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palmer amaranth growth rates

Spray Palmer amaranth before it reaches 4 inches high

Geez, the margin for error shrinks constantly, especially with weed resistance. The latest bulletin on this theme relates to Palmer amaranth’s tight control window—before 4 inches high. That gives you about 10 days.

This picture may as well be captioned, “Welcome to my nightmare.”

weed height after 16 days

Be sure to spray it before it reaches 4 inches, says University of Illinois Weed Scientist Aaron Hager. Better yet, use full rates of pre-emerge residual herbicide labeled for Palmer. And then, scout through the growing season, Hager says.

It took just 10 days for Palmer to reach this critical 4-inch height in a recent University of Illinois experiment.

And, don’t rely on what waterhemp’s taught you. If that is your frame of reference for weed control, you’ll miss the boat on Palmer economic thresholds, Hager adds. The photo says it all: You have much less leeway to control Palmer, than you do for other weed species.

Palmer amaranth is a faster-growing and more competitive cousin of common waterhemp, and it’s frequently resistant to glyphosate and other herbicides.

“Palmer amaranth has to be managed in an integrated system,” Hager says. “You can’t just throw one herbicide at it.” The key elements for control are:

•       Plant into a clean seedbed

•       Use full rates of a pre-emergence residual herbicide that kills Palmer amaranth

•       Apply timely postemergence herbicide before Palmer pigweeds are 3 inches tall

•       Tank mix another residual herbicide with application to extend control

•       Come back with a second postemerge application if needed

•       Remove surviving plants by hand or mechanically

See proven recommendations for Palmer amaranth management from University of Illinois weed scientists.

Nab Palmer at its most vulnerable stage — germination, says Travis Legleiter, Purdue University weed specialist. “Pre-emerge residual herbicides are critical in both corn and soybeans.” 

It’s difficult to control, especially in soybeans, says Christy Sprague, Michigan State University weed scientist. “In fields where it’s not correctly identified or managed, a few plants can rapidly multiply to several hundred plants in just a year’s time.”


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