, little research has been done to track its distribution patterns or impact on yield, both of which can affect action thresholds.

First author Jason Norsworthy and his colleagues state that all seed production should be prevented for weed species that exhibit prolific seed production, high competitiveness with the crop, and rapid dispersal, especially if resistance has evolved. They hypothesized that Palmer amaranth may be in that class but wanted to prove it through testing how fast it could spread and how quickly it could impact crop yield.

The scientists spread 20,000 Palmer amaranth seeds in a one square meter area in the south side of each of four fields in February 2008. The introduction was intended to mimic a very conservative estimate of seed dispersal from one single Palmer amaranth plant in 2007. None of the fields had any prior history of Palmer amaranth infestation.

By October 2008, Palmer amaranth had moved downslope about 118 meters (approximately 387 feet) in one field, forming a distinct and rapidly growing patch the next year. In 2008, the only factor involved in seed movement was rainfall, as tillage and rebedding had been done prior to seed introduction. In the other fields, seeds moved about 52 feet or less from the point of origin during the first year.

Most Palmer amaranth movement occurred along the length of the beds rather than across them. Weeds did not affect cotton lint yields the first year; however, yields decreased each successive year, with a total crop failure in 2010. In 2008, Palmer amaranth infested only 0.56% of the fields; by 2009, infestation was up to 20% and had spread from one field edge to the other.

By 2010, Palmer amaranth had infested about 95% of the total area of all fields. In addition to noting a low cotton yield because the crop couldn’t compete, researchers couldn’t even harvest the crop because of the likelihood that the weed would damage the equipment.

On average over the four years, cotton lint yields were reduced at a level of 17 kg ha-1 for each Palmer amaranth. In the right environment, researchers conclude, a single Palmer amaranth can have as much or more impact on a crop than a small group of Palmer amaranth in a less than ideal location.

The experiment proved that even one Palmer amaranth weed can impact an entire cotton field in less than two years and cause total crop failure in three years. Because of those results, Norsworthy and colleagues recommend a zero tolerance threshold for Palmer amaranth in the first year, eradicating any plants before they can reproduce:

“No Palmer amaranth should be allowed to reach reproductive maturity, meaning that multiple means of control will be needed over an extended growing season due to the season-long emergence of Palmer amaranth….The spatial approach we implemented in this study was extremely valuable in understanding the pattern of within-field dispersal of Palmer amaranth and demonstrating that a single escape is way too many to allow for this species, justifying the need for a zero-tolerance approach in managing this weed.”


Norsworthy, J.K., Griffith, G., Griffin, T., Bagavathiannan, M., and Gbur, E.E. (2014). In-field movement of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) and its impact on cotton lint yield: evidence supporting a zero-threshold strategy. Weed Science. 62(2):237-249, DOI: http://dx.doi.org/10.1614/WS-D-13-00145.1.

Also see Paul Hollis’ article about the study in Southeast Farm Press.

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