Wallaces Farmer

Yes, Palmer amaranth lurks in Iowa

Soybean Source: Identifying this pugnacious pigweed is the first step to manage it.

Rebecca Vittetoe

June 24, 2024

3 Min Read
Palmer amaranth plants
DENSE CANOPY: Palmer amaranth plants tend to have a denser canopy, which often looks similar to a poinsettia.Meaghan Anderson

Palmer amaranth received much press several years ago. While it seems like we haven’t heard as much about it recently, it doesn’t mean we shouldn’t keep our eyes open for this weed.

Just last year, it was identified in a crop field in Boone County in central Iowa. If a new infestation is found, it is still possible to prevent the establishment and spread of Palmer amaranth in a field. To do so, it must be identified early, and farmers must be diligent in controlling it. Just as with waterhemp, a single Palmer amaranth plant can be a prolific seed producer.


Several pigweed species exist across Iowa. They include redroot pigweed, smooth pigweed, Powell amaranth, spiny amaranth, waterhemp and Palmer amaranth.

Rebecca Vittetoe - Palmer amaranth leaves may have a silverish watermark

When trying to differentiate among these different species, look first for hairs or pubescence on the stems. Redroot pigweed, smooth pigweed, and Powell amaranth all have hairy stems.

Palmer amaranth, waterhemp and spiny amaranth have hairless stems. Spiny amaranth can easily be differentiated from waterhemp and Palmer amaranth by its sharp half-inch spines that are found where the leaves attach to the stem.

Differentiating between waterhemp and Palmer amaranth can be more challenging prior to flowering. In the vegetative stage, traits that differentiate between these species include:

  • Canopy shape. Palmer amaranth generally tends to have a denser canopy of leaves, similar to a poinsettia plant. Waterhemp has a more open canopy

  • Leaf shape. Palmer amaranth leaves tend to be wider and ovate-or diamond-shaped — compared to waterhemp leaves, which tend to be longer and narrower.

  • Leaf watermark. Some Palmer amaranth leaves may have a silverish watermark on them. but not all will. This trait can also rarely be found on some waterhemp leaves.

  • Leaf petiole. The petiole (part that connects the leaf blade to the stem) on Palmer amaranth will be longer than the leaf blade. This is one of the more reliable vegetative traits used to distinguish between Palmer amaranth and waterhemp.

When Palmer amaranth starts to flower (July and into August), it becomes easier to distinguish from waterhemp. Both species have separate male and female plants. If you rub the inflorescence (seedhead) of a plant and find small black seeds, it is a female plant. The male plants only produce pollen.

Meaghan Anderson - Palmer amaranth with long (more than 12 inch) terminal inflorescences

To differentiate between Palmer amaranth and waterhemp during flowering, Palmer amaranth tends to have thicker inflorescences and longer terminal branches. Female Palmer amaranth plants also have long (a quarter-inch in length), sharp, green bracts (modified leaves at the base of the flowers). Bracts on female waterhemp plants are only an eighth of an inch long and are not sharp to the touch.

Where to scout

When scouting for Palmer amaranth, target high-risk areas, which include farms that use livestock feed or bedding from Southern states, fields receiving manure from those farms and farms where out-of-state equipment has been used.

If you suspect that you find a plant that may be Palmer amaranth, reach out to your local Iowa State University Extension field agronomist at extension.iastate.edu/ag/crops

About the Author(s)

Rebecca Vittetoe

Rebecca Vittetoe is an Iowa State University Extension field agronomist in south-central and southeast Iowa. Her areas of expertise include agronomy, field crop production and management of corn and soybeans, Integrated Pest Management, and crop scouting. Vittetoe joined Iowa State University Extension and Outreach in 2015.

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