Palmer amaranth was first identified in Iowa in Harrison County in 2013, shortly afterwards isolated infestations were found in Fremont and Page Counties. In subsequent years I visited these sites and made casual assessments as to whether the population was increasing or decreasing and looked for evidence it was spreading outside of the initial areas. My last tour was in 2021, and I thought I had moved on to other things. However, playing in the Iowa Senior Four Ball Golf Tournament in Council Bluffs on September 9-10 provided an opportunity to revisit the infestations after a five-year hiatus. The findings were as disappointing as my play in the golf tournament.
Harrison County
This was the most depressing stop on the tour. Palmer populations had been declining in the source field since the infestation was first identified in 2013. This year there were heavy populations in both the original field (corn) and the field across the road (soybean). More troubling was finding Palmer amaranth growing along a train track in Modale about two miles from the field. In prior tours, I have never observed the weed more than 200 yards from the original infested field.
Fremont County
This infestation was originally found in a two-acre field used for show plots. It was later found in a small community sweet corn patch across the road and in adjacent gravel storage areas. As in previous years the 'show plot' field was Palmer-free, but there were a few plants in the grass area surrounding the field. The sweet corn patch had been plowed under a few weeks prior to my visit, but there were a few plants in the storage area. Although the small number of plants was good news, it is disappointing that it would have taken less than an hour to pull the escaped plants and stop seed production. This has been a recurring theme at this site.
Page County
Palmer amaranth had been found on the grounds of a commercial ag site and the adjacent crop field more than a decade ago. The population had been continuously decreasing since I started following the infestation, and this year I did not find any plants in either area.
Mills County
In 2019 I came across a heavy infestation along the shoulder of a road between I-29 and a large soybean processing plant (approximately 1/2 mile). Palmer was also present in 2020, but in 2021 the area was Palmer-free. My hope was that the county was taking appropriate measures to eradicate the Palmer, but this year the shoulder was as heavily infested as when I first detected the infestation. I also spotted a few plants on the ramp to I-29. This area has always intrigued me since I've never seen waterhemp infest a similar habitat.
Pottawattamie County
An industrial/retail area of Council Bluffs has numerous small fields (<10 acre) maintained in ag production until being developed. We have followed two fields that had significant Palmer amaranth infestations. One of these was in soybean this year and continued to have a severe Palmer infestation. The other field had been left fallow since 2021 and this year I was unable to find any Palmer amaranth in the field or adjacent areas.
Summary
Sometimes life can be disappointing, and this year's Palmer amaranth tour was one of those times. My first stop was Page County where Palmer amaranth wasn't present, so I moved on to the other sites with optimism thinking that people were taking the threat of this new weed seriously. But things went downhill after that. While I don't think Palmer amaranth poses an immediate threat to crop production in Iowa, allowing it to persist will allow it to adapt to Iowa conditions. When this happens, the high competitiveness of Palmer amaranth will pose a much greater risk to corn and soybean yields than waterhemp. Now is the time to eradicate Palmer amaranth before it establishes a permanent seedbank.
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- Retired Weed Scientist and Professor of Agronomy