Palmer amaranth

Editor’s note: The following was written by Meaghan Anderson, Iowa State University Extension field agronomist, and Bob Hartzler, professor of agronomy, for the university’s Integrated Crop Management News website Aug. 1.

Now is a great time to scout for Palmer amaranth in Iowa crop fields.

As of late 2018, this species had been identified in over half of Iowa’s 99 counties. While new identifications have waned since the widespread introductions in 2016, Palmer amaranth is a species to watch out for in virtually any Iowa crop field.

A native of the American southwest, Palmer amaranth is more competitive than common waterhemp, a pigweed native to Iowa. Both species are known for fast development of herbicide resistance, prolific seed production (more than 500,000 seeds per plant possible) and prolonged emergence.

The addition of Palmer amaranth to Iowa’s noxious weed law as of July 1, 2017, highlights the importance of this weed to Iowans and its potential impact on Iowa agriculture. Early identification is key to eradicating this weed from Iowa fields.

Eradication cannot happen without early detection and appropriate response soon after it invades an area. Palmer amaranth is reaching the growth stage where distinguishing it from waterhemp is easier due to the presence of flowers.

In addition to fields where Palmer amaranth has been found previously, other priority areas to scout include farms that utilize feed and bedding from southern states, fields receiving manure from those farms, and farms where out-of-state equipment has been used.

Palmer amaranth and waterhemp lack pubescence (hair) on plant parts like stems, petioles and leaves, while other common amaranth (pigweed) species have hair on stems and/or leaves.

Early in the growing season, Palmer amaranth is difficult to differentiate from waterhemp due to the high variability in both species. Leaves on Palmer amaranth often have a petiole that is longer than the leaf blade, this is the most reliable vegetative trait to differentiate the two species.

Leaves on Palmer amaranth are often clustered tightly at the top of the plant. People often observe Palmer amaranth as a denser-canopied weed as well.

Once they flower, Palmer amaranth and waterhemp produce male and female flowers on separate plants. Identifying males from females should be relatively simple due to the small, black seed produced by female flowers or the presence of pollen on male plants.

Female Palmer amaranth are easy to distinguish from waterhemp due to their long, sharp bracts surrounding the flowers on tall terminal inflorescences.

If you discover this weed, steps should be taken to remove all plants to prevent seed production.

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