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Palmer Amaranth Heads Northward

This pugnacious pigweed member has been confirmed in South Dakota and Wisconsin, and is threatening states like Minnesota and North Dakota.

If you think Palmer amaranth is a weed that farmers in the South and Mid-South only have to worry about, think again.

It’s spreading northward and has been confirmed in Wisconsin, Iowa, Illinois, and Nebraska (2011–2013), South Dakota (2014), and other northern states. This year, it was discovered in newly seeded CRP land in Iowa, including Clayton County, just one county away from southeastern Minnesota. Moreover, wildlife and pollinator habitat ground in some Iowa fields are showing up with Palmer amaranth.

That has weed scientists in the Upper Midwest greatly concerned. “If using seed mixtures for pollinator or wildlife habitat, cover crops, or any other objective, know the seed source and potential of unintentional introduction of pernicious and devastating weeds,” says Richard Zollinger, North Dakota State University Extension weed specialist. 

Back in 2011, Larry Steckel, University of Tennessee Extension weed specialist, calculated that glyphosate-resistant weeds — primarily Palmer amaranth — cost Tennessee farmers $120 million annually. Failure to anticipate the seriousness of Palmer amaranth infestations may produce a similar impact in a northern state.

Here’s some background on Palmer amaranth and management tips that have been compiled by University of Minnesota crop and weed scientists and communications specialists that include Lisa Behnken, Fritz Breitenbach, Jeff Gunsolus, Phyllis Bongard, and Liz Stahl.

Why the Worry?

Like waterhemp, this pigweed family member is a season-long emerger from May through August. Palmer amaranth is more aggressive, though, than waterhemp and can grow 2 to 3 inches per day.

It also quickly adapts to herbicide management tactics that lack diversity in effective herbicide sites of action (SOA). This limits control options. In northern states, expect Palmer amaranth to resist SOAs like glyphosate (SOA group 9) and ALS inhibitors like Pursuit (SOA group 2).

Palmer is dioecious, with male and female plants. Thus, outcrossing rapidly spreads herbicide resistance.

Like waterhemp, Palmer amaranth is prolific. A single female plant typically produces 100,000 to 500,000 seeds.

One piece of good news is that Palmer amaranth may not persist in areas being established for conservation habitat. That’s because native perennial vegetation could crowd it out once established. What’s concerning, though, is that Palmer amaranth may produce enough seed to establish a seedbank in those fields and move into neighboring corn and soybean fields.
 

How To Identify It

Here are a few characteristics that help to identify Palmer amaranth.
• No hairs on stems and leaves on Palmer vs. redroot pigweed, which does have the hairs.
• Petiole is as long or longer than older leaves.
• Ovate- to diamond-shape leaves, which gives it a poinsettia-like appearance.
• Prolific seed producer with a long main terminal seed head up to 3 feet long.

(For more information, go to this Iowa State University link at http://crops.extension.iastate.edu/cropnews/2016/08/palmer-amaranth-now-identified-least-nine-iowa-counties.)
 

How to Halt It

Identify early and remove and destroy plants. Don’t allow Palmer amaranth to go to seed!

In some states like Minnesota, Palmer amaranth is on the state’s noxious weed list as an Eradicate weed. According to Minnesota statutes, landowners must attempt to eradicate any Palmer amaranth found by destroying all the above- and below-ground parts of the plants. For more details, refer to http://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist.aspx. The Minnesota Department of Agriculture also requests the reporting of any weeds found on the Eradicate list. (Call 888-545-6684.)

Full-labeled rates of residual herbicides with multiple SOAs are a must in corn and soybeans. If a field does become infested, it should be rotated to a crop where management options are more diverse (alfalfa, small grains, or even corn).

Early postemergence herbicide applications (Palmer amaranth plants less than 3 inches high) are critical in managing this weed. Remember, Palmer can grow 2 to 3 inches in a day, so it can quickly exceed heights where herbicides provide acceptable control.

 

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