Redroot pigweed Amaranthus retroflexus L.
Life cycle: Annual
Native status: Native to North America
Habitat: Crop fields, waste areas
General description: Erect, branched plant reaching heights of 6 ft.
Key ID traits: Stems hairy; leaves are egg-shaped and often reddish underneath and with wavy margins; first leaf usually has notch at tip. Linear cotyledons. Inflorescences has short (<6"), thick (1/4-1/2") branches with large bracts.
Similar species: Both smooth and redroot pigweed have hairy stems, whereas common waterhemp and Palnmer amaranth have smooth stems. The seedhead of redroot pigweed is more compact than those of smooth pigweed and common waterhemp.
Miscellaneous: Redroot pigweed was the dominant Amaranthus species in Iowa prior to the 1980’s, but was replaced by common waterhemp following the introduction of ALS inhibiting herbicides and the spread of herbicide resistance. Redroot pigweed and smooth pigweed are monoecious, whereas waterhemp and Palmer amaranth are dioecious.