Palmer amaranth Amaranthus palmeri L.
Family: Amaranthaceae (pigweed)
Life cycle: summer annual
Native status: Native to the southwestern USA
Habitat: crop fields, disturbed habitats
General description: Erect plant reaching heights of 6 ft or greater. Stems and leaves with few or no hairs. Petioles are often longer than the leaf blade. Leaves occasionally have a V-shaped, ‘thumb-print’. Like waterhemp, this species is dioecious. The terminal inflorescence can reach lengths of 1 to 2 ft; female plants have large (1/3” long), stiff bracts. Plants are highly variable in shape.
Key ID traits: Due to the diversity in both waterhemp and Palmer amaranth it can be difficult to differentiate the two species in vegetative stages. Leaves tend to be ovate in shape compared to lanceolate shape of waterhemp, and some leaves usually have petioles that are longer than the leaf blades. The long petiole is the most reliable vegetative trait. Long terminal inflorescences and large, sharp bracts on female flowers are key traits for identifying Palmer amaranth.
Similar species: Redroot and smooth pigweed, waterhemp and Palmer amaranth are all in the Amaranthus genus. The two pigweeds are monoecious, whereas Palmer and waterhemp are dioecious. The pigweeds have hairs on the stems and leaves, whereas the dioecious species generally are glabrous.
Miscellaneous: Palmer amaranth is native to the southwest USA, and has been spreading into the Cornbelt since 2010. Palmer amaranth was first documented in Iowa in 2013, planting of native seed mixes contaminated with Palmer seen in 2016 greatly increased introductions of the weed in Iowa. At this time it does not appear to have a competitive advantage over waterhemp in Iowa, but in states south of Iowa with warmer temperatures early in the season Palmer amaranth is more aggessive.