Is it waterhemp or Palmer amaranth? Though identification can be difficult, University of Illinois associate professor of weed science Aaron Hager said early and accurate identification of Palmer amaranth plants coupled with implementation of an integrated management program are essential to reduce the potential for crop yield loss due to interference of this broadleaf weed.
Accurate identification of weedy Amaranthus species during early vegetative stages can be difficult because many look very much alike, Hager explained. “During the 1990s, waterhemp provided an excellent example of how difficult it can be to differentiate among the various Amaranthus species, especially when plants are small,” he added.
To assist weed management practitioners in accurately identifying Palmer amaranth, Hager said researchers in the U of I crop sciences department will accept tissue samples from suspected Palmer amaranth plants and use tools of molecular biology to identify whether the sample is Palmer amaranth or another species of Amaranthus. Information on how to collect and submit tissue samples from suspected Palmer amaranth plants is described on the Palmer amaranth identification form, found here. Download the form, provide as much information as possible and submit it along with the tissue samples to the address listed at the top of the form.
“Proper management of Palmer amaranth populations can help reduce the potential for seed production that will augment the soil seedbank and perpetuate the population in future growing seasons,” he said.
Palmer amaranth (Amaranthus palmeri) is a summer annual broadleaf weed species taxonomically related to other pigweed species (waterhemp, smooth, redroot) common in Illinois agronomic cropping systems. Hager said Palmer amaranth is not indigenous to Illinois, but rather it evolved as a desert-dwelling species in the southwestern United States, including areas of the Sonoran Desert. “Genotypic and phenotypic adaptability have allowed Palmer amaranth to expand its distribution and colonize the vastly different agricultural landscapes across much of the eastern half of the United States, including Illinois,” he explained.
Research has demonstrated that Palmer amaranth has a higher growth rate and is more competitive than other pigweed species. “Growth rates approaching 3 inches per day and yield losses of 78% (soybean) and 91% (corn) attributed to Palmer amaranth interference have been reported in the scientific literature,” Hager said. “Seed production capability of female Palmer amaranth plants is similar to that of female waterhemp plants.”
Hager described the cotyledon leaves of Palmer amaranth as being relatively long compared with other Amaranthus species. “Like all weedy Amaranthus species in Illinois, the true leaves — those produced after the cotyledon leaves — of Palmer amaranth have a small notch in the tip. Occasionally, a single hair can be found in the leaf notch. This hair may not be present in each leaf notch of a Palmer amaranth plant, and tends to be less common on leaves of waterhemp plants. The stems and leaves have no or few hairs and the stems feel smooth to the touch. Leaves are alternate on the stem and are generally lance-shaped or egg-shaped with prominent white veins on the underside,” he said. “As plants become older, they often assume a poinsettia-like appearance and sometimes have a white or purple chevron on the leaves. Leaves are attached to the stem by petioles that are usually longer than the leaf blade.”