Farmers in the Midwest are finding out what producers in the southwest United States have known for some time: Palmer amaranth is a tough weed that can be tough to control.

Managing the weed requires regular and timely field scouting, and a combination of cultural practices and herbicide programs, says Travis Legleiter, a Purdue University weed science program specialist.

“We want those producers who are not yet dealing with Palmer amaranth to realize how much of a major nuisance Palmer is and that they need to be able to identify it when scouting fields, and take action immediately when they do find it on their acres,” Legleiter says. “This is a weed that requires a zero-tolerance policy of not allowing any seed production.”

Palmer amaranth, often called pigweed but different than other amaranth species that also are often called pigweeds, is native to the desert Southwest and northern Mexico. The weed has slowly infiltrated the southeast U.S. and, recently, the Midwest.

Left unchecked, the weed can grow 2-3 inches per day to a height of more than 6 feet and produce hundreds of thousands of seeds. Those seeds can be spread by farm equipment — specifically combines — grain, seed or feed contamination, and wildlife. Emergence extends well into typical crop seasons and even after crop harvest.

“What makes Palmer amaranth such a problem is that most populations are resistant to glyphosate and ALS herbicides,” Legleiter says. “Producers who encounter Palmer amaranth should treat the population as if it is ALS- and glyphosate-resistant, because the original transplanted seed came from the South where the majority of Palmer populations are resistant to those herbicides.”

Purdue’s Bill Johnson stands next to Palmer amaranth

When field scouting for Palmer amaranth look for the weed’s differentiating characteristics: wider and ovate to diamond-shaped leaves than other amaranths; leaf stems (petioles) as long or longer than the leaf blade itself; rosette-like appearance; long seed head structures — up to 3 feet — on female plants; white chevron- or V-shaped watermarks on some leaves; and the absence of hair on leaves.

“The main management goals should be to reduce early season competition with crop plants and prevent all plants from producing seed, and to avoid spreading the weed to other areas,” Legleiter says. He says producers should consider adopting the following cultural practices if they find Palmer amaranth in their fields:

• Rotate crops, to allow the use of herbicides with additional modes of action and help slow further herbicide resistance.
• Use deep tillage, which buries Palmer amaranth seed below its preferred emergence depth.
• Plant cereal rye cover crops, which serve as a mulch to suppress Palmer amaranth emergence.
• Pull weeds by hand and remove them from the field so they cannot reroot and continue to grow and produce seed. Compost or burn the pulled weeds.
• Monitor ditches and borders along field edges, to prevent Palmer amaranth from spreading seed into fields.
• Harvest heavily infested fields last and then carefully clean the combine, to reduce spreading seed to other areas.

As far as herbicides go, there are a number of pre- and post-emergence products available that effectively control Palmer amaranth in corn and provide alternate modes of action.

“However, when growing corn for multiple years, take care not to rely heavily on single modes of action, because atrazine- and HPPD-resistant Palmer amaranth populations have previously been confirmed,” Legleiter says. “In soybean the number of herbicides available to replace glyphosate for Palmer amaranth control is limited. Furthermore, the herbicides must be applied at appropriate weed size for consistent control.”

For best control, herbicide applications should be made before the weed reaches 6 inches in height, Legleiter says. Applications made when the plant is 6-8 inches tall provide only marginal control, and herbicide effectiveness decreases significantly beyond 8 inches, he says.

For tables of herbicides that can be used to control Palmer amaranth in corn and soybeans and additional information, read Purdue Extension publication WS-51, Palmer Amaranth Biology, Identification, and Management , by Legleiter and Bill Johnson, Purdue weed scientist. The publication is a free download.

One application method that has shown to work well on Palmer amaranth and other emerged weeds is Weed Wiper . The long, cylindrical sponges, made by Smucker Manufacturing Inc. and available through SpraySmarter.com, can be mounted on sprayer booms, pull-type sprayers, tractors, ATVs and front buckets. There also is a hand-held model. As the Weed Wiper moves through a field herbicide is continuously applied to the sponge. As the name implies, the sponge wipes herbicide on weeds as it passes over them.

By Steve Leer

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