PPDL Picture of the Week for

January 12, 2015

Spiny Amaranth- The lesser known pigweed

Travis Legleiter, Weed Science Professional Assistant, Botany & Plant Pathology Department

The last couple of years the pigweeds have predominated the weed science focus in agronomic crops, although there is one pigweed that prefers to compete with forages and grass pastures rather than corn and soybeans. If you have every been around a cattle, horse, or other livestock grazing pasture there is a high likelihood that you’ve seen this weed. Spiny amaranth or pigweed (Amaranthus spinosus) is one of the most predominate weeds in pastures and feed lots, especially in areas that are overgrazed. This pigweed looks a lot like the other pigweeds we are used to seeing in our agronomic field, but with a few distinct characteristics. Spiny amaranth has ovate to lanceolate leaves that grow in an alternating pattern and will be completely hairless, much like common waterhemp and Palmer amaranth. Although spiny amaranth has a shorter and bushier or more highly branched growth habit than the other pigweeds with heights usually not occurring much past 4 to 5 feet in height. Also spiny amaranth has sharp spines, hence the name, occurring at each leaf axial and reaching lengths of 0.5”. 

Spiny amaranth is as competitive as its close agronomic weed relatives and can be especially troublesome in areas that are overgrazed or undermanaged. Control of spiny amaranth can be accomplished with timely applications of growth regulator herbicides such as 2,4-D, dicamba, or triclopyr. Much like the troublesome agronomic pigweeds it is important to make herbicide applications when plants are 4 to 6 inches in height. Proper grazing, fertilization, and other practices to promote a healthy pasture are the most effective methods to keep spiny amaranth from effectively competing with your favorable forages.

Further in-depth details of spiny amaranth biology, history, and control can be found in the Purdue WS-44 publication.

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