Posted on February 27, 2014 at 12:09 PM by Iowa Corn
Palmer amaranth, an aggressive and invasive weed that is native to the southwestern United States, has been expanding its range for decades.
Bob Hartzler, Iowa State University Department of Agronomy, reports that the first confirmed finding of Palmer amaranth in Iowa was near Modale in Harrison County last August. It appeared the Palmer amaranth was introduced in two fields where sludge has been repeatedly applied due to the soils being unsuitable for crop production. The sludge was imported from Nebraska, but it does not appear to be a likely source of weed seed. We suspect the seed came as a hitchhiker on trucks bringing the sludge into Iowa. It is likely the Palmer has been present at this site for several years, and it has spread to several adjacent fields.
A second, much smaller infestation was later found approximately 40 miles from the initial site. While it probably is too late to eradicate the Palmer at the Modale site, significant efforts are being made to contain the infestation.
|Known infestations of Palmer amaranth. Feb. 2014.
Muscatine County was the next confirmed Palmer amaranth infestation. The field is on a sandy soil in the flood plain of the Cedar River. The likely source for Palmer amaranth at this site was swine feed. The infestation appears to be limited to a single field and the adjacent ground. The farmer is taking the problem seriously and there is a likelihood of eradicating the weed from this location.
Two counties in the southwest corner of Iowa (Fremont and Page) were the next findings, both adjacent to commercial grain elevators. The likely source of Palmer amaranth at these sites is grain trucks that have been to areas in Nebraska or Missouri with Palmer amaranth.
The final report of Palmer amaranth in Iowa was received late in 2013 from a farmer in Davis County. He reported that the operation brings in cotton seed as a feed supplement for a cattle operation and believes this is where the Palmer amaranth originated.
Due to long-distance movement of equipment, grain and other agricultural materials, it is inevitable that new infestations of Palmer amaranth will be discovered.
Knowing how to identify Palmer amaranth and keeping an eye out for ‘odd pigweeds’ is the best tool to limit the rate of spread of Palmer amaranth.
Waterhemp is often mistaken for Palmer amaranth. The most reliable characteristic to differentiate the species is the size of the bracts at the base of flowers. Bracts are modified leaves, and on Amaranthus species they are narrow, triangular and ending in a sharp point. The bracts of Palmer amaranth are up to ¼ inch in length (3 to 7 mm) and extend far beyond the tepals (modified petals and sepals of Amaranthus flowers). On female plants, the bracts become stiff as they mature and are painful to the touch. The bracts on waterhemp are less than 3 mm in length and rarely longer than the tepals.
Contact ISU Extension and Outreach if you find Palmer amaranth. They’d like to hear from you.