Proper identification of pigweeds is essential for choosing adequate management schemes. Pigweeds are most difficult to identify as seedlings when weed management treatments are applied. Identification in the adult stage is easier and useful in deciding which treatments to use the following year.
To aid the producer in identifying pigweeds, we have provided an illustrated table of their distinguishing characteristics and written descriptions of common waterhemp, redroot pigweed, smooth pigweed, Powell amaranth, and Palmer amaranth, including U.S. distribution, seedling and adult descriptions, technical characters, management schemes, and pertinent notes. Each species is designated by its common name as well as its scientific name and Weed Science Society of America (WSSA) approved abbreviation.
The first step in using this guide is to look at the Identification Table. The table provides comparative characters for seedlings as well as mature individuals. When using the table remember to consider as many of the characters as possible before making an identification. Once you have reached an identification turn to the species write-up. If the species does not seem to match any of these descriptions, turn to the section on Less Troublesome Pigweeds. The technical key and characters on this page are not necessary for identification, but are provided as an extra tool for interested readers with some botanical experience.
Traditionally, waterhemp has been divided into two species, the common waterhemp (Amaranthus rudis) found from Nebraska south to Texas and the tall waterhemp (Amaranthus tuberculatus) found from Indiana east to Ohio. Waterhemps from Iowa, Illinois, and Missouri are often intermediate between these species, but seem to respond similarly to weed treatments regardless of which name is placed on them. Currently, waterhemp in Iowa is designated generally as common waterhemp; ongoing taxonomic studies should resolve this issue.
An excellent field guide is the Kansas State University and USDA/ARS publication "Pigweed Identification: A Pictorial Guide to the Common Pigweeds of the Great Plains." Available from the Kansas State University Cooperative Extension Service Distribution Center, 16 Umberger Hall, Kansas State University, Manhattan, KS 66506-3406.
One of the problems with pigweed identification is that most of the technical differences are based on flower characters. Pigweed flowers are very small (1-4 millimeters) and most of the characters require magnification to see clearly. The following nontechnical characters are used in the table for identification. They can be divided roughly into two categories, vegetative and floral/seedhead characters.
Color is not a useful character because pigweeds vary greatly in coloration even within a single population (Figures 7 and 8). All attempts to separate species based on color have failed, although some color trends are noted in the descriptions.
Vegetative characters are less reliable than floral and seedhead characters. However, they are important in identifying immature plants and some general trends can be noted.
Pigweed seedlings differ in shape. In general, common waterhemp has oar-shaped seed leaves while redroot and smooth pigweeds have long, narrow seed leaves. The table includes drawings illustrating these differences.
Stem, leaf, and seedling texture and appearance can be altered by the presence of hairs. The texture varies from rough to very smooth. The lack of hairs on smooth plants often gives the leaves and stems a glossy appearance. Vegetative smoothness is distinguished from a smooth seedhead (see table).
Pigweed leaves vary greatly in size and shape on a single plant. The table illustrates general trends in species.
Floral and seedhead characters
All pigweeds have separate male (pollen-producing) and female (seed-producing) flowers. The flowers are minute and can be analyzed only with a hand lens or greater magnification. Male flowers are very similar across all species, varying mainly only in size. Female flowers are much more variable and detailed illustrations are provided for each species. Both male and female flowers have bracts (modified leaves), the size and shape of which can alter the appearance of the seedhead (see below).
Male and Female Plants
Common waterhemp and Palmer amaranth have separate male and female plants (dioecious). The other species have both male and female flowers on a single plant (monoecious). This may be difficult to determine, as the ratio of male to female flowers in plants with both sexes can be as low as 1 male flower to 100 female flowers, causing them to resemble female plants. The best way to determine this character is to look for male plants in the population.
Seedhead shape varies among the species in length, width, and appearance. Lengths are described as short, long, and very long. Widths are described as slender or thick. Appearance is affected by the length of the bracts and is recorded as smooth, slightly prickly, prickly, or very prickly. A comparative illustration of seedheads is provided in Figure 1.
A short section detailing the technical flower characters used to distinguish amaranth species is included along with the drawings. Distinguishing these characters will require use of a hand lens or greater magnification.
The key is a formal botanical guide to the species and may be useful in addition to the table. The characters refer to the technical characters described and illustrated under the species write-ups. The keys do require some level of botanical expertise to use. However, the table of non-technical characters is meant to stand alone and the technical key is provided for additional information.
Technical Key to Species
1a. Plants dioecious
2a. Plants pistillate (female)
3a. Tepals 0-1; bracts short, 0.5-2.5 mm; stigmas 3...........................Common waterhemp
3b. Tepals 5; bracts long, 3-7 mm; stigmas 2......................................Palmer amaranth
2b. Plants staminate (male)
4a. Bracts short, 0.9-2.8 mm; all tepal midribs short...........................Common waterhemp
4b. Bracts long, 2.3-5 mm; first tepal midrib long................................Palmer amaranth
1b. Plants monoecious
5a. Bracts long, 4-7mm; at least one tepal longer than the fruit
6a. Tepals 5, all longer than the fruit, apices obtuse or retuse, sometimes reflexed........Redroot pigweed
6b. Tepals 3-5, commonly only two longer than fruit, apices acute or acuminate, straight..........Powell amaranth
5b. Bracts short 3-5mm; all tepals shorter than the fruit.............................Smooth pigweed