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Palmer Amaranth Management

Kacey Birchmier 04/04/2014 @ 5:33pm Agronomy & Conservation Editor for Successful Farming and Agriculture.com. I grew up on a fourth-generation farm in central Iowa. Follow me on twitter - @KaceyBirchmier.

Palmer amaranth is spreading throughout the country, and it is going to require careful management. This weed has the ability to grow two to three inches per day. The effectiveness of  foliar-applied herbicides decrease when Palmer amaranth plants are taller than four inches, leaving only days to get ahead of the weeds.

“Palmer amaranth is a weed species that must be thoughtfully and carefully managed; simply attempting to control Palmer amaranth often leads to ineffective herbicide applications, substantial crop yield loss, and increasing weed infestations,” says Aaron Hager, University of Illinois weed scientist.

If not effectively managed, corn and soybean yields can be reduced to near zero. The University of Illinois developed recommendation for the management of Palmer amaranth.

“These recommendations were developed in accordance with the somewhat unique growth characteristics of this weed species. The goals of the recommendations are twofold. One, to reduce the potential for Palmer amaranth to negatively impact crop yield, and two, to reduce Palmer amaranth seed production that ultimately augments the soil seed bank and perpetuates the species,” says Hager.

Follow the three general principles for Palmer amaranth management:

1) Prevention is preferable to eradication.

“Prevention refers to utilizing tactics that prevent weed seed introduction and weed seed production,” says Hager. “The myriad of ways in which Palmer amaranth seeds can be transported, however, makes preventing seed introduction extremely challenging.” 

Once Palmer amaranth populations become established, utilizing any and all tactics to prevent seed production becomes of paramount importance.

2) It is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established. 

“There are simply no soil- or foliar-applied herbicides that will provide sufficient control of Palmer amaranth throughout the entire growing season,” says Hager.  

In areas where Palmer amaranth is already established, it’s common to have three to five to five herbicide applications per growing season.

3) Control of Palmer amaranth should not be less than 100 percent. 

“Female Palmer amaranth plants produce tremendous amounts of seed and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field,” says Hager.

Germination and emergence
“Palmer amaranth seed germination and seedling emergence are best described as continuous,” says Hager. “Similar to waterhemp, multiple Palmer amaranth emergence events are possible throughout much of the growing season.” 

Research has demonstrated Palmer amaranth seed has a higher germination rate than most other Amaranthus species (including waterhemp), and demonstrates a germination percentage higher than waterhemp at both low and high temperatures. Because Palmer amaranth that emerges before waterhemp in the spring and later in the growing season after waterhemp emergence has stopped, it has a competitive advantage over waterhemp and most other weed species, says Hager.

Steps to handling germination and emergence
1) Have 100% control of emerged Palmer amaranth before planting.    

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