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Integrated Crop Management
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Smooth pigweed

Smooth pigweed - Amaranthus hybridus L.

Family: Amaranthaceae (pigweed family) 
Life cycle: Annual 
Habitat: Cultivated fields, gardens 
General description: Erect summer annual up to 7 ft tall. 

Key ID traits: Stems usually sparsely pubescent; leaves simple, ovate, up to 5 in long and 2.5 in wide, long petioled. Inflorescence consists of a cluster of numerous slender spikes approximately 0.4 in diameter and up to 6 in long.

Similar species: Both redroot and smooth pigweed are monoecious, whereas waterhemp is dioecious. Stems of waterhemp are glabrous, redroot and smooth pigweed have hairy stems. The spikes of redroot pigweed are shorter and thicker than those of smooth pigweed.

Fun facts: Smooth pigweed was the first Amaranthus to develop resistance to the triazine herbicides; however, it was misidentified as redroot pigweed in the first research papers reporting the resistance.

elongated spikes of smooth pigweed
Elongated spikes of smooth pigweed
vegetative differentiation of redroot and smooth pigweed is difficult
Vegetative differentiation of redroot and smooth pigweed is difficult
Authored by:
Bob Hartzler
Bob Hartzler
  • Retired Weed Scientist and Professor of Agronomy
Dr. Bob Hartzler is a former professor of agronomy and a former extension weed specialist with a Ph.D. in Crop Physiology from Iowa State University. He conducted research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also taught undergraduate classes in weed science and weed identification.
Meaghan Anderson
Meaghan Anderson
  • Field Agronomist in Central Iowa
Meaghan Anderson is a field agronomist serving nine counties in central Iowa and an extension specialist with Iowa State University Extension and Outreach. Areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and integrated pest management.

Meaghan holds a bachelor's degree in agronomy and a master's degree in crop production and physiology (focus: weed science) from Iowa State University.
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