Amaranthus spp.

The genus Amaranthus includes some of the world’s most common and troublesome weeds that are not only causing economic loss but also posing a threat to global food security. This chapter attempts to summarize and discuss the key biological characteristics and management options for major weedy Amaranthus species including A. palmeri, A. tuberculatus, A. retroflexus, A. spinosus, and A. hybridus. Due to changing agricultural practices in the 20^th century, A. plameri and A. tuberculatus have risen from relative obscurity to become the most problematic weeds in the United States. The weedy Amaranthus spp. exhibit high phenotypic plasticity and genetic variability that contribute to their persistence in diverse climatic conditions. Rapid growth rate, high fecundity, limited seed dormancy, prolonged emergence pattern, and evolving herbicide resistance helped these weeds to build up a persistent seedbank. As the number of multiple herbicide-resistant Amaranthus population is on a steady rise, implementing integrated weed management is now more important than ever.