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.

Identification

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

WATCH FOR WATERMARK: Palmer amaranth leaves may have a silverish watermark. (Rebecca Vittetoe)

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:

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.

LONG INFLORESCENCE: Palmer amaranth tends to have a thicker inflorescence, with long (more than 12-inch) terminal inflorescences. Additionally, female Palmer amaranth plants have longer and sharper bracts compared to those on female waterhemp plants. (Meaghan Anderson)

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.