Amaranthus hybridus | Smooth pigweed

Amaranthus hybridus or smooth pigweed originates from various parts of the Americas, but has since spread and naturalised in many countries across the world, where it is considered a weed but also used as a leafy vegetable and even as medicine. It is a summer annual, with separate male and female flowers that occur on the same inflorescence with pollination mainly by wind. An abundance of seed is produced from late spring to summer. Seed generally germinate in the top 2 cm of the soil profile and germination is stimulated by temperatures between 25 and 30 °C.

The name refers to a complex of different species that are difficult to distinguish morphologically due to their high levels of hybridization. The plants can therefore vary in colour and size but is generally much taller than other Amaranthus species, growing up to 2.5 m high. Smooth pigweed is probably one of the most common weeds across South Africa in many different crops. They compete very strongly for water and nutrients. When grown in nitrogen-rich soils leaves and stems can accumulate high levels of nitrates as well as oxalates that may pose a risk of nitrate poisoning in ruminants, if large amounts of immature leaves and stems are consumed.

Amaranthus is known to be and alternative host parasitic nematodes (Meloidogyne) the tobacco mosaic virus as well as the fungus that cause anthracnose in tomato fruit and cotton seedlings.

Control

Seedlings of smooth pigweed is easily controlled by cultivation, but older plants can recover from mechanical damage by producing axillary branches and inflorescences and should therefore be removed from the field.

Smooth pigweed is also readily controlled aby almost all the standard soil and foliar applied broadleaf herbicides with the exception of bendioxide (Bromilow, 2018). Glyphosate and triazine resistance have also been confirmed in some populations in South Africa (Heap 1997). Due to intermittent germination throughout the season, the application of residual soil herbicides is needed or sequential post-emergence treatments. It is important to treat plants when they are small as chemical control is less effective on large plants.