The U.S. ‘No. 1 weed problem’ heads north

Other weeds are higher priority for now, but be on the lookout for Palmer amaranth

It can grow two to three inches per day and reach eight feet tall. Each plant can produce more than a million seeds that can emerge right through the growing season. It has reduced yield up to 91 per cent in corn and 79 per cent in soybeans. It’s resistant to glyphosate and multiple other herbicide modes of action. It’s forced some U.S. cotton growers to resort to hand weeding at $150 per acre.

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

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And it’s heading north, almost within sight of the Canada-U.S. border.

While Canadian weed specialists say Palmer amaranth may not thrive here as well as it does in the southern U.S., it’s clearly worth monitoring its spread, and if possible, stopping its march northward.

Palmer amaranth has been found in several North Dakota counties, says Brian Jenks, a North Dakota State University (NDSU) extension weed scientist.

The first case was reported in McIntosh County in August when a farmer out hand-weeding kochia happened to find two plants that looked unusual. Less than a week later, another plant was found a few miles away. Since then, samples have been found in other counties in the central and southern parts of the state, says Jenks.

“We haven’t received confirmation from the lab but I’m 100 per cent sure that it’s Palmer,” he says.

It was only a matter of time until Palmer amaranth made it into the northern states. But so far no cases have been reported north of the border, and Manitoba weed specialists caution that other weeds are a higher priority for the time being.

“While we do need to be aware of it and concerned about it, it isn’t currently as adapted to our area as it is farther south,” says Tammy Jones, weed specialist for Manitoba Agriculture. “We’ve seen it creeping north but we haven’t seen it move as fast as some people expected.”


Palmer amaranth, a type of pigweed, is fast-growing and highly adaptable. Populations are resistant to at least five modes of action in the U.S., including glyphosate, says Rob Gulden, University of Manitoba weed scientist.

Gulden says that if Palmer amaranth is growing in North Dakota it’s already adapted to Canada’s cooler temperatures. “It seems to be surviving in that climate, so it might not be quite as competitive, relatively speaking, but judging by the pictures (circulating online) it’s going to be formidable weed to contend with,” he says.

“The real question in terms of what that means on the adaption side is how early it produces viable seed,” he adds. “If it takes well into the fall after most crops are off to produce seed, there’s a way to manage it. On the other hand, if it produces seed early, that complicates management.

“It’s relatively new to the northern states so how it behaves is unknown — it will take a few seasons to understand it,” Gulden says.

While he agrees that other weed species such as waterhemp, Powell amaranth (also known as green pigweed) and glyphosate-resistant kochia should be higher on producers’ radar, he says they should be aware of what’s happening south of the border.

Scouting is key for all of these weed species, and while most pigweeds “stick out like a sore thumb” late in the season because of their relative height, producers should look out for any plants that look unusual, intensifying scouting after herbicide application to see what wasn’t killed. “It’s not a fair assumption that the herbicide kills everything,” he warns.

Producers who find unusual weeds can reach out to Gulden, Jones or any Manitoba weed supervisor to help with identification.

“The amaranth species look fairly similar as seedlings. Nobody wants to wait until they’re in seed to figure that out. When in doubt, ask an agronomist,” says Jones.

Palmer amaranth and tall waterhemp are both Tier 1 noxious weeds in Manitoba, meaning they are considered serious threats and must be destroyed without conditions.

Control options

The dwindling number of chemical control options in the U.S. is one reason it’s important to be vigilant in scouting for Palmer amaranth north of the border.

Jenks says his focus in North Dakota is on identification. “We’re not even talking control yet. We’ll talk control this winter to those counties where we’ve found it,” he says.

The North Dakota samples were all found in soybean fields, says Jenks, and because they were found late in the year it’s possible that they emerged after herbicide applications.

Any Palmer amaranth population that arrives in Western Canada will likely already have resistance built in, says Gulden, pointing to the fact that when waterhemp entered Ontario it was already resistant to glyphosate.

Gulden says that since Palmer amaranth isn’t yet a weed of concern to Canadian producers, it isn’t on many herbicide labels, although products that tackle redroot pigweed would likely be somewhat effective against Palmer.

Pre-emergents could be useful if they broaden the number of modes of action used against the weed, but Gulden stresses that since Palmer amaranth can emerge at any point during the growing season, pre-emergents could have limited impact.

In other words, producers should be thinking about integrated weed management when it comes to weeds like Palmer amaranth and manage their crops as competitively as possible, says Gulden.

Palmer could set foot in any crop, he says, but Roundup Ready soybeans are automatically vulnerable because they’re a slow-to-start crop typically grown in wide rows and most new weed threats have been glyphosate resistant.

“Wide rows aren’t a good idea if there’s no inter-row tillage, because that forces the herbicide to do all the work. Set up the crop to compete as well as possible,” he says.

In the case of Palmer amaranth, an ounce of prevention is worth much more than a pound of cure. The seed can spread by wildlife via contaminated cattle feed or on equipment.

Producers should be on the lookout for Palmer amaranth in any seed imported from the U.S., although because the seeds are tiny and look like other pigweed seeds only a lab test can make a positive identification.

Gulden says it’s absolutely critical to ensure machinery moving across the border is clean when it arrives and then is cleaned again in the farmyard before it goes out to the field.

Jones adds that producers shouldn’t be afraid to talk about herbicide-resistant weeds they’ve found in their fields.

“I’ve been at meetings where, when you ask people if they have herbicide-resistance weeds they don’t really want to talk about it, but it’s so much better if they do talk about it so that we can be more effective in helping control it,” Jones says. “Knowledge is power.”

About the author


Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]



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