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Identifying the Enemy

September 7, 2001
Accurate identification of Amaranthus (pigweed) species can be very difficult, especially during early vegetative development because many of these species exhibit similar morphological characteristics (i.e., many look very similar). So if a pigweed plant has a red root, this does not always identify the plant as redroot pigweed! However, by late summer most pigweed species have initiated reproductive development ,and when flowering structures are present, accurate identification becomes much easier. So if you have "pigweeds" growing in fields and aren't quite certain which species they are, the following discussion will (hopefully) aid in the identification of various Amaranthus species. For those who have access to the web version of the Bulletin, pictures of the inflorescence (a picture is often worth a thousand words) of these six species are presented.

Approximately 10 Amaranthus species are regarded as weedy pests across the Corn Belt. These are either monoecious (male and female flowers on the same plant) or dioecious (separate male and female plants) species. Monoecious species include 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). Dioecious species are common waterhemp (A. rudis), tall waterhemp (A. tuberculatus), Palmer amaranth (A. palmeri), and sandhills amaranth (A. arenicola). Of these, redroot pigweed, smooth pigweed, Powell amaranth, common and tall waterhemp, and Palmer amaranth are most frequently encountered in Illinois agronomic production systems. We'll focus the discussion on these six species.

Although 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 using molecular biology techniques are also being employed. Instead of delving into molecular biology, we'll keep the following discussion restricted to separating the Amaranthus species based on floral characteristics.

We'll need to define some of the terms that will be used in the discussion, so we'll start with the outer parts of a flower and work inward to the seed.

Inflorescence--flowers collectively. Although many people associate the term "flower" with the colorful plants growing around the home, we'll use this term to refer to the reproductive structures of the plant. Male flowers produce pollen, while female flowers produce seed. So think of the flower (at least for the purpose of this article) as that part of the pigweed plant where the seed is 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. How the utricle fractures (breaks apart) has been the basis for differentiating between common and tall waterhemp.

Seed--small, hard, black, and often glossy.

Redroot Pigweed

Redroot pigweed has a highly branched but compact terminal (at the top of the plant) inflorescence.

The branches of the inflorescence are about the size of your thumb and much thicker than the inflorescence branches of smooth pigweed. Female flowers have five tepals (the structures that surround the seed), which are spatulate-shaped with rounded tips that curve outward and are about twice the length of the fruit (the tiny black seed). When you rub the inflorescence against the palm of your hand, you'll see the ends of the tepals tend to reflex or bend back, hence the species name retroflexus. The tepals of redroot pigweed are about twice the length of smooth pigweed tepals.

Smooth Pigweed

Smooth pigweed has a highly branched terminal inflorescence (more branched than the inflorescence of redroot pigweed or Powell amaranth) that is frequently dull green or red in color.

The branches of the inflorescence are generally about the thickness of a pencil or slightly thinner and much thinner but usually longer than the inflorescence branches of redroot pigweed. Female flowers have five tepals that do not curve outward, are about the same length as the seed, and are sharply pointed. When you rub the inflorescence against the palm of your hand, you'll see tepals that are straight and do not bend back. The seed just fits within the tepal, and the tepals are much shorter than those of redroot pigweed.

Powell Amaranth

The inflorescence of Powell amaranth is less branched than redroot pigweed or smooth pigweed.

Branches are 4 to 10 inches in length and thicker than a pencil. Tepals (three to five total) are sharply pointed, and one or two are generally longer than the seed.

Palmer Amaranth

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 grows larger than waterhemp.

Common and Tall Waterhemp

Flowering structures of common waterhemp are much more open and located near the top of the plant and at tips of branches.

Pistillate (female) flowers generally have none or one tepal. Differentiation between common waterhemp and tall waterhemp has been based on how the utricle breaks apart. If the seed capsule (utricle) breaks into two regular sections, it is common waterhemp; if the utricle breaks into irregular sections, it is tall waterhemp. For all practical purposes, differentiation of common and tall waterhemp is not necessary, as no evidence exists to suggest these species respond differently to management practices. Indeed, some have recently proposed that in lieu of two discreet species, waterhemp exists as a single, widely variable species.

If you're still feeling some uncertainty about accurately identifying Amaranthus species, don't feel too bad. Weed scientists and plant taxonomists have also had difficulty identifying these species for many years. Hybridization among species has long been known to occur, and hybrid offspring from these crosses generally do not fit "neatly" into these categories. The following quote, taken from a paper (Uline, E.B. and W.L. Bray. 1894. A Preliminary Synopsis of the North American Species of Amaranthus. Botanical Gazette 19:267­272) published in 1894 illustrates the difficulty in identifying Amaranthus species:

"Likewise many of the species approach dangerously near to one another; and the complex question of adaptation and modification of adventive forms together with the still greater uncertainty which prevails in regard to hybridization among certain groups of species has rendered the question of specific limitation one of peculiar difficulty and uncertainty."

Two excellent pigweed identification guides can be viewed at the following Web pages: Control in Crops

Additionally, Dr. Kenneth Robertson, of the Illinois Natural History Survey, a recognized expert in Amaranthus identification and taxonomy, has a picture of Powell amaranth, redroot pigweed, and smooth pigweed tepals that can be seen at

Although the characteristics and descriptions outlined in this article may seem like a lot to comprehend, they are very useful when attempting to identify these species. Pay close attention to the pictures of individual species included in the web version of the Bulletin, and perhaps putting all these species side by side will help you see the differences (at least the differences in appearance of the reproductive structures).

--Aaron Hager, Loyd Wax, and Christy Sprague

Author: Aaron Hager Loyd Wax Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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