Amaranthus hybridus
(smooth pigweed)

A. hybridus is a native riverbank pioneer of eastern North America and parts of Mexico, Central America and northern South America (Sauer, 1967). It is much more common in the eastern than the western half of the USA. Its range has expanded to Africa, south-central Asia and Australia, possibly because of its use as a green vegetable.

A. hybridus is found on a wide variety of soil types and textures. It is a common weed of cultivated fields, gardens, waste places, roadsides, riverbanks, and other open, disturbed habitats where annual weeds predominate. It is seldom found in closed or shaded communities (Weaver and McWilliams, 1980).

A. hybridus is a common weed of many field and horticultural row crops throughout the temperate areas of the world.

A. hybridus is a summer annual herbaceous weed that reproduces solely by seed with maximum emergence in late spring or early summer (Weaver and McWilliams, 1980; Anderson, 1994). Plants which emerge in late May generally begin to flower in late July or early August, and produce ripe seed by the beginning of September (Weaver, 1984). Its small seeds are easily dispersed by wind, and germinate primarily from the top 2 cm of soil in disturbed habitats. Germination is stimulated by light and/or high temperatures, with greatest germination occurring at alternating temperatures of 20/35°C in the light (Weaver and McWilliams, 1980; Weaver, 1984).

A. hybridus possesses the C4 pathway of photosynthesis, a low carbon dioxide compensation point, little photorespiration, and its optimum temperature for net photosynthesis lies between 30 and 40°C (Patterson, 1976). Separate male and female flowers occur on the inflorescence, and pollination is generally by wind. A single vigorous plant may produce as many as 100,000 seeds. Seed longevity has not been reported, but seeds of the closely related species A. retroflexus and A. powellii, can remain viable for many years when buried in soil.

A. hybridus is a host plant for a variety of insect pests and diseases which attack crops, however damage is rarely severe enough to serve as biological control.

Animal feed, fodder, forage

• Fodder/animal feed

Genetic importance

• Related to

Human food and beverage

• Vegetable

Medicinal, pharmaceutical

• Traditional/folklore

A. hybridus is similar in appearance to A. retroflexus and A. powellii, particularly in the vegetative state. Flowering plants of A. retroflexus differ in having shorter, thicker branches on the terminal inflorescence, sepals rounded or blunt and often reflexed, bracteoles less distinctly spiny. A. powelli also has a somewhat thicker inflorescence, the bracteoles are longer (up to 6 mm), and stamens usually 3, not 5.

A. caudatus and A. hypochondriacus are ornamental, domesticated plants, with larger terminal inflorescences, often bright red in colour, with a utricle equalling or exceeding the bract and sepals in length. The utricle of A. hybridus, A. retroflexus, and A. powellii is shorter than the bracts and sepals. The seeds of the weedy amaranths are usually dark brown, whereas many forms of the cultivated grain amaranths have ivory seeds.

Many other Amaranthus species are superficially similar. Of those included as data-sheets in this compendium, A. spinosus has spines, A. viridis has much smaller flowers, A. bitum has indented leaf tips, and A graecizans and A. blitoides have axillary inflorescences. Further species can occur as weeds on a local basis and reference to local floras is therefore recommended.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

Seedlings of A. hybridus can be controlled by cultivation, but older plants often recover from mechanical damage by producing axillary branches and inflorescences.

Chemical Control

A. hybridus is readily controlled by almost all the standard soil-applied and foliar-applied herbicides used for controling broadleaved weeds, including atrazine, simazine, metribuzin, linuron, bromoxynil, 2,4-D, dicamba, imazethapyr, thifensulfuron-methyl, rimsulfuron, nicosulfuron, acifluorfen, fomesafen and pendimethalin (Weaver and McWilliams, 1980; Manley et al., 1996; Robinson et al., 1996). Its pattern of intermittent germination throughout the growing season, however, make the application of residual soil-applied herbicides, or sequential post-emergence treatments, necessary in heavily infested fields. Lorenzi (1984) indicates that A. hybridus is only moderately susceptible to oxyfluorfen, butylate or vernolate.

Populations of A. hybridus resistant to triazine herbicides have been reported in the USA, France, Switzerland, Italy, Spain, Israel and South Africa (Heap, 1997). Populations resistant to the imidazolinone herbicides have been found in the USA (Heap, 1997).

Biological Control

Reputed biological control agents for A. hybridus are the pathogenic bacterium, Erwinia carotovora var. rhapontici [Erwinia rhapontici], (Gonzalez-Mendoza and Rodriguez, 1990), and insects Herpetogramma bipunctalis and Conotrachelus seniculus (Perez Panduro et al., 1990), all in Mexico.