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Palmer Amaranth Management

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.    

Burndown herbicides or thorough tillage are effective tactics to control emerged Palmer amaranth plants before planting. Keep in mind, however, that glyphosate will not control glyphosate-resistant Palmer amaranth and growth regulator herbicides (such as 2,4-D or dicamba) are most effective on Palmer amaranth plants less than 4 inches tall. If preplant scouting reveals Palmer amaranth plants taller than 4 inches, consider using tillage instead of herbicides to control the plants, advises Hager.

2) Apply a full rate, based on label recommendations for soil texture and organic matter content, of an effective soil-residual herbicide not sooner than seven days prior to planting no more than three days after planting. 

“Many soil-residual herbicides that are effective for controlling waterhemp are also effective for controlling Palmer amaranth,” says Hager.

Palmer amaranth growth rate
Palmer amaranth’s growth rate exceeds those of other Amaranthus species. “Whereas waterhemp can add one inch of new growth per day under good growing conditions, Palmer amaranth can add two to three inches per day.  The effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than four inches,” says Hager.

Glyphosate should not be solely relied on in controlling the weed.  

Steps to handle Palmer amaranth growth rate
1) Begin scouting fields within 14–21 days after crop emergence. 

“We recommend this interval even for fields previously treated with a soil-residual herbicide applied close to planting,” says Hager.

2) Remember foliar-applied herbicides must be applied before Palmer amaranth plants is taller than four inches.

The effectiveness of foliar-applied herbicides decrease once the weed is taller than four inches.

“Postemergence herbicides that demonstrate control or suppression of Palmer amaranth include synthetic auxin herbicides (dicamba, 2,4-D), diphenylethers (acifluorfen, lactofen, fomesafen), glufosinate, glyphosate, and HPPD inhibitors (mesotrione, tembotrione, topramezone),” says Hager. “Palmer amaranth can germinate and emerge over an extended period of time, so there is often a wide range of plant sizes by the time postemergence herbicides are applied.” 

If you have problems with spray interception by smaller plants under the canopy, make adjustments in spray volume and pressure to change the coverage.

3) Include a soil-residual herbicide during the application of the foliar-applied herbicide. 

“A soil-residual herbicide applied with the foliar-applied herbicide can help control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds,” says Hager.

4) Fields should be scouted 7–14 days after application of the foliar-applied herbicide to determine:

- Herbicide effectiveness
- If the soil-residual herbicide included with the post application is providing effective control.
- If additional Palmer amaranth plants have emerged.

“If scouting reveals additional Palmer amaranth plants have emerged, make a second application of a foliar-applied herbicide before Palmer amaranth plants are four inches tall,” says Hager.

Species Biology: Palmer amaranth seed production

“Palmer amaranth is a dioecious species, meaning plants are either male or female,” says Hager. “Male plants produce pollen and female plants produce seed, which makes Palmer amaranth an obligate outcrossing species. Outcrossing species tend to have more genetic diversity then self-pollenated species and this can hasten the evolution of herbicide resistance.”

Because of the transfer of herbicide-resistance traits is by pollen, the resistance can spread quickly.

Handling Palmer amaranth seed production

“Physically remove any remaining Palmer amaranth plants before the plants reach the reproductive growth stage,” says Hager. “Plants should be severed at or below the soil surface and carried out of the field. Severed plants can root at the stem if left on the soil surface, and plants can regenerate from stems severed above the soil surface.” 

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