Discoveries of Palmer amaranth in conservation plantings, such as Conservation Reserve Program ground and cover crop acres, has created the need to develop management plans to reduce the likelihood of movement of this difficult to control weed into row crop fields. 

The map accompanying this article shows 20 counties in Iowa where Palmer amaranth has now been discovered that can be traced back. The red ‘X’ represents introduction via establishment of native seed mixes for conservation purposes.

SCOUT FOR PALMER: Fields planted with native seed mixes earlier this spring such as cover crops, CRP acres, buffers and grass waterways, should be scouted as soon as possible to determine if Palmer amaranth is present.

Reducing or preventing Palmer amaranth seed production should be a high priority, says Bob Hartzler, Iowa State University Extension weed specialist. “The maturity of Palmer amaranth varies considerably in the fields we have observed,” he says. “While it is likely that some viable seed of amaranth is already present, the amount of seed produced can still be dramatically reduced with appropriate control measures.”

What are your management options for Palmer amaranth?

Physical removal of individual Palmer amaranth plants is the preferred management option in conservation plantings, says Hartzler. Palmer amaranth plants collected in multiple Midwest and mid-south states retained 95% to 100% of their seed at soybean maturity, research shows (Schwartz et al. 2016). Hand removal and destruction of plants yet this fall should be a viable tactic to remove seed from both crop and native perennial plantings.

Hand rogueing, however, may not be viewed as feasible in some cases due to the expense or labor availability. Thus, the second option for managing Palmer amaranth in conservation plantings would be mowing. 

Mowing may not provide effective control of big Palmer plants

Unfortunately, mowing may not be highly effective on large Palmer amaranth plants present in conservation plantings that were not mowed earlier this year, Hartzler says. “Mowing large plants may sever some stems, but we’ve observed plants that are simply pushed over by the mower and quickly rebound. The effectiveness of mowing large plants will vary with mower type and growth habit of the Palmer amaranth.”

Effects of mowing during establishment year of planting

Most agronomists and researchers who are experience with covers crops recommend repeated mowing during the establishment year. This repeated moving of native plant mixes used in conservation plantings will reduce competition and seed production by weeds. No information is available regarding the effect of single or multiple mowing events on Palmer amaranth seed production in perennial habitats.

Researchers at the University of Georgia studied the effect of incomplete physical control of flowering Palmer amaranth plants on survival and seed production (Sosnoskie et al. 2014). They reported 35% mortality of Palmer amaranth plants cut 6 inches above the soil level. Surviving plants were reduced 51% in height and produced 73% fewer seeds per plant than uncut plants. 

Palmer amaranth cut to 1.2 inches above the soil surface had 64% mortality. Survivors of the 1.2-inch cutting had an 82% reduction in plant height and a 94% reduction in seeds produced. This study demonstrates the value of mowing in reducing seed production in Palmer amaranth.

Scouting and prevention is needed to fight Palmer amaranth

The establishment of Palmer amaranth in the planting of conservation acres across the state of Iowa is unfortunate. However, the long-term impact of the contaminated seed mixes can be minimized by responding accordingly, say Hartzler and ISU Extension field agronomist Meaghan Anderson, who is also closely studying the Palmer amaranth situation in Iowa fields. Seed mixes that were purchased from several seed producers/vendors have been the source of Palmer amaranth in some cases. 

Fields planted with native seed mixes earlier this spring should be scouted as soon as possible now to determine if Palmer amaranth is present, say the ISU weed control specialists. 

“If you find this weed on your conservation acres there is still time to minimize seed production and the risk of Palmer amaranth spreading into crop fields in the area,” emphasizes Hartzler. “Since some viable Palmer amaranth seed is likely to be present, you need to take appropriate precautions to prevent moving seed from source fields with mowers or other equipment.”