Weed of the Month: Palmer Amaranth

Published: July 5, 2010

Figure 1. Palmer amaranth seedlings. Notice the long narrow cotyledons and first true leaves that are ovate with a notched tip.

Within the pigweed family there are ten species which can be encountered in the Midwestern United States: redroot pigweed (Amaranthus retroflexus L.), smooth pigweed (Amaranthus hybridus L.), Powell amaranth (Amaranthus powellii S. Wats.), tumble pigweed (Amaranthus albus L.), prostrate pigweed (Amaranthus blitoides S. Wats.), spiny amaranth (Amaranthus spinosus L.), common waterhemp (Amaranthus rudis Sauer.), tall waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer.], Palmer amaranth (Amaranthus palmeri S. Wats.) and sandhills waterhemp (Amaranthus arenicola I.M. Johnst.).

In Missouri, the most common pigweed species encountered in corn and soybean production is common or tall waterhemp (we just generally refer to these plants as "waterhemp" because of the vast degree of hybridization that has now occurred between the two species). Redroot and smooth pigweed used to be much more common weeds before waterhemp developed into a problem in the 1980's and 90's, but these species can still be found sporadically in some corn and soybean fields throughout Missouri. Of the remaining pigweed species, spiny amaranth is probably the most common; however this species is typically only found in pasture and hayfield settings. Powell amaranth, tumble pigweed, prostrate pigweed, and sandhills waterhemp are fairly rare in Missouri and only found in isolated areas. Palmer amaranth (also called palmer pigweed) is a species we thought was confined to the southernmost counties of the boot heel of Missouri but recently we have discovered sporadic infestations of palmer amaranth in corn and soybean fields in central and northwestern Missouri.

Palmer amaranth is a summer annual C4 weed that is one of the most problematic weeds of cotton and soybean production in the southern United States. Current research we are conducting in Missouri has shown that Palmer amaranth is at least twice as competitive as waterhemp, and that this species can grow 2- to 3-inches per day during the peak portions of the growing season.

Figure 2. Waterhemp seedling. Notice the much longer and narrower leaves of waterhemp.

Identification of Palmer amaranth is difficult in the early stages of seedling growth as many species within the pigweed family look similar. Once members of the pigweed family are mature, identification becomes less difficult. Palmer amaranth seedlings have cotyledons that are narrow and green to reddish in color. The first true leaves of palmer amaranth seedlings are ovate in shape, with few or no hairs present. Leaves often have a slightly notched tip and the leaf petioles are usually as long as or longer than the leaf blades (Figure 1). Mature Palmer amaranth plants are without hairs, with leaves that are diamond or egg-shaped in outline, and petioles that are usually longer than the leaves (Figure 3). The leaves of Palmer amaranth have a poinsettia-like leaf arrangement when viewed from above and an occasional v-shaped variegation or watermark on the upper surface of the leaf. Mature palmer amaranth can grow to more than seven feet in height.

Figure 3. Palmer amaranth on left and waterhemp on right.

Waterhemp, our most common pigweed species, has seedlings with leaves that are generally longer and more lanceshaped than any of the other pigweeds. Waterhemp seedlings are also hairless and have a waxy or glossy appearance (Figure 2). Stems of waterhemp and Palmer amaranth are hairless, whereas redroot and smooth pigweed have hairy stems. Depending on environmental conditions, waterhemp can range from 4 inches to 10 feet in height, but generally grows to about 4 or 5 feet in height in most agronomic settings. The leaves of mature plants are elongated and narrow (lance-shaped) and, like seedlings, have a waxy or glossy appearance. Leaves are arranged alternately on the stem and are without hairs. Stem and leaf color tend to be shades of green, but often within a population some plants will have distinctly red stems and/or leaves.