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Defeating Palmer Amaranth

Palmer Amaranth: Bedeviling Farmers Like No Other Weed

Whatever you call the Devil, several religions describe the Evil One as nearly indestructible as he silently swipes human hearts while spawning global mayhem.

The weed world seems eons away from the demonic one. Still, it has a similar dark prince (and princess, too) preying on your crops.

Indestructible? Well, this weed may grow 10 feet tall with a circumference as thick as a baseball bat. It can reroot itself if pulled and tossed in a field.

Spreader of evil? Certainly. With no surrounding crop, just one plant can produce 1 to 1.8 million seeds.

A silent lurker of debauchery? Of course. Just when you think this wicked weed has finished its season-long germination and emergence pattern, it can still poke up through a hole in a September soybean canopy.

Swiping souls? Even a weed demon can’t do everything. It does steal yields, though.

Data collected by Purdue University researchers show this weed can ravage yields by 78% in soybeans and 91% in corn.

“It’s the only weed I’ve seen that can drive a farmer out of business,” says Bill Johnson, Purdue University Extension weed specialist.

All hell breaks loose

“Its common name,” says Aaron Hager, University of Illinois Extension weed specialist, “is Satan.”

It’s known as Palmer amaranth, too.

This scoundrel, having worked its evil through the Mid-South, is now heading full steam into the Midwest.

“We started seeing some herbicide-resistant Palmer around here about three years ago, and the number has been increasing,” says Justin Knopf, who farms with family near Gypsum, Kansas. “At first, we thought maybe it was a fluke and that weather conditions were promoting Palmer. But last year was the worst.”

“All hell broke loose last year (in Iowa),” says Bob Hartzler, Iowa State University (ISU) Extension weed specialist. Conservation seedings contaminated with Palmer amaranth seeds joined fields contaminated by traditional farming practices to the point where 49 of Iowa’s 99 counties now have confirmed Palmer amaranth infestations.

Worried? You should be. Chin up, though. There are both chemical and cultural strategies farmers may use to keep it in check. Just don’t let it sneak up on you.

“It’s a weed to be concerned about, but it’s not the end of the world,” says Hartzler. “Everyone in agriculture will have to pay attention to what’s in the fields.”

“Palmer seems to have a better will to survive than any other weed,” says ISU’s Bob Hartzler.

One reason is that it is a prolific seed producer. Even with crop competition, this weed can still produce 250,000 to 500,000 seeds per plant, says Mike Weber, senior technical service representative for Bayer CropScience.

It also germinates nearly the entire growing season, from early May into mid-August, says Weber.

It can even exceed these parameters. “Last year, an early soil warm-up started emergence (in central Iowa) in mid-April,” he says.

It's in yankeeland

Palmer amaranth is well established in mid-South states like Arkansas and Tennessee. It’s also invaded states as far north as Minnesota, Iowa, and Michigan.

The good news is, Palmer is not yet adapted to conditions in more northern states like Iowa, says ISU’s Bob Hartzler. “It typically takes weeds 20 years to adapt to a new environment,” he says. “It should be easier to manage now than it will be in 15 to 20 years.”

Still, be wary, especially if you have problem fields with waves of waterhemp.

“Palmer can easily be overlooked because of waterhemp’s prevalence,” says Hartzler.

Equal Rights, Equal Debauchery

One thing you can say about Palmer amaranth: It’s not a member of the old boys’ club. Palmer amaranth is dioecious, meaning there are male and female plants. “That makes for lots of genetic variability,” says Bayer’s Mike Weber. This can make it easier to create biotypes that resist herbicides and enhance survival.

Know the Enemy

If you can identify Palmer amaranth, you’re one step closer to controlling it in your fields.

Unfortunately, it’s not easy, particularly in early-growth stages. Palmer amaranth closely resembles other pigweed species like waterhemp, particularly in the seed, seedling, and even vegetative stages. The following three factors help separate it from waterhemp.

Height. Palmer amaranth may grow up to 10 feet tall. Often, though, it reaches just 6- to 7-foot heights, says ISU’s Bob Hartzler. Still, Palmer amaranth often towers over waterhemp.

Seed Heads. Female Palmer amaranth has sharp bracts on its seed head that can extend up to 2 feet long or more.

“If you grab a sharp spiky seed head, it is Palmer amaranth,” says Hartzler. “Sometimes, waterhemp will look like Palmer, but it won’t have sharp bracts on its seed head.”

Watermark. Look for a white chevron- or V-shape watermark on Palmer leaves. This isn’t always a given, though. Palmer amaranth doesn’t always sport this watermark. Meanwhile, Hartzler has seen waterhemp sport a watermark on rare occasions.

Quick Grower

Palmer amaranth can grow up to 3 inches per day. Earlier in the season, though, it grows at a more manageable ½ inch per day, says ISU’s Bob Hartzler.

