State investigators have tracked down the source of a harmful and invasive weed that was found growing for the first time in Minnesota last fall.

All of the contaminated seed mixes came from Green Valley Seed & Supply in Cottonwood, Minn., the state said. On its Facebook page, Green Valley says it is a small, family-owned company that specializes in the retail and wholesale distribution of lawn and farm seed, wildflower seed and Minnesota native prairie seed.

The company did not respond to requests for comment. It has agreed to $4,000 in fines and pledged to follow several corrective actions.

The investigation stems from finding Palmer amaranth, one of the most prolific and devastating weeds in the country for corn, soybeans and other row crops, at 30 planting sites owned by 13 landowners in Yellow Medicine and Lyon counties in western Minnesota.

The weed seeds were mixed with native grass and flower seeds and inadvertently planted on ­conservation land intended to produce habitat for bees and other pollinators.

"We determined that one of the blends in the [Green Valley] mix that was sold was black-eyed Susan, and it was labeled as being of Minnesota origin when in fact it came from Texas," said Geir Friisoe, director of the plant protection division for the Minnesota Department of Agriculture.

Texas has lots of Palmar amaranth, said Friisoe, and some of the weed seeds apparently contaminated the black-eyed Susan seeds, and then in turn were blended with other native Minnesota grass and flower seeds that were sold to farmers.

Friisoe said that Green Valley Seed violated four requirements of the Minnesota Seed Law. It did not have a current permit to sell seed in the state, mislabeled the origin of the seed, incorrectly labeled laboratory testing dates for components of the seed mix and did not keep samples on file for each lot of seed that it sold in the state.

Friisoe said Green Valley cooperated with the state in the investigation, enabling officials to find all 30 sites where the contaminated seed was planted. The worry is not so much that Palmer is going to take over conservation land, he said, but that it's going to spread into nearby row crops.

Minnesota has many reputable seed vendors, ­Friisoe said, but the incident with Green Valley Seed has increased concern that similar problems could occur in the future.

"We've put the seed industry on notice that we will be greatly increasing our surveillance of these particular types of seed mixes," he said.

If people have concerns or suspicions about seed mixes, they should notify the state Department of Agriculture, Friisoe said.

University of Minnesota weed specialists have said that a single female Palmer plant can produce more than 250,000 seeds, and can grow up to 8 feet tall with a woody stem thick enough to damage farm equipment that tries to mow it down. The weed has caused significant crop yield losses and higher ­herbicide costs for farmers in the South, and has gradually been spreading northward, to states including Illinois and Iowa.

Some of the Palmer plants found in Minnesota were incinerated late last year to destroy their seeds, and many of the 30 sites, which total about 175 acres, will be treated with herbicides this spring and summer.