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Invasive weed creeps northwest from cotton country

Palmer amaranth can shoot up as high as 7 feet, and just one plant can produce up to a million seeds. Photo courtesy of Richard Old.

Palmer amaranth can shoot up as high as 7 feet, and just one plant can produce up to a million seeds. Photo courtesy of Richard Old.

A weed that can cause deep losses in corn and soybean yields is making its way from the South into the Midwest.

Palmer amaranth can shoot up as high as 7 feet, and just one plant can produce up to a million seeds. Herbicide is increasingly futile against it, and the weed’s thick stems and deep roots make it hard work to clear by hand. It can slash yields and profits when it gets out of control.

Midwestern weed scientists are sounding the alarm because the destructive plant recently turned up in Iowa.

“This is not just a nuisance. This is a game-changer,” warned Purdue University weed scientist Bill Johnson, whose state has well-established pockets of the plant.

Cotton growers in the South already spend about $100 million a year to try to keep it out of their fields, University of Georgia scientist Stanley Culpepper said.

“This is a crop robber,” said W.C. Grimes, who farms 1,600 acres of cotton, peanuts and corn near Twin City in eastern Georgia. “It will steal your profit. It will choke your cotton out, and anything else you’re trying to grow.”

Grimes said he was losing up to 200 pounds of cotton per acre until farmers learned the key to overcoming Palmer amaranth’s resistance to glyphosate — sold under brand names like Roundup — was to continuously change herbicides.

One thing that makes Palmer amaranth so much tougher than other weeds is that one plant can produce 500,000 to 1 million seeds. A combine can scatter seeds from a couple plants across an entire field, Johnson said.

“I think it would do quite well in Idaho, particularly in irrigated annual crops,” said Prof. Richard Old, a consultant who has been identifying weeds for the University of Idaho and Washington State University for decades.

And there’s little to stop the weed from its westward march.

“Weeds like this tend to travel as contaminants of crops, crop seed, with animals and animal feed, as well as on machinery,” Old said. “Our level of detection of new species is extremely poor, and this species is superficially similar to some we already have, which makes detection even more difficult.”

Palmer amaranth probably took root in Kendell Culp’s fields near Rensselaer in northwestern Indiana last year, but he wasn’t aware of it until a seed salesman spotted it this summer. Culp pulled it up by hand — filling a pickup truck bed from one spot and a half load from another.

“Unfortunately I think it’s going to be a pretty difficult weed to control for us,” Culp said. He’s working with a consultant on strategies for deploying herbicides on his 1,750 acres of corn, soybean and wheat.

The infestation found this August in two western Iowa soybean fields probably got there by truck, Iowa State University weed scientist Bob Hartzler said.

Despite those fields being adjacent to a stretch of flood plain with poor soil where sludge from a Nebraska company has been spread as fertilizer, he said there’s no reason to think the sludge contained Palmer amaranth seeds. His suspicion is that the seeds were stuck in mud on trucks that hauled the sludge.

Given the weed’s resistance to glyphosate, which is typically applied after weeds sprout, farmers need pre-emergent herbicides to kill the weed earlier in its growing cycle. Those have a much narrower window of effective application time.

Palmer amaranth likes long growing seasons and hot, sunny weather, Culpepper said, so it may not be quite as aggressive in colder states. However, he said it’s still going to be “the baddest boy on the block.”


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