Rapid growth makes timely post-emergence herbicide applications crucial. Liberty’s label, for example, says a 29-ounce-per-acre rate of the chemical needs to be applied by the time Palmer amaranth reaches a 4-inch height. A two-day rainy spell when the weed is 4 inches high could render Liberty ineffective when a sprayer is next able to enter a field.

Rogue Resistance

“One weed we saw last year was glyphosate-resistant Palmer amaranth,” says Jayme Dick-Burkey, who farms with his wife’s family near Dorchester, Nebraska. “We will be working hard to control it.”

It’s especially scary when Palmer biotypes exist that resist multiple herbicide sites of action (SOA) like:

  • ALS inhibitors/Group 2 (Pursuit, Scepter)
  • Triazines/Group 5 (atrazine)
  • PPO inhibitors/Group 14 (Flexstar)
  • HPPD inhibitors/Group 27 (Balance Pro, Callisto)

So far, no biotypes have been found that resist the synthetic auxin/Group 4 SOA. These are the herbicide components for new dicamba-tolerant (soybeans) and 2,4-D-tolerant (corn and soybeans) systems. Palmer also does not yet resist glufosinate/Class 10 (Liberty).

As with other SOAs, though, repeated use will stifle effectiveness, says ISU’s Bob Hartzler.

Conservation Conundrum

Last July, ISU’s Bob Hartzler fielded a call from a chemical company representative who noticed a Madison County (Iowa) field that had been planted to pollinator habitat.

“He saw pigweed that didn’t look quite right,” says Hartzler. A closer examination revealed Palmer amaranth.

A week later, an Iowa crop consultant found it in a bird habitat seeding for waterways. Subsequent reports surfaced in CRP plantings.

“One landowner had Palmer amaranth that looked like cedar trees every 50 feet or so,” says Hartzler. Findings like these accelerated previous discoveries in Iowa to the point where Palmer amaranth now infests 49 of Iowa’s 99 counties.

This mimicked what happened in Ohio and Indiana in previous years. “One way it came into Indiana was through pollinator habitat seedings,” says Purdue University’s Bill Johnson.

So why the spread?

Lower corn and soybean prices compounded by excellent CRP bids spurred high demand for Iowa CRP plantings in 2016.

“Cash rental rates decreased, making CRP more appealing,” says Meaghan Anderson, ISU Extension field agronomist. The formula used to calculate CRP rates uses average cash rental rates from previous years, when grain prices were higher than in 2016.

“CRP bids looked really good to what rented ground was going for, especially pollinator habitat that had a signing incentive payment,” she says.


In a normal year, state seed producers were able to fill demand locally for seed production. Not so in 2016.

“To get all the grasses/forbs in it, they had to outsource and go to other states,” says Anderson. “Some out-of-state seed appears to have come from fields infested with Palmer amaranth.”

Wasn't the seed tested?

Sure. It’s just that some likely slipped past the goalie. “There was some flawed testing,” says ISU’s Bob Hartzler.

But here’s the deal. It’s difficult to clean Palmer amaranth out of the myriad native forb sizes and shapes found in these mixes. Even if a standard purity seed test found an Amaranthus species, it’s impossible to visually distinguish Palmer amaranth from the approximately 48 other such species.

Meanwhile, Palmer amaranth wasn’t even considered a noxious weed with enforcement protection in Iowa, he adds.

“So even if it was found, it wasn’t illegal to sell it,” says Hartzler.

Multiple Sources

Palmer amaranth arrives in multiple ways.


One way Palmer invaded Indiana was from dairies buying cottonseed originating from mid-South states. ‘We have advised farmers not to take manure from an animal operation that has used cottonseed as feed,” says Bill Johnson, Purdue University Extension weed specialist.

Palmer amaranth-infested hay also came in from Kansas and Oklahoma, he adds.

Harvest is an opportune time for Palmer amaranth to move, too. “Combines are great spreaders,” says Johnson. “Cleaning up and blowing out combines after harvesting infested fields can prevent the spread of Palmer amaranth.”

What we found

For this story, we submitted a weed check test for a wetlands mix (CP 23 Floodprone Mix) from a central Iowa landowner to the ISU seed testing laboratory that was labeled Palmer amaranth-free. The ISU purity test, which cost $125, showed this was, indeed, the case.

The test did find weed seeds, one of which – quackgrass – is a noxious weed in Iowa. It’s listed as Elymus repens on the following report. Other nonnoxious weeds found include:

  • Rumex crispus (curly dock)
  • Setaria pumila (yellow foxtail)
  • Setaria faberi (giant foxtail)

Noxious weed listing

Not having Palmer amaranth listed as a noxious weed is akin to a priest not talking about sin in his homilies. Yet, that’s the case in most states except Delaware, Minnesota, and Ohio. At press time, a bill had been introduced in the Iowa Legislature to declare Palmer amaranth a noxious weed in Iowa.

Palmer’s status as a noxious weed in Minnesota gave state agencies authority and resources to immediately respond when it surfaced in conservation seed mixes in 2016, says Bob Hartzler, ISU Extension.

