Palmer amaranth threat intensifies

Palmer amaranth is getting close.

So close, it could already be in Western Canada.

The weed, a member of the pigweed family, is now established in North Dakota and it may soon move across the border into Manitoba or Saskatchewan.

Weed scientists in North Dakota are worried because palmer, which can reach a height of two metres or higher, is a nasty and aggressive weed.

“Palmer amaranth’s prolonged emergence period, rapid growth rate, prolific seed production, and propensity to evolve herbicide resistance quickly makes this the most pernicious, noxious, and serious weed threat that North Dakota farmers have ever faced,” said Rich Zollinger, retired North Dakota State University extension scientist for weed control.

Joe Ikley, Zollinger’s replacement at NDSU, didn’t use such hyperbolic language.

But he is concerned.

Palmer has been detected in four counties south of Fargo and in the town of Devils Lake, in the north-central part of the state.

Its spread to more counties in North Dakota is almost guaranteed because it can grow five to seven centimetres per day and a single plant can produce a million seeds.

Most problematic, palmer rapidly develops resistance to herbicides, making it almost impossible to control.

“It will eventually cross the border (into Canada),” Ikley said.

He is particularly worried about pulse crops in North Dakota.

Palmer and waterhemp — a weed that is flourishing in the state — could become very difficult to control in peas and lentils.

“They (the two weeds) are game changers…. They produce a lot of seed and can quickly get herbicide resistance,” he said. “We still have some options in corn and soybean. But if we look at our pulse crops … because of the resistance that palmer and waterhemp have, that will basically leave us with no in-crop (herbicide) options.”

Palmer is an emerging threat in North Dakota, but this summer waterhemp is the biggest weed problem in the state.

The southern half of the state has been wet and, on certain farms, waterhemp is out of control.

“There are some fields that are edge to edge waterhemp,” Ikley said, noting farmers are using three herbicide applications to kill waterhemp.

“And there are still plants going to seed.”

With waterhemp exploding in North Dakota, Manitoba’s challenges with the weed could soon get much worse.

Waterhemp first appeared in the province two years ago. It has now been confirmed in four Manitoba municipalities — three east of Winnipeg and one to the south.

Contaminated combines, other farm equipment and water can transport waterhemp and palmer seeds from field to field. But scientists have also found that geese and ducks are a vector as they eat seeds in one region and deposit the seeds in an entirely different region.

“It genuinely looks like maybe wildlife is one of the main contributors to spread, at least here in Manitoba,” Tammy Jones, Manitoba Agriculture weed specialist, said in a webinar on waterhemp. “A couple of the fields (with waterhemp) are in the middle of nowhere, with no water nearby.”

Jones and other weed experts are doing what they can to keep waterhemp in check. Under Manitoba’s Noxious Weed Act, tall waterhemp is designated as a Tier 1 noxious weed.

That designation requires landowners to destroy waterhemp, even if there is one plant in a field.

As well, farmers should clean machinery and equipment, like combines and quads, if it’s been used in a field with waterhemp.

“The legal expectation is it needs to be cleaned and all presence of that weed needs to be removed … before it (the equipment) is moved,” Jones said. “That’s pretty significant.”

Manitoba has designated tall waterhemp as a Tier 1 noxious weed for a number of reasons, including how quickly it develops resistances to herbicides.

Tests must be done, but Jones suspects the waterhemp in Manitoba is already resistant to Group 2, Group 5 and Group 9 (glyphosate) herbicides.

“It’s just a monster to try and control,” Jones said. “You’re looking at 45 to 60 bucks an acre to control this weed … in a crop production system.”

Manitoba Agriculture has created a tall waterhemp fact sheet, with pointers on identification and other information.

It can be found here.

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