Amaranthus species are among the most troublesome weed species in agronomic production systems. In corn and soybean they can cause yield loss and have a propensity to evolve resistance to various herbicides. Several Amaranthus species are regarded as weedy pests across the Great Plains, including the monoecious species (male and female flowers on the same plant) redroot pigweed (A. retroflexus), smooth pigweed (A. hybridus), Powell amaranth (A. powellii), tumble pigweed (A. albus), prostrate pigweed (A. blitoides), and spiny amaranth (A. spinosus), and the dioecious species (separate male and female plants) common waterhemp (A. tuberculatus) and Palmer amaranth (A. palmeri). Among these, smooth pigweed, redroot pigweed, Powell amaranth, Palmer amaranth, and waterhemp are most common in Illinois corn and soybean fields.
Palmer amaranth is perhaps the most "aggressive" Amaranthus species with respect to growth rate and competitive ability. It is most common in the southern third of Illinois but, similar to waterhemp's expansion during the 1990s, Palmer amaranth may be expanding its range northward. Its growth rate and competitive ability exceed that of other Amaranthus species. Horak and Loughin (2000) conducted a two-year field experiment to compare several growth parameters of Palmer amaranth, waterhemp, and redroot pigweed. Their research demonstrated that Palmer amaranth had the highest values for plant volume, dry weight, and leaf area of all species, as well as the largest rate of height increase. Klingaman and Oliver (1994) found that soybean seed yield was reduced between 17% and 68% from Palmer amaranth interference at densities between 0.33 and 10 plants per meter of crop row. Biotypes of Palmer amaranth resistant to glyphosate, triazines, dinitroanilines, or ALS-inhibiting herbicides are known to exist in the United States.
Recently a glyphosate-resistant population of Palmer amaranth was reported in southern Illinois. Its occurrence in Illinois (and its widespread existence in other states, such as Arkansas, Tennessee, and Georgia) should raise awareness among Illinois farmers because this example provides additional evidence that resistance to glyphosate can occur in a summer annual weed species that is very competitive with corn and soybean and because Palmer amaranth occurs in many fields in the southern portions of Illinois.
Accurate identification of weedy Amaranthus species during early vegetative stages can be difficult because many exhibit similar morphological characteristics (they look very much alike). Hybridization between the species creates additional difficulty in identification. 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. The following descriptions are provided to help in identifying Palmer amaranth.
While many people tend to identify weeds based on the look of the entire plant, more accurate identification can be achieved by examining parts of the flowers. Historically, taxonomic separation of Amaranthus species has been based on differences in floral characteristics, but new methods utilizing molecular biology techniques also are being employed.We will restrict the discussion to floral characteristics.
We'll first define some of the terms to be used, starting with the outer parts of a flower and working inward to the seed.
Inflorescence—flowers collectively. While many people associate the term flower with colorful plants growing outside the house, we use it to refer to the reproductive structures of a plant. Male flowers produce pollen, while female flowers produce seed.
Bract—a modified leaf associated with flowers. It differs from foliage leaves in shape, color, size, texture, or some other feature.
Tepal—leaflike scales that encircle the outer flower parts. Some people refer to these structures as sepals when describing Amaranthus species flowers. When you brush the inflorescence of a mature pigweed plant against your palm, the tan-colored structures that fall into it are the tepals.
Utricle—a membranous bladderlike sac enclosing an ovary or fruit (seed). The utricle is contained with the tepals, and the seed is enclosed by the utricle.
Seed—the unit of reproduction. Seeds of Amaranthus species typically are small, hard, black, and glossy.
The cotyledon leaves of Palmer amaranth are relatively long compared with other Amaranthus species. As with 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. The stems and leaves have no hairs or just a few, resulting in a smooth feel. Leaves 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 often have a white V-shaped mark on the leaves. Leaves are attached to the stem by petioles that are usually as long as or longer than the leaf.
Palmer amaranth plants are either male or female; male plants produce only pollen, while female plants produce only seed. The terminal inflorescence of male and female plants is generally unbranched and very long. Female Palmer amaranth plants have a long terminal inflorescence (10 to 24 inches), with flowers containing five spatulate-shaped tepals. The tepals are about twice the length of the seed, and the seed capsule (utricle) breaks into two regular sections when fractured. Grabbing the inflorescence of a mature female Palmer amaranth plant with your bare hand is not recommended, as the bracts are very stiff and sharp. Palmer amaranth is an aggressively growing species that often reaches 6 to 8 feet in height.
Horak, M.J., and T.M. Loughin. 2000. Growth analysis of four Amaranthus species. Weed Science 48:347-355.
Klingaman, T.E., and L.R. Oliver. 1994. Palmer amaranth (Amaranthus palmeri) interference in soybeans (Glycine max). Weed Science 42:523-527.
Associate Professor of Weed Science
Professor of Molecular Weed Science