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Issue No. 24, Article 6/November 11, 2005

Palmer Amaranth: Today's Pigweed of Concern

Amaranthus species are among the most troublesome weeds in agronomic production systems. They are considered troublesome for corn and soybean because of the ability to cause yield loss. Several Amaranthus species are regarded as weedy pests across the Great Plains region, including the monoecious (male and female flowers on the same plant) species 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 (separate male and female plants) species common waterhemp (A. rudis), Palmer amaranth (A. palmeri), and sandhills amaranth (A. arenicola). Of these, smooth pigweed, redroot pigweed, Powell amaranth, Palmer amaranth, and the waterhemps 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. Palmer amaranth is most common in the southern third of Illinois but, similar to waterhemp during the 1990s, it appears to be expanding its range northward. The growth rate and competitive ability of Palmer amaranth exceed those of other Amaranthus species. M.J. Horak and T.M. Loughin (2000, "Growth analysis of four Amaranthus species," Weed Science 48:347-355) conducted a 2-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. T.E. Klingaman and L.R. Oliver (1994, "Palmer amaranth (Amaranthus palmeri) interference in soybeans (Glycine max)," Weed Science 42:523-527) found 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 ALS-inhibiting herbicides are known to exist.

Recently, weed scientists in Georgia have reported the occurrence of a glyphosate-resistant population of Palmer amaranth. The population was identified growing in a field of glyphosate-resistant cotton, and University of Georgia weed scientists estimate this biotype currently infests about 500 acres. This occurrence in Georgia, along with suspected cases in Tennessee and North Carolina, should raise awareness among Illinois farmers because it illustrates that resistance to glyphosate can occur in a summer annual weed species that is very competitive with corn and soybean and because Palmer amaranth is indigenous to Illinois. If glyphosate-resistant Palmer amaranth can occur in other states, it seems likely that it can occur in 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). Additional identification difficulty arises due to hybridization between dioecious and monoecious species. 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 descriptions here are provided to help you identify Palmer amaranth.

While many people tend to identify weeds based on "how the plant looks," 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 are also being employed. But rather than delve into molecular biology, I'll keep my discussion restricted to floral characteristics.

Let's 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 the colorful plants growing around the home, I'll use it to refer to the reproductive structures of the plant. Male flowers produce pollen, while female flowers produce seed. So with pigweed, think of the flower as the part of the plant where pollen and seed are produced.

Bract--a modified leaf associated with flowers. It differs from the 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 the palm of your hand, the tan-colored structures that fall into your hand 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--small, hard, black, and often glossy.

Immature Plants

The cotyledon leaves of Palmer amaranth are 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.

The stems and leaves have no hairs or only a few, resulting in these plant parts feeling smooth to the touch. Leaves are alternate on the stem and are generally lance- 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.

Mature Plants

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.

--Aaron Hager

Aaron Hager

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