“Minnesota is likely to reap large dividends by minimizing the future establishment, spread, and economic impact of the weed. It (listing it) would have saved a lot of trouble in Iowa.”

Conservation seeding mixes

If you’re thinking about seeding CRP, pollinator, or other conservation acres, ISU’s Bob Hartzler gives these guidelines about how to ensure palmer
amaranth isn’t part of the mix.

  • Purchase locally produced native seed and communicate with those producers.
  • Be wary of feed and bedding from infested states, such as Arkansas and Tennessee.

What do I do if I have it?

“Identified fields are the  tip of the iceberg,” ISU’s Meaghan Anderson says. “We will have some permanent infestations because of that.”

In CRP and pollinator mixes, mowing and hand-roguing are options. Neither is easy.

“Mowing is not always effective,” says ISU’s Bob Hartzler. “If you do mow, you need to mow it regularly at a 6- to 8-inch height. Some Palmer amaranth will survive, but it hopefully will reduce seed production in the field.”

Hand-roguing is more difficult. “In Crawford County, one landowner had eight people work for 11 hours to pull it and then put it in a barrel afterward and burn it,” says Hartzler.

Talk to federal agencies regarding management of Palmer amaranth in infested conservation seedings. Contract violation could occur if an implemented control program alters the vegetation type specified in the contract, Hartzler says.

Scrutinize Seed Testing Labs

In the future, ask where the seed is tested and check if it is a reputable testing lab, advises ISU Extension field agronomist Meaghan Anderson. “There is a lack of oversight in the industry,” she says.

There is some good news.

A DNA seed test is on tap for 2017. It has been developed by the California Department of Food and Agriculture and Eurofins BioDiagnostics. The Minnesota Department of Agriculture Plant Protection Division Seed Program also supported it. The DNA sequencing method differentiates Palmer amaranth from other Amaranthus weed species.

However, the DNA test will only be available on a limited basis for 2017. Besides the DNA test, seed producers may also use a grow-out method. In Illinois, the Illinois Crop Improvement Association uses a grow-out method to evaluate whether Amaranthus weed seeds are indeed Palmer amarant

Cropland Infestations

Equally disconcerting is the migration of Palmer amaranth from conservation seedings to cropland. That’s what farmers whose fields were contaminated by livestock feed or manure containing Palmer amaranth have experienced.

“In Iowa, we now have enough Palmer amaranth that no field is safe from invasion,” says ISU Extension weeds specialist Bob Hartzler.

In fields suspected or confirmed to have glyphosate-resistant Palmer amaranth, here’s what University of Illinois’ Aaron Hager advises:

  1. Plant a Liberty Link soybean into a weed-free seedbed either through tillage or through a preplant herbicide.
  2. Apply an effective soil-residual herbicide within 14 days of planting.
  3. Apply 32 ounces per acre of Liberty when Palmer amaranth is 4 inches or smaller.
  4. Scout the treated field seven days after application. If control is not complete or another flush has emerged, re-treat.

No to H.O.S

“Too many soybeans are managed under the H.O.S system – herbicide-only system,” says Bayer CropScience’s Mike Weber.

Cultural practices play a large role, too. They include:

  • Narrow soybean rows. “We have way too many 30-inch rows for soybeans,” says Weber. “Narrow rows (from 10 to 20 inches) enable soybeans to canopy quickly and prevent sunlight from getting to the weeds. One exception is a field with a white mold history.
  • A Managed seed bank. “Do not allow them to go to seed,” says Weber. Puling a stray Palmer amaranth plant here and there poking through the soybean canopy prior to harvest can save future headaches.
  • At harvest, some Australian farmers use seed collectors like the Harrington Seed Destructor. Such combine-mounted systems smash chaff and weed seeds as they exit the combine. Research shows it consistently destroys 95% of weed seed present in chaff, says Richard Zollinger, North Dakota State University Extension weed specialist.

Cover up with cover crops

Besides narrowing soybean rows, Nebraska farmer Jayme Dick-Burkey and his family have used multiple mixes of cover crops.  “We have found that cover crops have been effective at holding back weeds,” he says.

Bury the past

Farmers can also bury bad memories of a Palmer amaranth infestation by burying it via tillage. Granted, tillage can erase all the benefits that reduced tillage or no-till bring. If Palmer amaranth escapes, though, it’s a way to push the reset button and start from scratch with a Palmer amaranth management program.

In a University of Missouri study, no-till left 95% of pigweed seeds like Palmer amaranth and waterhemp on the surface. Meanwhile, deep tillage left just 18% of them on the soil surface. Deep tillage that buries seeds 4 to 6 inches deep ensures they won’t germinate. Once buried, seeds die after four to five years, says Kevin Bradley, University of Missouri Extension weed specialist.

Don't give up

Most of the world’s religions teach that, ultimately, good triumphs over evil. It’s the same way with Palmer amaranth.

It will take a mix of control measures, though. “We will not spray our way out of this,” says Mike Owen, ISU Extension weed specialist. “Look at alternative strategies of weed management.”

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