Chasing Palmer: Minnesota tries to stop high-cost invasive weed

On a steamy morning earlier this month, Shane Blair walked methodically through 25 acres of waist-high grass on a restored prairie in west-central Minnesota.

He criss-crossed the field along an invisible grid just a few feet wide, searching for the telltale leaves of one of the most-wanted weeds in the state.

It’s tedious work: The plants are nearly identical to the common pigweed, or water hemp, two plants that are ubiquitous from fields to gardens across the state. And, this time of year, the plants grow close to the ground.

“There might be a Palmer amaranth plant that’s 8 inches tall. We’re not going to see it until we get a few feet from it,” Blair said.

Blair is Minnesota’s lead Palmer amaranth detective — officially, a noxious weed eradication specialist with the Minnesota Department of Agriculture — and he’s spending his summer on fields like this one, in Douglas County, on the front lines of the state’s search-and-destroy mission for the invasive plant that, if it takes hold, has the potential to wreak havoc with Minnesota’s commodity crops like corn and soybeans.

This field was taken out of crop production and planted with native prairie seeds three years ago. But the new landowner had no idea that what they’d planted had Palmer amaranth seeds mixed in.

The Agriculture Department found out the seed mix was contaminated when another landowner reported Palmer amaranth sprouting in a new planting. That prompted the department to survey all the fields in the state where that seed mix had been planted. They hit pay dirt on this plot.

"We're talking, like, 80 plants just from here to that wood line” about a quarter mile away, said Blair. "That may not sound like a lot, but one female [Palmer amaranth plant] can produce up to a half-million seeds. So, 80 times a half-million, that's a lot of seed."

The Palmer amaranth plants the Agriculture Department’s team found here were quickly burned using propane torches, before they could go to seed. So far, Blair has found no sign of the invasive plant. He’ll be back in about a month to walk this field again.

“Once we haven't found Palmer, we'll survey it for three consecutive years,” he said. “And if all three of those years come up with a negative — no Palmer — then we have essentially done our part and it's up to the landowner to continue."

Palmer amaranth is on the Minnesota Noxious Weed List as an “eradicate” weed. That legal status means the plant must be destroyed and that no transportation, propagation or sale of this plant species is allowed.

Most of the Palmer amaranth in Minnesota so far has been found in prairie plantings like this on conservation land planted with contaminated seed mixes.

The state department is taking these cases so seriously because so far the plant hasn’t yet spread to corn or soybean fields. Palmer amaranth can grow up to 8 feet tall, and spreads rapidly if it’s allowed to mature long enough to disperse its seed. It destroys crops by sheer growth, crowding out everything else in a field.

"Once that happens, then it's a whole different ball game,” said Anthony Cortilet, who leads the department’s noxious weed program. “Corn and soybeans, even small grains, can't compete with that, so as a farmer, your only option when you get these big populations established is to till it under and then start over."

Palmer amaranth hasn’t yet taken hold in Minnesota, but it’s caused crop losses as high as 80 percent in southern states like Arkansas and Nebraska by growing so thickly it chokes out the crops. And it's often resistant to common herbicides.

Should it make its way into the state’s crop fields, Cortilet said, the potential economic losses for Minnesota farmers could be in the millions of dollars.

“That's the stakes, and that's huge,” he said. “I don't know of any other pest right now that you could say that about."

Minnesota is taking a more aggressive approach to Palmer amaranth prevention than neighboring states, he said, but states are also collaborating to prevent the weed’s spread through seed or livestock feed.

"Are we going to keep it all out? No, this is constantly going to keep coming into the state because it's the way ag works, it works across borders," said Cortilet. "So, we're not foolish to believe that you know we're going to win the battle where we're gonna eradicate Palmer completely."

But Cortilet by taking a zero-tolerance approach to the weed, he said, the Agriculture Department is sending a message to farmers that this is an important threat to their livelihood, and it’s important for them to stay vigilant against it. And if they can slow the spread, it buys time to develop a better method for treating infestations in farm fields. So far, the best method for dealing with large outbreaks of Palmer amaranth is to till it under, and the crop along with it.

It's been nearly three years since Palmer amaranth was first found in Minnesota. And it's starting to get more attention.

"We're getting more calls,” Blair said. ”People are saying, 'Hey, I've got this weed. I think it might be Palmer.' Nine times out of 10, it's probably not Palmer amaranth, but it is a pigweed."

Because the plant looks so much like pigweed, which is ubiquitous in fields and gardens, Ag Department specialists send samples of any suspect plant for genetic testing at labs including the National Agricultural Genotyping Center in Fargo, N.D.

Because the invasive weed is difficult to identify by sight, the lab is seeing an increase in samples thought to be Palmer amaranth, said Megan O'Neil, who manages the lab.

"Using the DNA, it's an objective test rather than a subjective identification, so it takes a lot of the gray area out of it," said O'Neil. "It's important they can identify these really prolific weeds so they can save the farmland."

The lab is designed to quickly process large quantities of genetic samples. A quick turnaround is important because depending on how mature the plants are when they’re found, landowners might have a limited window of opportunity to try to control the rapidly growing plant.

Lab testing can confirm what Shane Blair sees when he’s searching for Palmer amaranth in fields around the state, but he’ll still walk many miles this summer and fall.

He has at least 70 sites across the state, covering hundreds of acres, to survey in hopes of finding any Palmer amaranth plants before they have a chance to spread hundreds of thousands of seeds across the landscape.

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