Palmer Amaranth: A Weed on the Move in Illinois

The bane of the south is making its way northward.

The question at hand says University of Illinois Extension Weed Scientist Aaron Hager is not if, but when and where Palmer Amaranth will show up in Illinois fields. Maybe more importantly, how much damage will it do to the yield of a growing crop. He wrote about it in The Bulletin online. “The Bulletin” is ILLINOIS’ crop science and extension outreach webpage.

Hager notes research done by Adam Davis, USDA-ARS plant ecologist at the University of Illinois, has examined these questions.  Results demonstrate there are few landscape-level barriers to the
establishment of Palmer amaranth populations in Illinois, and that these populations, once established, are competitive with crop species.  In other words, these results indicate that it’s not a question of if Palmer amaranth will become established in Illinois, but rather when and where it will become established.
For the past two seasons, weed scientists at the University of Illinois and Southern Illinois University have conducted surveys (field and laboratory) to determine where in Illinois Palmer amaranth now occurs. It shows up in about a quarter of the 102 counties. All but one of these is west of the Illinois River. However they stretch from Chicago to Cairo (kay-row). The weed is quite likely in many other counties, but simply not accounted for, yet.

Similar to female waterhemp plants, female Palmer amaranth plants produce an abundance of seeds.  These small black seeds are easily moved within and between fields in a myriad ways, including on harvesting and tillage equipment.  If a farmer can identify Palmer amaranth in one or more fields Hager has the following suggestions to keep it from moving.
  • Fields with Palmer amaranth populations should be 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.
  • Fields in which Palmer amaranth seeds were produced should NOT be tilled.  Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.
It’s not too early to begin planning an integrated Palmer amaranth management program writes Hager online.  He believes an integrated herbicide program should include soil-residual herbicides applied at full recommended use rates within two weeks of planting and followed by postemergence herbicides applied before Palmer amaranth plants exceed 3 inches tall.  Most herbicides that control waterhemp also control Palmer amaranth, but successful, long-term management of Palmer amaranth in Illinois will likely require more than herbicide.