Numerous weedy species in the pigweed family (Amaranthaceae) are found across Iowa, including waterhemp, redroot pigweed, smooth pigweed, Powell amaranth and others. At this time, Palmer amaranth has not been confirmed in the state but because of its presence in surrounding states it is suspected it may be here, or will appear in the near future (see April 24 ICM article). That's how Iowa State University Extension weed management specialists Bob Hartzler and Mike Owen sum up the situation.
Identifying infestations of Palmer amaranth when they first get started is the key to preventing its spread in Iowa, says Hartzler. It is difficult to differentiate vegetative plants of waterhemp and Palmer amaranth. Both have glabrous (hairless) stems and both species have variable leaf shapes.
Purdue University recently published an article and video providing information on differentiating Palmer amaranth and waterhemp. They state that Palmer amaranth frequently has a single hair in the notch found at the leaf tip, and that this trait is a reliable way to differentiate the two species. "We have examined waterhemp plants and photos of waterhemp, and find that this hair is commonly present on waterhemp in Iowa," says Hartzler. "Thus, we do not recommend this as a trait for differentiating the two species."
Early and correct identification of Palmer amaranth is essential in reducing crop yield loss
Palmer waterhemp frequently (but not always) produces leaves with a petiole much longer than the leaf blade. This probably is one of the most consistent vegetative traits for separating the two species but it also is variable. Plants with inflorescences present are best to identify and confirm the Amaranthus species.
"We are willing to aid in identifying any plants suspected of being Palmer amaranth," says Hartzler. "In most cases, digital images will be insufficient to differentiate vegetative Palmer amaranth and waterhemp. "We want people to contact us via email or phone to determine how to proceed in confirming the identification of any suspected Palmer amaranth."
Bob Hartzler and Micheal Owen are professors of agronomy and weed science Extension specialists with responsibilities in weed management and herbicide use. Hartzler can be reached at [email protected] or 515-294-1164. Owen can be reached at [email protected] or 515-294-5936.