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Posted 26 September 2014. PMN Crop News.

Palmer Amaranth Can Grow Out of Control Quickly

Important for farmers to take advantage of short management windows

Source: United Soybean Board Press Release.

Chesterfield, Missouri (September 11, 2014)--Any mention of “Palmer amaranth” is likely to bring a shudder from farmers who have been forced to deal with it. Common in the southern United States for several years, this devastating weed keeps creeping further and further north, making those negative reactions more and more widespread.


But what makes this weed so tough to tackle? University of Tennessee Row Crop Weed Specialist Larry Steckel, Ph.D., recently explained this weed’s capabilities to farmers at a field day 75 miles southwest of Chicago. The University of Illinois and Bayer CropScience co-hosted the event.

Steckel says Palmer plants can grow 3 inches in one day and can go from a seed to an 8-inch-tall plant in 13 days.

“The herbicides we’re trying to control this with now have an outside chance of controlling Palmer when the plant is 2 to 3 inches tall,” Steckel says. “We have no chance when the plant is 8 inches tall.”

As a result, farmers have a very small window of opportunity to spray Palmer plants when they’re a manageable size.

“You might have plants that are 2 or 3 inches tall, but then it rains and the next chance you get to spray, the plants are beyond control,” Steckel explains. “In a matter of days, it’s easy to go from having a chance to kill them to having to till a field and replant.”

Steckel also shared other facts about Palmer amaranth, including:

• It loves hot weather. Palmer plants have a taproot that can reach at least 5 feet deep, giving it outstanding drought tolerance and making it very comfortable in hot weather. “Actually, this year has been kind of hard on it,” Steckel says of the mild temperatures that much of the soybean-producing region of the country has experienced this summer.

• It looks like waterhemp. In young plants, it’s hard to tell the difference between Palmer plants and waterhemp. The best way is to look for a single hair growing out of the end of the Palmer leaf, pictured above, which waterhemp almost never has. However, later on, it’s easy to see the difference in mature plants. “If you measure the seed head in feet, it’s Palmer; if you measure it in inches, it’s waterhemp,” Steckel says.

• Yearlong germination. Palmer amaranth can germinate from March through October, so new plants are still coming up while crops are drying down. Farmers might need to keep spraying to keep a field harvestable and seed out of the seed bank.