Invasive Palmer amaranth in half of Iowa counties

The planting of native grasses intended to help butterflies and other pollinators has inadvertently fueled the rapid spread of a weed that threatens to out compete corn and soybeans.

Contamination of conservation seed mixes with seed of Palmer amaranth has resulted in the spread of the aggressively invasive weed from just five of Iowa's 99 counties at the beginning of 2016 to 49 counties at the end of the year, according to Bob Hartzler, professor of agronomy and extension weed specialist at Iowa State University.

That collateral damage, he said, 'will have a significant economic impact for Iowa farmers.”

The weed, a relative of common pigweed and water hemp, is suspected to be in as many as 80 Iowa counties and could well be found in all Iowa counties by the end of 2017, Hartzler said.

'The surveillance got better with more people looking for it, but most of the rapid expansion was due to the widespread planting of conservation seed mixes contaminated with Palmer amaranth,” he said.

During 2016, 64,000 acres of pollinator habitat mix were planted in Iowa, which compares with 20,000 acres in Illinois and just 1,500 acres in Minnesota, according to Hartzler.

'We have so much Palmer amaranth in the state, there's no doubt it will establish and move,” Hartzler said.

Hartzler said he knows of no lawsuits that have been filed in Iowa to recover damages from the providers of the contaminated seed, which is believed to have been harvested and packaged in southern states, where Palmer amaranth is much more prevalent.

Palmer amaranth, he said, has not been classified as a noxious weed in Iowa so its inclusion in conservation seed mixes would not be illegal.

Like water hemp, a weed commonly found in Iowa, Palmer amaranth quickly evolves resistance to herbicides, as it has in southern states, to the detriment of cotton yields, Hartzler said.

While standard herbicide treatments such as glyphosate, the active ingredient in the popular Roundup herbicide, are still effective in killing Iowa-grown Palmer amaranth, that will likely soon change, he said.

Hartzler calls Palmer amaranth 'a better version of water hemp,” which he describes as 'the number one weed problem” for Iowa farmers.

Both rapidly evolve herbicide resistance, but Palmer amaranth is 'about twice as competitive” as water hemp, he said.

Able to grow several inches per day, Palmer amaranth can deprive nearby crops of sunlight and nutrients.

Unlike other highly-competitive weeds, such as horseweed and cocklebur, Palmer amaranth also produces inordinate amounts of seed - often more than 400,000 per plant, Hartzler said.

While most farmers can tolerate some water hemp in their soybeans without suffering yield losses, a similar density of Palmer amaranth would sharply reduce soybean yields, he said.

'Farmers are worried about it and talking about it, and it will take some planning and adjustment to deal with it,” said Jim Greif, a Prairieburg farmer who custom applies herbicide.

As Palmer amaranth becomes more prevalent in Iowa and more resistant to glyphosate, farmers will switch to other effective herbicides and to genetically modified seeds that are tolerant of those herbicides, Greif said.

Farmers' level of concern will depend on the percentage of the weed with herbicide resistance, according to Ed Anderson, research director for the Iowa Soybean Association.

'It's still early enough in the game for effective efforts to slow its spread,” he said.

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