Encyclopedia Article

Waterhemp           Amaranthus tuberculatus (Moq.) Sauer

Family:  Amaranthaceae (Pigweed)       
Life cycle:   Annual
Native status:  Native to Iowa and western Cornbelt         
Habitat:   Crop fields, disturbed areas.  Prefers poorly drained soils.        

General description:  Erect plant up to 8 feet tall.   Leaves are glossy, alternate, ovate to lanceolate.  A dioecious species, thus plants are either male or female.  Highly variable in shape and color, ranging from yellowish green, dark green to reddish green.  Stems are hairless.

Key ID traits:  The stems of waterhemp are hairless, whereas redroot and smooth pigweed have hairs on stems.  Leaves are lanceolate in shape and tend to be glossier than those of redroot and smooth pigweed.  Waterhemp cotyledons are egg-shaped, whereas those of the pigweeds are more linear.  The inflorescences on waterhemp have slender branches (<1/4") that are typically less than 6".  Some male plants have longer, thicker terminal branches. The dioecious nature (separate male and female plants) leads to large variability in appearance of plants.

Similar species:  Redroot and smooth pigweed, Palmer amaranth

Miscellaneous:  Although native to Iowa, waterhemp was not a serious weed until the mid-1980s (story of waterhemp's rise). It is the most widespread herbicide resistant weed in Iowa, having developed resistance to triazines (HG 5), PPO (HG 14), HPPD (HG 27) and ALS (HG 2) inhibitors, and glyphosate (HG 9).  Populations resistant to growth regulators (HG 4) and VLCFA inhibitors (HG 15) have been identified in surrounding states.  Initially, two waterhemp species were described (common and tall), but now they are considered a single species.

Egg-shaped cotyledons on waterhemp seedlings.


Young waterhemp with lanceolate leaves.

Palmer amaranth (L) and waterhemp (R) seedlings. Small plants can be difficult to differentiate.  Some leaves on Palmer may have petioles longer than the leaf blade.


Stigma (pollen catching structures) on a female waterhemp.