Final Labor Day Palmer amaranth tour summary

September 6, 2016 2:00 PM
Blog Post

I took my final Labor Day Palmer amaranth tour on Saturday, visiting the first three known sites of Palmer amaranth in western Iowa. First stop was Harrison County, ground zero for Iowa Palmer amaranth. The Palmer was identified in the summer of 2013 in a 50 acre fallow field; because of the extensive infestation we believe the Palmer must have been introduced several years earlier. Since identification, the Palmer has been managed fairly well in this field while in soybean. Last year I was unable to detect any surviving Palmer in the field, but this year there were numerous small patches of Palmer present. However, when compared to waterhemp control in many surrounding fields the control wasn’t too bad. I was unable to see evidence of Palmer amaranth moving into adjacent fields, but roadside scouting isn’t real accurate.

Harrison County - small patch of Palmer amaranth escapes in soybean.

Second stop was in Fremont County. This is a unique infestation in that the Palmer is found in a small field within a small town. There were a few Palmer within the field used for show plots, but I also spotted Palmer in a few waste areas around town. 

Fremont County - Palmer growing in 'parking lot'

From there I swung through Page County and an infestation in a crop field and adjacent commercial storage area. Although I was able to find a few Palmer within the corn field, there seemed to be much less than three years ago when it was first found. There were a few Palmer present in the adjacent gravel area, but again, far fewer plants than when the Palmer was first identified in 2014.

The establishment of Palmer amaranth across the state in conservation plantings is disappointing, but it is not the end of the world. I was encouraged in that at the three sites I visited the quantity of Palmer amaranth was significantly less than when the weed was first identified. This shows that Palmer amaranth can be managed.

While I was happy to see that Palmer isn’t taking over the fields where it has been present for three or more years, it was disappointing in that two of the areas are so small that it wouldn’t take much effort to eliminate any surviving Palmer amaranth. Allowing these plants to survive will sustain the population at these areas and increase the risk of Palmer amaranth moving to new sites in the area.  

There is no doubt that Palmer amaranth can survive in Iowa, but I believe that at this time it is not well adapted to our soils and environment. This, and the fact that nearly all crop fields are being treated with a program targeting a pigweed species, will slow the rate that Palmer spreads in the state. This doesn’t mean we shouldn’t be concerned, but rather that through diligent scouting and targeted management the spread of Palmer amaranth across Iowa can be greatly impeded.

Odds and Ends
One of the benefits of spending 12 hours driving across Iowa is chance encounters with new/unusual weeds. I was very excited to encounter a weed I had never seen before, impressed myself that I knew it. Pitted morningglory usually has entire, not lobed leaves, but the cotyledons present on younger plants keyed me in. While driving through downtown Vail I spotted a flowering Japanese knotweet growing against a beautiful brick wall. And finally, I found a kochia about ready to snap off at the base. I was upset because I wanted to find a Russian thistle to use in an exam for my weed ID class, but failed to encounter any of those.

Pitted morningglory in Fremont County.

Japanese knotweed in Vail, Iowa.


Kochia getting ready to convert into a tumbleweed.