Short Version: 493 miles in 12 hours; 2 out of 3 known Palmer amaranth sites well managed; no new infestations found.
Long Version: I went to the three Palmer amaranth infestations in western Iowa that I am aware of. This is the third year I’ve visited the Harrison county site, and the second for the others.
Harrison County: Last year I was disappointed in that one of the infested fields had what I considered a complete control failure, and adjacent non-crop areas had significant Palmer amaranth populations. This year I didn’t see any Palmer amaranth in the crop fields or non-crop areas. While there were a few small plants producing seed along the road, I was very impressed with the level of control considering the infestation the previous two years.
Fremont County: This was the one disappointment of the day for two reasons. First, it is a very small field - less than 2 acres. The second is that the field is used for show plots for a seed company. It wouldn’t take much effort to adopt the zero threshold approach for Palmer amaranth management at this location. While there weren’t large numbers of Palmer amaranth present in the field, there were enough to sustain the population and act as a seed source to spread the weed to new fields. There also was a healthy population of Palmer amaranth in an adjacent ditch.
Fremont County Palmer amaranth infestation
Page County: The final stop of the day found a few escapes in the crop field and adjacent areas, but the Palmer amaranth population was down significantly compared to 2014. The pigweed population in the area was dominated by waterhemp rather than Palmer amaranth, whereas in 2014 the population was split equally between waterhemp and Palmer amaranth, if not skewed towards Palmer amaranth.
Summary: The 2015 tour reinforced my view that Palmer amaranth may not be completely adapted to Iowa’s soils and environment at this time. The ability of growers to stay ahead of the infestations indicates Palmer amaranth is unlikely to run rampant across the state in the next few years. Casual roadside surveys of nearby fields failed to spot any new infestations of Palmer amaranth. One thing probably working in our favor is that it often takes weeds several years to adapt to new environments, a period referred to as the lag phase. I don’t intend to downplay the need for vigilance regarding this new threat. Palmer amaranth is much more competitive than waterhemp, thus when/if it becomes more widely distributed in Iowa it will have a much bigger economic impact than our current number one weed problem. Palmer amaranth needs to be aggressively attacked at this time while it is at a disadvantage with waterhemp, other weeds, and the crop.