Palmer Amaranth

Note from the GROW team: This page is being updated in an effort to provide you with the most accurate and timely information. In the meantime, please visit the rest of our website to learn more about the tactics to manage herbicide-resistant weeds.

Photo Credit: University of Missouri

Palmer Amaranth is a highly competitive pigweed species with herbicide resistant populations that are problematic in many states. ManyPalmer amaranth populations in the US are resistant to glyphosate, and it is common for plants to be resistant to ALS-inhibitor herbicides as well.

In the past few years, herbicide resistance has been reported not only to these two families but to many others which recently included 8-way multiple resistant populations. The most recently herbicide-resistant populations found were both auxins (2,4-D) and long-chain fatty acid inhibitors (S-metolachlor).

This species grows very rapidly, and even effective herbicides are not likely to kill Palmer seedlings that are over 6 inches tall. If it is allowed to grow above 6 inches, farmers must move to other tactics including hand-pulling, mowing, burning, HWSC, and cultivation. if the plants are not terminated somehow, they will produce up to 600,000 seeds per plant that survive in the soil and continue the cycle of weed issues next year.

Therefore, it is essential to follow best management practices for Palmer amaranth:

  1. Start clean
  2. Use effective residual herbicides at planting
  3. Scout to identify this weed early before it grows too tall
  4. Cultivate or use an effective postemergence herbicide while plants are at susceptible stage
  5. Use multiple effective herbicide sites of action to effectively manage Palmer amaranth and prevent/delay weed resistance to different sites of action herbicides. Palmer amaranth has a prolonged germination period so postemergence herbicides may need to include a residual herbicide; cover crop residue often starts to degrade before crops form a dense crop canopy, more than one cultivation maybe needed.
  6. Employ a zero tolerance policy – make sure no plants are left by harvest: Hand pull, remove from the field, and burn or bury.
  7. Combines and other equipment can spread Palmer amaranth seeds. Avoid using combines that have recently been in fields with Palmer amaranth, as they will spread the infestation to other fields. Contaminated combines should be cleaned after harvesting a field with Palmer. Consider not harvesting heavily infested areas, because harvesting the crop and spreading weed seeds from that part of the field may not be economical compared to the increased weed management cost of controlling a larger infestation.
  8. Prevent adding to the soil seed bank.

Use integrated weed management tactics that target Palmer amaranth, for instance:

  • Use narrow rows where appropriate
  • Plant competitive varieties
  • Follow good agronomic practices to achieve a competitive crop canopy as quickly as possible
  • Use cover crops and terminate them close to planting to maximize their impact

Herbicide resistant populations:

  • Group 4
  • Group 2
  • Group 3
  • Group 5
  • Group 9
  • Group 15
  • Group 27
  • Group 9 + 14
  • Group 5 + 27
  • Group 5 + 9
  • Group 2 + 14
  • Group 2 + 9
  • Group 2 + 5 + 9
  • Group 2 + 9 + 27
  • Group 2 + 5 + 27 + 9 + 4
  • Group 2 + 14 + 9 + 3 + 15

For more information on palmer amaranth management, visit:
https://iwilltakeaction.com/weed/palmer-amaranth

Hero Photo credit: University of Missouri. Hero Circle photo credit: University of Illinois

IWM best practices are constantly changing and it is GROW’s goal to keep you up to date on the science and advice that best combat herbicide-resistant weeds. Sign up below to stay up to date.

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