Farmers Urged To Fight Palmer Amaranth Before It Grows Worse

Farmers Urged To Fight Palmer Amaranth Before It Grows Worse

Farmers still have a fighting chance to stop this tough to control, yield-robbing weed from spreading in Iowa.

The Iowa Soybean Association and Iowa State University Extension are working together to provide information to keep Palmer amaranth at bay. Native to the southwest United States, this hard to control weed is new to Iowa. It was first officially identified in Iowa last September. There are now documented cases of it in fields in Harrison, Page, Muscatine, Fremont and Davis counties. Herbicide resistance, primarily to glyphosate, is an issue.

ATTACK IT NOW: ISU weed scientists are advising farmers that early identification and action is needed to mitigate the spread of Palmer amaranth, a new weed in Iowa which resembles waterhemp but is harder to control.

"We're at a point that we can really restrict how quickly it spreads," says Mike Owen, ISU Extension weed specialist. "If we ignore it, in the next 10 years this weed could be infesting half the row crop acres in Iowa." If that is allowed to happen it would decimate corn and soybean yields and would cost farmers a lot of money in increased weed control costs.

Owen and colleague Bob Hartzler at ISU suspect there are more infestations of this weed in Iowa fields that haven't been found and identified yet. Late May is the time to start scouting but it's hard to tell the difference between Palmer and it's "cousin" common waterhemp, when the weeds are small.

If uncontrolled this weed will decimate yields and the bottom line
Palmer amaranth outbreaks in the Southern U.S. in recent years have caused complete crop failures. A mild to moderate infestation can result in soybean yield losses of up to 30%, with a potential revenue hit of more than $200 per acre.

Early identification and action is paramount to mitigate the spread of Palmer, which resembles waterhemp, in Iowa. "It is increasingly important that you know what weed species you have in your fields in order to figure out better prescriptive control methods," says  Ed Anderson, ISA senior director of supply and production systems.

ISU weed specialists are available to help farmers and crop consultants identify Palmer Amaranth. Correct identification of the weed is vital for appropriate planning of your management strategy, says Hartzler. Late May is the time to start scouting for Palmer, but it will germinate throughout the growing season.

Best strategies for control include using soil-applied herbicides
Recommended control strategies for Palmer include soil-applied, residual herbicides and using a herbicide program that provides multiple effective modes of action. Group 3, group 15 and some group 14 herbicide products work well. Post-emergence herbicide products are limited in their effectiveness due to herbicide resistance, and should be used sparingly, says Owen. Cover crops and spot cultivation are also effective measures to control this difficult weed.

Waterhemp is a "relatively wimpy weed" and can be controlled with weaker herbicide doses unlike Palmer, which makes identification so important. Contact Owen at (515) 294-5936 or [email protected] for management help. For more take-action tips to manage Palmer Amaranth, go to

A poster describing waterhemp and Palmer Amaranth is available for downloading on the ISU Weed Science website. There are also a number of other articles on the ISU weed science website providing information about identification and management of Palmer amaranth.

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