Palmer amaranth tower above the soybean canopy
PROLIFIC PALMER: Long inflorescences on Palmer amaranth tower above the soybean canopy and produce lots of weed seed. One plant can produce up to 500,000 seeds.

What Bob learned on Palmer amaranth tour

ISU’s Bob Hartzler encourages farmers to stay vigilant in their efforts to control this invasive weed.

For the past four to five years, Bob Hartzler, Iowa State University Extension weed management specialist, has visited the first three known infestations of Palmer amaranth in western Iowa to track the status of the populations in these fields. This year on Sept. 3 he visited these fields in Harrison, Fremont and Page counties.

First stop was ground zero for Palmer in Iowa, a field near Modale in Harrison County. He encountered the first Palmer plant in the ditch about a third of a mile down the road from what is believed to be the source field. “The field was in corn for the first time since I’ve been visiting it, so it was difficult to compare the infestation severity to previous years,” he says. “However, there seemed to be fewer plants along the field edges than in earlier years. Unfortunately, there were a few good-sized plants in a ‘waste-area’ in a corner of the field. An area in perennial grass across the road that had a healthy Palmer infestation from 2011 to 2015 was Palmer-free.”

The Harrison County field is bordered by an Iowa Department of Natural Resources public hunting area on one side. “As I scouted that side of the field, I came across a tree with hundreds of roosting monarch butterflies that took flight as I drove by,” says Hartzler. “My photography skills failed to catch the scale of the monarchs leaving the tree. I did not find any Palmer in this area.”

BIG PALMER PLANT: Palmer amaranth is shown growing in a waste area in the corner of a field with no competition from crops or other weeds.

Second stop on the Palmer amaranth tour was Fremont County. This is an interesting site in that the infestation of this weed is inside a small town. Palmer was first found in a small field under 2 acres used for seed show plots, and the field has always has had a low number of escapes (less than 50 plants). “The benefit of having Palmer classified as a noxious weed was evident at this site,” says Hartzler. “The weed commissioner had contacted the seed company and informed them of the infestation in late August. The hand-pulled plants were lying in the grass areas adjacent to the field.

Unfortunately, there was evidence that Palmer has moved from the initial field. “A sweetcorn patch across the road had a significant infestation of Palmer, and I found a small patch — approximately 200 square feet — in a ditch about a third of a mile down the road. I had never noticed Palmer in that area before,” says Hartzler.

Zero tolerance for Palmer
Final stop was Page County. This infestation is in a crop field adjacent to an ag retailer. In previous tours, Hartzler found Palmer in both the crop field and on the grounds of the retailer. The amount of Palmer in both areas has declined over the course of his surveys. “This year I did not find any Palmer in the field, and only a single plant in the retail area,” says Hartzler. The Palmer infestation in the field had always primarily been in drainage ways where there was also a healthy waterhemp population. Over the four years, the percentage of Palmer in these areas compared to waterhemp has declined.

It has been encouraging to see that the Palmer amaranth populations are not “exploding” in these early infestations, notes Hartzler. However, it is disappointing that the land managers haven't adopted a zero threshold, he adds. “The Fremont County infestation is concerning because there is evidence that Palmer is spreading. I haven't observed that at the other locations. Hopefully, having Palmer amaranth classified as a noxious weed will provide the incentive for land managers to go the extra mile to stop the spread of Palmer amaranth.”

Add Osceola County to map
A farmer in Osceola County recently found a single Palmer amaranth plant in a soybean field. He removed and burned the offending plant. This brings the official ISU count of infested counties in Iowa to 50. “As we’ve stressed before, Palmer amaranth is likely in many more counties than reported to us,” says Hartzler.

The finding of this plant reinforces the importance of increased vigilance to find new infestations of Palmer amaranth, he says. Palmer amaranth is now widely established across the state, so it's likely new infestations will randomly appear in new fields at an increasing frequency. While pigweeds do not have inherent dispersal mechanisms, the abundant small seeds are easily transported by farming operations and wildlife. Every field in the state is at risk of being invaded. A new infestation in the opposite corner of Fremont County than the initial detection was also found in mid-August.

Late summer is probably the simplest time to locate Palmer amaranth in soybean fields, although it is preferable to identify the weeds earlier in the season. 

“The terminal inflorescences of Palmer amaranth are typically thicker and longer than those of waterhemp, thus plants can be identified from a considerable distance if a person is observant,” says Hartzler. “While it is likely some seed may have already shed, removing female plants in mid-to-late August and into September can minimize future populations and enhance the likelihood of eradicating the weed from individual fields. As Smoky said, ‘Only you can prevent the spread of Palmer!’”


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