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Hill and Furrow

Make Preparations for Encountering Palmer Amaranth During Harvest

Scouting from the Cab. While crop producers are encouraged to scout their corn and soybean fields throughout the entire growing season, the view of a field from the vantage point of a combine cab can be eye-opening, particularly when harvesting corn.

One thing that can become abundantly clear is just how well the weed management strategy for the year kept weeds in check.

This clarity is important for multiple reasons:

  • Changes need to occur when weed management strategies have proven ineffective.
  • Field areas with large weed populations will have weed issues for years to come due to additions to the weed seed bank – areas of future target for additional scouting and weed hook attention.
  • Female Palmer amaranth plants should not be run through harvest machinery.

Where is Palmer amaranth? Several years ago University of Illinois weed scientists published a map of the state with individual counties shaded one of three colors indicating that Palmer amaranth had either been confirmed, that glyphosate-resistant Palmer had been confirmed or that no Palmer had yet been confirmed in the county. Perhaps it is a positive development that no such map has been published recently. While scientists always cautioned against assuming that Palmer amaranth wasn't in a county just because the county wasn't shaded on the map, a false sense of security can nonetheless set in.

Now that Palmer amaranth is in the state, and with the recent revelations that additional introductions may have occurred due to contaminated seed used for pollinator habitat CRP plantings, all must have a plan in place to respond appropriately upon encountering this weed.

Why Should One Care? Under ideal conditions Palmer amaranth plants are capable of growing up to 3 inches per day with each female plant producing up to 1 million seeds. It is the combination of its fast rate of growth and tremendous reproductive capacity that is responsible for reports of this weed being responsible for yield reductions of up to 90 percent in corn (Massinga et al. 2003) and 80 percent in soybean (Klingaman and Oliver 1994).

Preparing to Show Palmer 'Zero Tolerance' with a Little Homework. Before setting out to harvest, be sure to have already brushed up on how to identify Palmer amaranth and to 'mark' points with your GPS. Along with everything else that you plan to bring into the combine cab, bring along flags and sturdy garden bags (this will soon make more sense). The following are elements of the Palmer amaranth 'zero tolerance' policy recommended by University of Illinois weed scientists and were originally outlined in a Bulletin article written by Dr. Aaron Hager:

  • "Fields with Palmer amaranth populations should be the last fields harvested this fall and the last fields planted next spring.
  • Mark or flag areas where Palmer amaranth plants have produced seed. These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.
  • Do not mechanically harvest mature Palmer amaranth plants with crop harvesting equipment. Physically remove the plants immediately prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field. Bury or burn the bags in a burn barrel as soon as possible.
  • Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring. Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores."

Additional Resources

For great pictures of female Palmer amaranth flowers check out this September 2014 Bulletin article written by Dr. Aaron Hager, "Remain Vigilant for Palmer Amaranth".

Hager, A.G. 2014. Management of Palmer amaranth in Illinois. University of Illinois Department of Crop Sciences Weed Science publication.


Massinga, R.A., Currie, R.S. & Trooien, T.P. 2003. Water use and light interception under Palmer Amaranth (Amaranthus palmeri) Corn (Zea mays) competition. Weed Sci.51:521-531.

Klingaman, T.E. & Oliver L.R. 1994. Palmer amaranth (Amaranthus palmeri) interference in soybeans (Glycine max). Weed Sci. 42:523–527.