Is it Waterhemp or Palmer Amaranth?
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. Palmer amaranth is not indigenous to Illinois, but rather 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.
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. Seed production capability of female Palmer amaranth plants is similar to that of female waterhemp plants.
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 Palmer amaranth. Proper management of Palmer amaranth populations can help reduce the potential for successful seed production that will augment the soil seedbank and perpetuate the population in future growing seasons.
The cotyledon leaves of Palmer amaranth are relatively long compared with other Amaranthus species (Figure 1).
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 of Palmer amaranth (Figure 2). 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. As plants become older, they often assume a poinsettia-like appearance and sometimes have a white or purple chevron on the leaves (Figure 3). Leaves are attached to the stem by petioles that are usually longer than the leaf blade.
Accurate identification of weedy Amaranthus species during early vegetative stages can be difficult because many exhibit similar morphological characteristics (i.e., they look very much alike). 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 (Figures 4 and 5).
To assist weed management practitioners in accurately identifying Palmer amaranth, you may send us tissue samples from suspected Palmer amaranth plants and we will 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 can be found in the “Palmer Amaranth Identification” form that accompanies this article. Please download this 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.