Amaranthus retroflexus (weed)

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Amaranthus retroflexus (click to enlarge)
Source:Wikimedia Commons

Amaranthus retroflexus (weed) L. (redroot pigweed)

This is a wide-spread and important annual weed which affects crops like cereals, potatoes, maize, soybean or beans. In maize and soybean, yield losses can be larger than 50% if the weed occurs at a high density.

It usually germinates late in the spring and prefers light soil, rich in nutrients. It is highly competitive under warmer conditions, low moisture and strong light. Up to 100,000 seeds can be produced per plant. The seeds are dispersed by wind, water, animals and farm machinery. Fields often contain many thousand seeds per m2.

Vernacular names
• Deutsch: Rauhaariger Fuchsschwanz
Zurückgebogener Amarant
• English: redroot pigweed
redroot amaranth
rough pigweed
• Français: amaranthe réfléchie
amarante à racine rouge

A. retroflexus is difficult to control once a larger seed-bank has established in a crop, as the seeds survive up to 20 years. Prevention of establishment and the use of herbicides are the most common control methods.

The plant usually grows to a height of 30-40 cm but can reach 1 m or more. The leaves are elliptical, the roots are thick and reddish. The flower-heads are erect and the plant produces many, small and shiny black seeds. A. retroflexus has female and male flowers on the same plant, and the female flowers predominate.

For a review and for a comparison with A. hybridus and A. powellii see Costea et al., 2004.