Keep close watch for Palmer amaranth

By Mark Loux, Ohio State University Extension weed control specialist

Palmer amaranth (aka Palmer pigweed) has been fairly accurately characterized as “pigweed on steroids.” In addition to the glyphosate resistance, this weed’s rapid growth, large size, extended duration of emergence, prolific seed production, and general tolerance of many post- herbicides makes it a much more formidable weed to deal with than the other Amaranthus species. The post- herbicides that have activity on Palmer amaranth — Flexstar, Cobra, Reflex, Ultra Blazer, and Liberty — must be applied when

Resistant pigweed plants can produce 500,000 seeds per plant that can spring up in fields the following year.

the weed is less than three inches tall.

Palmer amaranth has overall more potential to reduce yield if not controlled well, compared with the other pigweeds. We have at this point confirmed the presence of Palmer amaranth in one large field in southern Ohio, and this population is resistant to both glyphosate and ALS inhibitors. We also have been informed of another possible site just south of Columbus. As far as we can tell, the source of the first population may have been a CREP/wildlife type seeding, where the seed of the desirable species was apparently contaminated with Palmer amaranth seed. There are also substantial infestations of Palmer amaranth in Michigan and Indiana, and the source of

Palmer amaranth taking over a cotton field.

these appears to be contaminated cottonseed shipped from the southern U.S. for use as animal feed here in the Midwest. Manure from these animal operations was spread on fields, and the Palmer amaranth then became established.

A major goal over the next decade for Ohio agribusiness and growers, and OSU weed scientists, has to be the prevention of additional infestations of Palmer amaranth and waterhemp. Both of these weeds have more potential to impact the profitability of our corn and soybean production than our other resistance problems. More information on

Palmer amaranth from a southern cotton field.

these weeds is available on the OSU weed management website,, and the websites of other land grant universities in the Midwest and South. Steps we need to take to accomplish this:

1. Identification of new infestations as soon as possible after they occur. We have posted a brief Powerpoint video on our website that covers identification of pigweeds. A key characteristic for identification of Palmer amaranth early in the season may be the presence of a very dense population where there were only a few plants the previous year. Identification needs to occur when the plant is small enough that herbicides are still effective. OSU weed scientists can help with identification, either via digital photos emailed to us, or with visits to fields. Special attention should be paid to fields receiving applications of manure from animal operations using cottonseed products as feed. We would appreciate being informed of any new infestations of Palmer amaranth as soon as possible (

2. Implementation of effective management programs. Both waterhemp and Palmer amaranth have extended periods of emergence, and a combination of preemergence and postemergence herbicides is required. We assume that most populations are resistant to glyphosate and ALS inhibitors (although there are waterhemp populations in Ohio that are just ALS-resistant), and this should be factored into the choice of herbicides. Waterhemp is included in the soybean herbicide effectiveness table in the “Weed Control Guide for Ohio and Indiana.” Growers in the south are using an approach for Palmer amaranth that includes preplant/preemergence application of residual herbicides, and then a combination of postemergence and residual herbicides when the weeds are less than three inches tall (and in some cases another application of residual herbicides).

3. Prevention of further seed production. As new infestations of Palmer amaranth develop in Ohio, there are likely to be instances where the problem is identified too late to implement effective control measures (this is typical for any new resistance problem). Where this occurs, it is essential that growers take all steps possible to prevent weed seed production. This can include tillage, mowing, and also removal of surviving plants by hand. Effective removal of plants by hand requires more than just cutting them off. Experiences in the south are that the plant must be uprooted and removed from the field. Paying a crew of people to remove plants in mid-season should be considered a viable solution, even at a relatively high cost. The result of not doing so could be a substantial loss of income in future years.

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