Now is a good time to scout for Palmer amaranth in Iowa 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 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 (over 500,000 seeds produced per plant is 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 farmers and its potential impact on Iowa agriculture. “Early identification is key to eradicating this weed from Iowa fields,” says Iowa State University Extension weed management specialist Bob Hartzler. Hartzler and ISU Extension field agronomist Meaghan Anderson offer the following information and recommendations to correctly identify and effectively control Palmer amaranth.
Eradication cannot happen without vigilance, early detection and appropriate response soon after it invades an area. Palmer amaranth is now 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 use 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 such as stems, petioles and leaves, while other common amaranth (pigweed) species have hair on stems 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 weeds often have a petiole that is longer than the leaf blade; this is the most reliable vegetative trait to differentiate the two species. This photo of a Palmer amaranth leaf shows this distinguishing trait.
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 male plants from female plants 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 plants are easy to distinguish from waterhemp due to their long, sharp bracts surrounding the flowers on tall terminal inflorescences. If you discover this weed in a field or anywhere, steps should be taken to remove all plants to prevent seed production.
“Continued vigilance is imperative to slow the speed with which Palmer amaranth invades our state,” Anderson says. If you observe a plant that you think may be Palmer amaranth, please don’t hesitate to contact Bob Hartzler at 515-294-1164 or firstname.lastname@example.org, or Meaghan Anderson at 319-331-0058 or email@example.com.