Eyes Peeled For Palmer

Pigweed Out of Gate Early

Palmer amaranth seedlings look a lot like waterhemp. A reddish tint and more rounded leaves are clues you've found the renegade. (Photo courtesy of Aaron Hager, University of Illinois)

DECATUR, Ill. (DTN) -- Weed scientists across the Midwest are calling on farmers to keep eyes peeled this spring for an aggressive pigweed species called Palmer amaranth. "We're finding seedlings and they are growing fast," said Aaron Hager, University of Illinois weed scientist. "Knowing if you have the weed and being proactive before it goes to seed is the best defense against Palmer."

Last year Illinois reported 27 counties with confirmed Palmer amaranth infestations. Glyphosate resistant Palmer amaranth was found in 12 of those counties. "I'm confident it is more widespread than that. Those are just the cases we know about," Hager said. The first confirmed Palmer amaranth infestations were found in Iowa late in 2013. The weed has now been confirmed in five Iowa counties, but Iowa State University agronomist Bob Hartzler said in a news release last week that there are likely more unknown infestations than known.

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. Hager said soybean yield losses approaching 80% and corn yield losses exceeding 90% have been reported in peer-reviewed scientific literature.

Originally a desert plant, the weed has plagued southern row crop farmers. Hopes that the plant might succumb to cold winters farther north have long since evaporated. Populations are well established as far north as northern Michigan.

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"Palmer amaranth can be effectively managed in Illinois, but the greatest likelihood for successful management is with systems that employ multiple management tactics," Hager said. "It takes a very integrated management approach."

One of the main challenges in identifying the problematic weed is that it closely resembles waterhemp, another common troublemaker. Hager said Palmer amaranth leaves frequently are wider in relation to their length compared to waterhemp. Palmer amaranth also tends to have a 'bushier' growth habit. Waterhemp is leggy and has an open canopy.

Another identifier is by the petiole, or the stem-like structure that connects the leaf to the main stem. Bend back the petiole, and for Palmer amaranth it will be as long or longer than the leaf itself. Waterhemp petioles are shorter than their lance-shaped leaves.

"The guys in the south will tell you to go out early when the morning sun is over your shoulder and the plant is still 3 inches or less. Palmer will have a red hue or red tint," Hager said. "That color tends to disappear as the plant gets bigger."

The return of warm air conditions coupled with ample soil moisture will help emerged Palmer amaranth plants grow very rapidly, Hager warns. Postemergence herbicides should be applied before Palmer amaranth plants are taller than 4 inches. "It is not advisable to delay the application of postemergence herbicides until additional Palmer amaranth plants emerge," he said. Delayed application, whether by choice or due to adverse weather conditions, can let plants get too large, with disastrous outcomes.

Palmer amaranth continues to germinate throughout much of the growing season, making it likely that additional control may be needed. "Consider including a soil-residual herbicide with the postemergence herbicide to control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds," Hager said.

For photos of Palmer amaranth to help in identification: https://www.extension.purdue.edu/…

http://bulletin.ipm.illinois.edu/…

Pamela Smith can be reached at pamela.smith@dtn.com

(GH\SK)

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