Amaranthus retroflexus
(redroot pigweed)

A. retroflexus is thought to be a native riverbank pioneer of the central and eastern USA and adjacent regions of south-eastern Canada and north-eastern Mexico (Sauer, 1967). It has become naturalized throughout the temperate regions of the northern and southern hemispheres. A. palmeri is also native to North America, but its distribution is confined primarily to the southern USA and Mexico (Sauer, 1955).

A. retroflexus is found on a wide variety of soil types and textures. It grows particularly well in fertile soils and has a high N requirement. It tolerates soil pH from 4.2 to 9.1 (Feltner, 1970), but is less common on acid soils, such as those of the south-eastern USA, where the related species A. palmeri is more abundant. It is common in cultivated fields, gardens, waste places, roadsides, river banks, and other open, disturbed habitats where annual weeds predominate. It is seldom found in closed or shaded communities (Weaver and McWilliams, 1980).

A. retroflexus has a worldwide distribution and is a common weed of most field and horticultural row crops in the temperate areas of the world.

A. retroflexus is an annual that reproduces solely by seed. It is a prolific seed producer, with single vigorous plants capable of producing between 230,000 and 500,000 seeds (Stevens, 1957). Seed production declines beneath crop canopies where light is limited (McLachlan et al., 1995). Germination requirements and dormancy patterns are highly variable depending on distribution and local climatic and ecological conditions and, as such, generalizations should be treated with caution. Recent research suggests that germination is stimulated by light and high temperatures (Gallagher and Cardina, 1997; Oryokot et al., 1997). The seeds are small and most germinate near to the soil surface, with optimum emergence from about 1 cm depth (Wiese and Davis, 1967; Siriwardana and Zimdahl, 1983). Weaver and McWilliams (1980) reported that seeds can remain viable in the soil for many years, but Egley and Chandler (1978) reported a 90% decline in viability after seed burial for 18 months. Seeds are dispersed by wind, animals and as contaminants of crop seeds or farm machinery.

A. retroflexus has the C4 pathway of photosynthesis, typical 'Kranz' leaf anatomy, a low carbon dioxide compensation point and high water use efficiency (Weaver and McWilliams, 1980; Tremmel and Patterson, 1993).

More detailed information on the biology and ecology of A. retroflexus is provided by Weaver and McWilliams (1980) and Holm et al. (1997).

A. retroflexus is a host plant for a variety of insect pests and diseases which attack crops, although damage is rarely severe enough to serve as biological control. However, several authors have suggested the pigweed flea beetle (Disonycha glabrata) (Tisler, 1990) and various pathogens (Burki et al., 1997) as potential control agents.

Animal feed, fodder, forage

• Fodder/animal feed

Human food and beverage

• Flour/starch
• Seeds
• Vegetable

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Traditional/folklore

Immature plants of A. retroflexus are similar in appearance to those of A. hybridus and A. powellii. Flowering plants of the latter two species have thinner, longer branches of the inflorescence, and straight, un-reflexed perianth segments. In A. hybridus, the perianth segments are acute and the bracteoles, although a little shorter than those of A. retroflexus, are distinctly more spiny to the touch. A. retroflexus also resembles A. palmeri and the waterhemps, A. rudis and A. tuberculatus, but the stems and leaves of the three latter species are smooth and hairless, and male and female flowers occur on separate plants. The latter three are all north American species. A. palmeri has not spread significantly outside the USA and Mexico (Sauer, 1955), but has become increasingly important weed in soyabeans, groundnuts and cotton in south-eastern USA (Webster and Cole, 1997).

Many other Amaranthus species are superficially similar to A. retroflexus. Of those included in this compendium, A. spinosus has spines, A. viridis has much smaller flowers, A. blitum has indented leaf tips, and A. graecizans and A. blitoides have mainly axillary inflorescences. Further species can occur as weeds on a local basis and reference to local floras or expertise may be necessary

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.

Control

Cultural Control

Seedlings can be controlled by cultivation, but older plants often recover from mechanical damage and produce axillary inflorescences. Soil solarization under plastic can provide control of A. retroflexus if high temperatures are achieved for prolonged periods of time (Mas and Verdu, 1996).

Chemical Control

A. retroflexus is readily controlled by most herbicides which inhibit photosynthesis, such as atrazine, simazine, metribuzin, linuron and bromoxynil. It is also highly susceptible to the synthetic auxin herbicides, such as 2,4-D or dicamba, and sulfonylurea and imidizolinone herbicides, such as imazethapyr, thifensulfuron-methyl, rimsulfuron and nicosulfuron. Most other herbicides for control of broad-leaved weeds also provide good control including acifluorfen, fomesafen and pendimethalin (Weaver and McWilliams 1980; Bauer et al. 1995; Carey and Kells 1995; Mamarot and Rodriguez, 1997). Its pattern of intermittent germination throughout the growing season, however, makes the application of residual soil-applied herbicides, or sequential post-emergence treatments, necessary in heavily infested fields. A. retroflexus can be controlled by the soil fumigant methyl iodide (Zhang et al., 1997).

Herbicide Resistance

Populations of A. retroflexus resistant to atrazine have been reported in the USA, Canada, France, Germany, Hungary, Switzerland, Spain, Poland, Chile and the Czech Republic (Heap, 1997). Many of these are cross-resistant to metribuzin and linuron (Daban and Garbutt, 1996). Populations resistant to ALS inhibitors (sulfonylureas and imadizolinones) have been reported in the USA and Israel (Heap, 1997). In the past 20 years biotypes resistant to 15 herbicide active ingredients have been reported in 15 countries (LeBaron and Gressel, 1982; Benbrook, 1991).

Biological Control

Biological control of A. retroflexus with fungal pathogens has been reported (Burki et al., 1997). The pigweed flea beetle, Disonycha glabrata, is native to the southern USA and South America. It has been promoted as a control agent of A. retroflexus (Tisler 1990), but it gives incomplete control in the field because beetle populations develop too slowly and too early in the season to prevent competition with the crop and seed production.