Amaranthus spinosus | Thorny pigweed

Thorny pigweed is a serious weed in tropical and subtropical regions of the world. Originally from Central America it has spread across the warmer parts of the world. It is a problem weed in agricultural areas (especially cotton and tobacco but less common in maize), pastures and orchards.

Thorny pigweed has a characteristic pair of spines at the base of the leaves which makes it unpalatable for grazing livestock and make hand weeding difficult. Leaves can also contain high levels of nitrates that can be poisonous to animals when converted to nitrites by microorganisms present in the rumen.

Amaranthus spinosus is drought tolerant and thou it can grow in wetter soils it grows well in dryer soils that are loamy and high in organic matter and contain sufficient nitrogen. Male and female flowers are born on the same plant and flowers are pollinated by wind and a single plant can make hundreds of thousands of seeds. The seeds are easily distributed by wind, water and animals contaminated crop or pasture seed as well as farm equipment.

Control

Mechanical control and hand weeding are effective on small plants but it can become hard to hand-wed once the spines have hardened. Thorny pigweed is generally easily controlled by most standard broadleaf herbicides (except bendioxide). As with all pigweeds development of herbicide resistance is a possibility with the repeated use of single herbicides and should be proactively managed to prevent this. Suspected cases of resistance should be reported immediately.