By Bob Hartzler, Department of Agronomy
Two new infestations of Palmer amaranth have been identified in Iowa since the initial finding in Harrison County in August. Another Harrison County field with Palmer amaranth was found, approximately 15 miles from the first area near Modale. The second area was not as extensive as the first, and there is a good likelihood of eradicating the weed in this area. There is a possibility that transport of materials from the Modale site is responsible for the newer infestation.
The newest infestation was recently identified in Muscatine County, near the Cedar River. The source of this infestation is unknown, but it may have been associated with swine feed since the patch appeared to have started near a compost pile serving a swine facility. While the weed has spread over several acres, the infestation is not extensive and with good management should be able to be contained.
Continued vigilance is required to identify new introductions of Palmer amaranth while there is a chance to prevent it from becoming a permanent component of the weed community. Now is a great time to spot these infestations since Palmer amaranth is in the reproductive stage. In addition to the large inflorescences and sharp bracts, Palmer amaranth plants are much less mature than waterhemp and still very green, whereas most waterhemp plants are beginning to dry down (Figure 1).
Figure 1. Inflorescences of Palmer amaranth (left) and waterhemp (right).
Bob Hartzler is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. He can be reached at firstname.lastname@example.org or 515-294-1923.