While growers wonder if Palmer amaranth populations will be able to adapt to the growing conditions of Illinois, results from a recent University of Illinois study suggest that it is a question of when and where, not “if” pigweed will become established in the state.

“Perhaps a more important question now concerns the damage niche of Palmer amaranth populations in Illinois agronomic cropping systems,” said Aaron Hager, a U of I associate professor of weed sciences.

Palmer amaranth (Amaranthus palmeri) has garnered much attention recently in both academic discussions and popular press releases, and with good reason, Hager said. Among the weedy species of Amaranthus, Palmer amaranth has the fastest growth rate and is the most competitive with the crops common to Midwest agronomic cropping systems. 

According to Hager, soybean yield losses approaching 80 percent and corn yield losses exceeding 90 percent have been reported in peer-reviewed scientific literature. 

“Palmer amaranth can be effectively managed in Illinois agronomic crops, but the greatest likelihood for successful management is with systems that employ multiple effective management tactics,” he said. “Palmer amaranth is perhaps the personification of a weed species that requires an integrated management approach.”

Unlike waterhemp, Palmer amaranth is not indigenous to Illinois. The pigweed evolved as a desert-dwelling species in the southwestern United States, including areas of the Sonoran Desert. However, genotypic and phenotypic adaptability have allowed Palmer amaranth to expand its distribution and colonize across much of the eastern half of the United States.

“It is likely that Palmer amaranth was introduced by seeds moved into Illinois from areas where Palmer amaranth has become the dominant pigweed species,” Hager said.

Competitive with crops

Results from experiments conducted by Adam Davis, a USDA-ARS plant ecologist at the U of I, have demonstrated 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. 

For the past two seasons, weed scientists at the U of I and Southern Illinois University have conducted field and laboratory surveys to determine where Palmer amaranth now occurs in Illinois. 

“The current known distribution of Palmer amaranth in Illinois based on 2012–2013 surveys shows that Palmer amaranth populations have been confirmed in 26 counties throughout the state, with some showing glyphosate-resistant populations,” said Aaron Hager  

Other counties have submitted samples, but analyses have not been completed 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 myriad ways, including harvesting and tillage equipment.

If Palmer amaranth is identified in one or more fields, what can growers do this fall to limit movement of the seed?

Hager offered the following suggestions, developed by weed scientists in Illinois, to help limit the movement of Palmer amaranth seed:

• 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. 

“It’s not too early to begin planning an integrated Palmer amaranth management program. An integrated herbicide program should include soil residual herbicides applied at full recommended use rates within two weeks of planting and followed by post-emergence herbicides applied before Palmer amaranth plants exceed 3 inches tall,” Hager said.

Most herbicides that control waterhemp also control Palmer amaranth, but successful, long-term management of Palmer amaranth in Illinois will likely require more than herbicides, he added.