By Bob Hartzler and Mike Owen, Department of Agronomy
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 we suspect it may be here, or will appear in the near future (see April 24 ICM article).
Identifying infestations of Palmer amaranth when they first get started is the key to preventing its spread in Iowa. 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. Thus, we do not recommend this as a trait for differentiating the two species.
Palmer amaranth 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. In most cases, digital images will be insufficient to differentiate vegetative Palmer amaranth and waterhemp. Contact us via e-mail 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 firstname.lastname@example.org or 515-294-1164. Owen can be reached at email@example.com or 515-294-5936.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 4, 2013. The information contained within may not be the most current and accurate depending on when it is accessed.