Dangerous invader: Palmer amaranth reaches Upper Midwest

Jason Bond has been battling Palmer amaranth for years. And he has this advice for Upper Midwest farmers encountering the weed for the first time. "If I was a farmer up there, and I see a small patch of Palmer amaranth coming up out of my crop, I...

Palmer amaranth plants grow 6 to 8 feet tall - some even as high as 10 feet - and can ravage crop yields. (Dallas Peterson, Kansas State University)
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 Jason Bond has been battling Palmer amaranth for years. And he has this advice for Upper Midwest farmers encountering the weed for the first time.

"If I was a farmer up there, and I see a small patch of Palmer amaranth coming up out of my crop, I'd stop my truck, walk out there, pull them up and go throw them in a ditch someplace," says Bond, weed science specialist with Mississippi State University's Delta Research and Extension Center.

"Zero tolerance," he says. "You need zero tolerance. It can get away from you quickly. Once you recognize it for the first time, take extreme measure to keep it from getting away from you."

Palmer amaranth, a huge and longstanding concern in the southeastern United States, is spreading into Agweek country. The weed, which already has been found in Minnesota and South Dakota, can damage farm equipment and devaste yields.


Yield losses of up to 91 percent in corn and 79 percent in soybeans have been reported.

Though the weed hasn't been discovered in North Dakota, farmers in the state - and anywhere else it hasn't spread - need to be vigilant, says Tom Peters, extension sugar beet weed specialist for North Dakota State University and the University of Minnesota.

"It's mind numbing what this weed can do to us potentially," he says, pointing to both the cost of controlling Palmer amaranth and the damage it can do to crop yields. "It's a game-changer."

The weed hasn't been reported in Montana, extension and U.S. Department of Agriculture experts tell Agweek, but scientists there also urge extreme caution.

No. 1 weed enemy

Palmer amaranth - voted the most troublesome weed in the U.S. by the Weed Science Society of America - has a unique combination of characteristics that make it particularly dangerous.

• It's a prolific seed producer, with a single plant producing as many as 1 million seeds.

• The seeds are extremely small, making them relatively easy for farmers to spread unintentionally.


• Seeds can lie dormant in the soil for years, waiting to germinate until growing conditions are favorable.

• The seeds are unusually competitive with most crops, including corn and soybeans.

• It can grow as much as 3 inches per day - and the bigger it is, the harder it is to control.

• Because it closely resembles pigweed and waterhemp, especially when small, farmers may misidentify it and take inadequate steps to control it.

• It's prone to developing herbicide resistance.

Scientists are wary of the term "super-weed," Bond says.

"But you can go to your weed science textbook and find what makes a weed a weed. (This) species has pretty much every single characteristic that makes a weed a weed," he says. "It's a very, very efficient weed."

Not just conservation acres


Palmer amaranth received national attention last fall, when seed mixes planted on conservation acres in eight states, including Minnesota, were found to contain the weed.

But the weed was spreading, putting Upper Midwest fields at risk, even before that happened, experts say.

Palmer amaranth seed is so small that farmers, no matter how careful they are, can spread it unintentionally with farm equipment, says Jeff Gunsolus, extension weed scientist with the University of Minnesota.

"We know it will move eventually," he says.

And waterfowl, particularly ducks and geese, spread Palmer amaranth seed, too, Peters says.

Migratory birds can eat the seed in one state and carry it hundreds or even thousands of miles, according to research from the University of Missouri.

Hard to control

Iowa farmers' experience with Palmer amaranth might be useful to producers in Minnesota, South Dakota, North Dakota and Montana.

The weed was first discovered in Iowa in 2013 and has been spreading in the state since then, says Bob Hartzler, extension weed specialist with Iowa State University.

Though herbicides are effective in controlling the weed initially, Palmer amaranth quickly builds up resistance to them, he says.

"If all we do is try to manage it with herbicides, we'll quickly run out of tools," he says.

Hartzler stresses "an integrated program" that includes both pre- and post-emergent products and "alternative tactics." Examples of the latter include narrow row spaces, to help growing crops compete better against the weed.

"We need to continuously shake things up. The weeds adapt very quickly," he says.

Iowa farmers, who grow primarily corn and soybeans, also can combat Palmer amaranth by diversifying their rotations, especially by adding small grains or forage crops, Hartzler says.

Wheat emerges relatively early in the growing season, giving it a competitive edge against Palmer amaranth. And multiple crops in a rotation allows farmers to use different types of herbicide, slowing the weed's ability to build resistance to it.

"A longer rotation - more than a couple of crops - will help. Adding wheat helps," says Gared Shaffer, weeds field specialist with South Dakota State University Extension.

Hard to identify, too

Palmer amaranth so closely resembles pigweed and waterhemp that farmers may misidentify it. Even weed scientists familiar with it can struggle with identification, especially when the plant is small.

Nonetheless, early identification is important, and farmers who think the weed may be present in a field should immediately contact an extension weed specialist, officials say.

Shaffer, for example, encourages farmers to send him photos of what they suspect might be Palmer amaranth.

"The awareness is critical. We'll need to be very proactive and assume the worst (that the weed is present in a field)," Peters says.

Agriculturalists also need to use a variety of tools, not herbicide alone, necessary to combat Palmer amaranth successfully, he says.

"Don't expect industry to provide a solution for Palmer amaranth. Don't expect industry to provide a new herbicide technology solution. We're going to have to fight this weed with the tools that we have in hand," Peters says.

Scouting, seed bank

In an ideal world, Palmer amaranth would never get started in a field. In the real world, accidental introduction is evitable in at least some fields, experts say.

That increases the need for farmers to do a better job of scouting their fields, both before and after herbicide applications, Hartzler and others say.

Identifying Palmer early, before it's established in a field, gives farmers a fighting chance of preventing the weed from taking over.

"Get them (Palmer amaranth plants) when they're just getting established. Generally you don't go from zero Palmer to complete loss in a field in one year," Bond says. "It usually happens in up to three years.

Weed scientists stress the importance of reducing the "weed seed bank density," or the reserve of viable weed seeds in the soil.

"Take a zero-threshold approach to this weed," Gunsolus says ."Be aggressive in not letting this weed go to seed."


How to identify the weed .

Costs to farmers

Palmer amaranth hurts farmers through both higher control costs and reduced yields, says Tom Peters, extension sugarbeet weed specialist for North Dakota State University and the University of Minnesota,

He supplies this example, which assumes a North Dakota soybean field that potentially would produce 50 bushels an acre that would sell for $8.50 per bushel.

• If Palmer amaranth is present in the field, the cost of weed control is $65 per acre - triple the $21 per-acre cost of controlling weeds if Palmer amaranth isn't there.

• If the weed isn't in the field, and yields don't suffer, the farmer will enjoy a return of $425 per acre. If Palmer amaranth is present, however, yields will tumble and the producer's return will be just $89 per acre.

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