The first confirmed infestations of Palmer amaranth were found in Iowa late in 2013. A difficult-to-control weed that has plagued farmers in the southern U.S. for years, it has recently migrated northward. "Infestations have been found in five Iowa counties so far, but we suspect there are more unknown infestations than known," observes Iowa State University Extension weed management specialist Bob Hartzler.
Virgil Schmitt, an ISU Extension field agronomist in southeast Iowa, and Hartzler visited an infestation in Muscatine County on May 9. There was an abundance of amaranthus seedlings present in the area that was infested with Palmer amaranth last fall. "We are fairly confident that these seedlings were Palmer amaranth because there was little or no waterhemp or other pigweed species present in the field last fall," says Hartzler. "However, I am not convinced that it is possible to distinguish the two species while they are in the seedling stage."
Visit ISU weed website to learn more about Palmer amaranth
While plants are in the vegetative stage, the long petioles on Palmer amaranth are probably the best trait use to differentiate Palmer amaranth and waterhemp, says Hartzler. Palmer amaranth leaves frequently are wider in relation to their length compared to waterhemp. Palmer amaranth also tends to have a 'bushier' growth habit, whereas waterhemp is leggy and has an open canopy. Another characteristic of Palmer amaranth is its very rapid growth rate.
"A poster describing the two weeds is available for downloading on the ISU weed science website and we have a limited supply of printed copies of the poster available," Hartzler adds. "However, there are a number of other articles on the ISU weed science website providing information about identification and management of Palmer amaranth."
Early detection of new weed infestations is critical
The simplest way to manage a weed is to prevent it from getting established. "While it is unlikely that all of the current Palmer amaranth infestations will be eradicated, the rate that Palmer amaranth spreads across the state can be reduced," notes Hartzler. "Early detection of new infestations is critical; unfortunately, the similarities between Palmer amaranth and waterhemp complicate this process."
He is advising farmers and crop consultants to closely monitor fields for any 'pigweeds' that behave unusually. Fortunately, it is relatively easy to differentiate the two species once they begin to flower. "We at ISU would appreciate being contacted if any fields are found with suspected Palmer amaranth in Iowa," says Hartzler. He can be reached at [email protected] or 515-294-1923.
Image 1. This is Palmer amaranth in the two-leaf stage of growth.
Image 2. This group of Palmer amaranth seedlings was found in an Iowa field this spring.