Although I am retired, I still am passionate about weeds and decided to continue my Labor Day Tour of the first known infestations of Palmer amaranth in Iowa. I started this ‘survey’ in 2014, links to earlier reports are found at the end of this article. My efforts do not involve a systematic sampling of Palmer amaranth infestations at these sites, but rather casual ‘hit and run’ observations. All but one of the sites involve less than 2 acres, so it is relatively easy to subjectively evaluate the prevalence of Palmer and observe changes in numbers from year to year.
Harrison County: This is the second straight year that I did not find any Palmer on the field edge or the adjacent roadside of the first field where Palmer was found in Iowa. However, the infestation in the alfalfa field across the road seemed greater than in 2020. A concern with this location is that transport of alfalfa could introduce seed to new areas. Grade: B
Pottawattamie County: In 2019 I was made aware of several ag fields inside an industrial area of Council Bluffs that had heavy Palmer infestations. This year the field I have followed had been left fallow and was dominated by annual weeds (the first step of old field succession). Marestail/horseweed was the most prevalent species, favored by its early germination, but Palmer amaranth, common ragweed and annual sunflower were holding their own. Ironically, marestail was suppressing Palmer amaranth better than the weed management program the previous two years since the Palmer population was lower this year than in 2019 and 2020. Grade: C-
Mills County: By happenstance, in 2019 I discovered a roadside infestation about ½ mile in length between I-29 and a large soybean processing facility. I suspect Palmer seed dropped from trucks bringing grain from Nebraska or other states with more significant Palmer amaranth infestations. The past two years there was a 6 ft wide strip of Palmer immediately adjacent to the road, but this year this area was solid annual grass weeds (foxtail, fall panicum, crabgrass). I suspect the county used multiple applications of a synthetic auxin herbicide (HG 4) to manage the Palmer. Grade: A+
Fremont County: There seemed to be more Palmer escapes in the managed perennial sod (regular mowing) adjacent to the small crop field where Palmer was initially found. However, there was a large reduction in plants across the road in an area that in previous years had a sweet corn patch, but this year was not planted and left ‘to the weeds’. Unlike Harrison County the area appeared to have been mowed regularly. In addition, I didn’t find Palmer in roadsides about ½ mile from the initial infestation where it had been in previous years. Grade: B
Page County: This site had some major construction over the summer, eliminating some of the areas where Palmer was found previously. However, there were small patches of Palmer growing amongst other native and naturalized weeds in unmanaged areas of the commercial ag site. Grade: C
Summary: As in previous years, I saw no evidence that Palmer amaranth is increasing in prevalence at these sites nor does it seem to be spreading. However, it continues to be frustrating to see the persons responsible for managing these areas aren’t going the ‘extra mile’ to prevent seed production and eradicate the newly introduced weed. They are missing the opportunity for ‘early detection and eradication’. Eventually Palmer amaranth will adapt to Iowa conditions and be much more difficult to manage.
Trip highlights: I always enjoy the opportunity to jump across the Missouri River and have lunch with my daughter, a zookeeper at the Henry Doorly Zoo in Omaha. But weed-wise, the highlight was finding several patches of Johnsongrass when driving north out of Hamburg. When I first came to Iowa, there were no known permanent infestations of this perennial weed in Iowa. Although infestations had been found, they usually died out after a couple years due to cold temperatures lethal to the rhizomes.