Worldwide: Native to eastern North America (USDA-ARS 2016Footnote 3) and introduced into Asia (Israel and Jordan), the western United States and Europe (CABI 2016Footnote 4).
Duration of life cycle
Seed or fruit type
Seed diameter: 0.8 - 1.3 mm
Seed is oval or ovate
Seed is glossy, smooth
Dark reddish brown
Hilum is near the top of the seed in a notch.
Seeds may have a persistent funiculus at the hilum.
The rim around the outside of the seed, common in Amaranthus spp. is diminished or lacking in this species.
Habitat and Crop Association
Cultivated fields, wet areas, including the margins of ponds, rivers and ditches, roadsides, and disturbed areas (Darbyshire 2003Footnote 2, FNA 1993+Footnote 5). A dominant weed of soybean and corn fields in the United States (Costea et al. 2005Footnote 6).
Tall water-hemp produces 35,000-1,200,000 seeds per plant and forms persistent seed banks under cultivation (Costea et al. 2005Footnote 6).
This species prefers warm, moderately moist, nitrogen-rich sites in full sunlight (Costea et al. 2005Footnote 6). Considering that both seed and fruits float and the species prefers to grow near water, (Costea et al. 2005Footnote 6) suggest that dispersal by water may be significant.
Redroot pigweed (Amaranthus retroflexus)
Redroot pigweed seed has a similar lenticular shape, notched hilum and glossy surface as tall water-hemp.
Redroot pigweed seed is larger by up to 0.5 mm, has a pronounced rim, and is black.
Tall water-hemp seeds do not have a rim and are reddish-brown.