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Meet the 'Osama bin Laden' of weeds

Jeff Caldwell Updated: 01/25/2012 @ 3:10pm Agricultural content creator and marketer.

Get to know Palmer amaranth now, because you may wind up with fields full of it this summer if you're not careful.

There are a lot of questions floating around right now about how to manage the weed alone, let alone if it develops glyphosate resistance. It's in the same family as waterhemp -- a weed with which most farmers are familiar -- but is much more aggressive, says University of Illinois Extension weed specialist Aaron Hager. And, that aggression is pushing the weed further north into the heart of the Corn Belt.

"This species has managed to drastically spread across the southern United States, so what says it won’t spread to northern Illinois as temperatures increase,” Hager says in a university report. "This could become a huge problem, especially as glyphosate resistance is identified."

One of the things that makes Palmer amaranth so tough to control is how quickly it grows. Conventional control measures, namely glyphosate, are effective against it when it's around 3 inches tall, but once it gets to 6 inches tall, it's tougher to knock down. But, here's the kicker: Palmer amaranth grows 3-6 inches per day. So, the treatment window's a narrow one. "Delaying applications of postemergence herbicides is not recommended," Hager says.

Because of that narrow window for postemergence treatment, Hager says it's best to use soil-applied preemerge herbicides. Postemerge herbicides, though not ideal, can work in some situations, as can mechanical cultivation. If you're well-prepared to stave off waterhemp, though, base your Palmer amaranth control on that, he recommends.

"The management practices for Palmer amaranth are very similar to those recommended for waterhemp," Hager says.

Glyphosate resistance is something to watch out for with Palmer amaranth, just like waterhemp. That's part of what makes it tough to control. So, if you do find it in your fields, be prepared to not only take quick action, but also be ready to adapt your approach.

"Palmer amaranth is the Osama bin Laden of weeds. It seems to embody adaptability and aggressiveness in its growth patterns," says Dave Rahe, a certified crop adviser with Soil-Right Consulting Services, Inc., in Hillsboro, Illinois. "As with other resistant pests, rotating chemical families and modes of action should be helpful."

Top 10 Bad Weeds of 2012

Hager says Palmer amaranth is just one of the weeds that could cause major headaches this year. He says the Top 10 weeds to watch for this year are:

  • Waterhemp

  • Giant ragweed

  • Palmer amaranth

  • Common lambsquarters

  • Giant foxtail

  • Annual morningglory

  • Velvetleaf

  • Common cocklebur

  • Fall panicum

  • Horseweed

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