Federal programs designed to help farmers set aside more habitat for bees and wildlife have produced a dangerous offshoot this year: Some mixes used to plant native grasses and flowers were contaminated with harmful weed seeds that have now been introduced onto hundreds of farms in several Midwestern states, including Minnesota.

The weeds include Palmer amaranth, one of the most prolific and devastating weeds in the country for corn, soybeans and other row crops. It has been a scourge for cotton and soybean farmers in the South and has slowly but steadily started to move north into the nation's Corn Belt. The inadvertent planting of weed seeds on conservation land has greatly accelerated their spread, leading to the discovery of Palmer in Minnesota for the first time.

"It's probably the most significant agronomic weed that we've seen over the last 30 years," said Tony Cortilet, noxious weed program coordinator for the Minnesota Department of Agriculture.

A farmer in Yellow Medicine County in western Minnesota reported Palmer amaranth in his newly planted conservation land in September. Minnesota agriculture officials have since confirmed the weed in 30 plantings by 13 landowners in Yellow Medicine and Lyon counties — all of them planted with contaminated seed traced to the same company. Officials have not identified the company because the matter is under investigation.

The worry is not so much that Palmer is going to take over conservation land, Cortilet said, but that it's going to spread into nearby row crops.

There's reason for concern, said University of Minnesota Extension weed scientist Jeff Gunsolus, because a single female Palmer amaranth plant produces more than 250,000 seeds, grows to a height of 6 to 8 feet and has a woody stem thick enough to damage combine cutter bars and other farm equipment that try to mow it down.

"It's almost chain saw material; it's that strong," Gunsolus said. Left untreated, it has taken over fields in southern states in as little as three years.

To get a handle on the Minnesota infestation, crews working for the state agriculture department used propane torches at five sites in late November and early December to incinerate weeds. That in itself won't eradicate the weeds, Cortilet said, but it will at least prevent their small, shiny black seeds from falling into soil and sprouting into thousands of plants next spring.

"This is a window of opportunity for us to put boots on the ground and try to stop this thing from getting out of control and moving into the row crops," he said.

The aggressive annual weed species has been found in other states, including Illinois and Iowa, on conservation land planted in 2015 or 2016. The weed is native to the arid Southwest, and where it has spread into states such as Arkansas and Tennessee, it has caused significant crop yield losses and higher herbicide costs for farmers struggling to keep it under control.

Extension and state officials in Minnesota will spend the winter figuring out a plan to try to eradicate the Palmer amaranth at its other known locations in the state next spring, both before and after it sprouts. It's not a one-year job, Gunsolus said, because the weed seeds may germinate at any time during the growing season or even during the next year or two. Nevertheless, he is optimistic that there's a chance for success unless the contaminated seeds were also planted in other areas that have not been discovered or reported yet.

That's not the case in Iowa, where Palmer amaranth and other weed seeds have been inadvertently planted on thousands of acres, making eradication impossible. Bob Hartzler, weed scientist at Iowa State University Extension, said Palmer amaranth was discovered in five counties in Iowa in 2013, probably introduced by used farm equipment sales or animal bedding or feed from Palmer amaranth-infested states just to its south.

In 2016, the weed spread to 49 counties, he said, most of it because of a sharp rise in demand for native seeds after hundreds of farmers enrolled more than 100,000 acres in new conservation programs, especially to increase habitat for bees and other pollinators.

"Because of that huge increase and need, the local producers of those seeds couldn't meet the demand and they had to purchase certain species from out of state," Hartzler said. "That's how we believe the Palmer amaranth got into those native seed mixes."

The seeds that dealers purchased and mixed with other native seeds likely came from Kansas, Texas and other areas where Palmer amaranth has long been established, he said. Several of the Iowa counties now infected are just south of Minnesota, increasing the chances that the spread northward could be years sooner than expected.

The federal Farm Service Agency, part of the U.S. Department of Agriculture, administers the conservation programs. Its state executive director in Minnesota said he is aware of the weed contamination problem, but the agency does not inspect seed mixes that vendors sell and that farmers plant.

The agency reviews the tags on seed bags, but not the seed mix itself. "We have to trust the vendor that the seed tags are accurate," Grant Herfindahl said.

If there's a problem, Herfindahl said, it's the responsibility of individual states to investigate based on their particular laws about seed inspection and noxious weeds. In Minnesota, Palmer amaranth was added to the state's noxious weed list in 2014, and last month it was declared a "prohibited weed seed." The changed legal status gives Minnesota more regulatory authority than many states to find and fight the weed, and it establishes a zero-tolerance policy that prohibits its sale and transport in any seed mix.

Some of the contaminated seed mixes sold and planted in Minnesota, Iowa and Illinois have been sent to the University of Illinois, where associate professor of weed science Aaron Hager is growing them in a greenhouse to provide positive identification of Palmer amaranth and other weeds.

"On some bags the tag indicated that the weed seed percentage was 0.00," Hager said. "I think somebody missed something."

Hager said it's not clear who's at fault or whether anything illegal happened.

"I seriously doubt this is confined to one seed company," he said, and many of the seed businesses were probably not aware that they were buying and mixing seeds contaminated with Palmer amaranth.

"Nobody thinks the pollinator habitat program is a bad idea," Hager said. "But here's something unintentional in that program that if it's not addressed, and addressed soon, could turn out to be a huge issue that goes well beyond the well-being of the pollinators."

Hager said prompt action is needed to stem the flow of contaminated seed. That may be difficult, he said, because states have different laws and lists about which weeds are noxious. Federal officials also need to send warning letters to farmers enrolled in conservation programs, Hager said, and they need to provide technical help about how to manage weed-infested land, since planted conservation plots often have restrictions on herbicide use and other practices.

Cortilet said that in addition to efforts to eradicate Palmer amaranth at its known locations in Minnesota, he and others face another task. "Native seed mixes are very complex," he said. "We're trying to find a way to better sample them before they get sealed or before they even get mixed so that we can ensure their integrity."

Tom Meersman • 612-673-7388