The first ‘official’ finding of Palmer amaranth in Iowa was August 2013 in Harrison County. Shortly thereafter the weed was identified in Muscatine, Fremont and Page County. These infestations were likely due to Palmer amaranth seed being transported on equipment/trucks involved in crop production, although the Muscatine infestation may have been associated with a swine facility.
The heyday for Palmer amaranth introductions in Iowa was 2016 with the planting of native seed mixes contaminated with Palmer amaranth. I am aware of more than 30 counties where Palmer amaranth was found in CRP fields following planting of contaminated seed mixes. Fortunately, Palmer amaranth is poorly adapted to survival in perennial plantings and failed to permanently establish in these fields. It is unknown if Palmer amaranth in these plantings led to any permanent infestations in more suitable habitats.
A few new Palmer amaranth infestations have been reported each year since the big influx in 2016. A variety of vectors for the seed are suspected, including purchase of used farm equipment from states with established Palmer amaranth, import of cotton seed for dairy operations, and grain transport. Continued vigilance is needed to identify new infestations when they first appear.
Since 2014 I have followed the three original infestations in western Iowa. On a positive note, the size of the infestations has declined at all sites since they were first reported, and I haven’t seen evidence that Palmer amaranth has spread to adjacent fields. On the negative side, two of the three infestations were small enough that they should have been easy to eradicate, but steps to achieve this haven’t been adopted. Also on the bad side, since starting these tours I’ve found two new sites to visit. I visited these sites over Memorial Day weekend, and Palmer amaranth was easily found at three of the five sites.
What does the future hold for Palmer amaranth in Iowa? The failure of the weed to rapidly spread from the initial introductions should not lead to complacency. Weeds typically undergo a lag phase of approximately 20 years when introduced to a new region. During the lag phase the weed spreads at a relatively low rate, but evolves to be better adapted to local conditions. Another factor limiting the current spread of Palmer amaranth is that most crop fields are treated with herbicide programs designed to control waterhemp, making it difficult for Palmer amaranth to invade fields. Waterhemp thrives in spite of these programs due to its large, established seed bank, something Palmer amaranth lacks at this time.
Rebecca Baker, a recent grad student, compared emergence and growth characteristics of Palmer amaranth to waterhemp in central Iowa. In 2020 emergence of two Palmer amaranth populations was similar to waterhemp, whereas in 2019 Palmer amaranth required one to two weeks longer to reach 50% emergence (Figure 1). Palmer amaranth seed was more persistent than waterhemp when buried within the upper half inch of soil, but persistence of the two species was similar at a six-inch soil depth. At a six-inch depth more than 50% of seeds of both species remained viable one year after burial. The prolonged emergence and persistence of Palmer amaranth seed suggest that Palmer amaranth will pose a management challenge similar to waterhemp if allowed to become established.
Palmer amaranth is renowned for its higher growth rate and greater competitiveness with crops than waterhemp. Under Iowa conditions, Palmer amaranth did not consistently produce more biomass or seed than waterhemp in Rebecca's studies. Palmer amaranth is native to the southwest, and thrives in high temperatures. Average temperatures early in Iowa’s growing season are well below the optimum for Palmer amaranth, thus negating the advantage it has over waterhemp in states south of Iowa.
A second factor that likely suppresses Palmer amaranth growth is early flowering; Palmer amaranth initiated flowering about two weeks earlier than waterhemp (Figure 2). Research with other weed species has found that early flowering biotypes typically produce less biomass than later flowering biotypes. I suspect Palmer amaranth in Iowa will adapt quickly and biotypes with later flowering will be selected if the weed is allowed to persist.
The presence of existing infestations and constant influx of seed from a variety of sources suggest that Palmer amaranth will be a permanent component of Iowa’s flora. While research and casual observations indicate it is less adapted to Iowa conditions than waterhemp at this time, one trait contributing to the success of the weedy amaranths is their ability to rapidly adapt. It wasn’t that long ago when waterhemp wasn’t viewed as a weed threat in Iowa (history of waterhemp in Iowa).
Palmer amaranth has not lived up to the hype presented upon its arrival in Iowa. However, Palmer amaranth will undoubtedly adapt to our climate, and therefore will pose a larger challenge than it currently does. While it is unlikely to be eradicated, early detection and rapid response should make it relatively easy to prevent new detections from becoming permanently established in fields.