Submitted to: Washington State Weed Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 1, 1995
Publication Date: N/A
Interpretive Summary: The pigweed (Amaranthaceae) family contains many genera and over 500 species. Four species of pigweed commonly found in Washington State are redroot pigweed, Powell amaranth, prostrate pigweed, and tumble pigweed. All are annual weeds and control methods are similar. The biology of pigweeds and methods of control are discussed. Some pigweed biotypes have developed resistance to certain herbicide classes. An integrated approach to weed control consisting of cultural, mechanical, and chemical will help control these herbicide resistant biotypes. A simple key is included to identify pigweed species in Washington State.
Technical Abstract: The pigweed (Amaranthaceae) family contains about 60 genera and over 500 species. The four species of pigweed commonly found in Washington State are redroot pigweed (Amaranthus retroflexus), Powell amaranth (A. powellii), prostrate pigweed (A. blitoides or A. graecizans), and tumble pigweed (A. albus). All are annuals. Pigweed germinates throughout the summer months with maximum germination at about 95 deg F. It is a prolifi seed producer, and an individual plant may contain seed with various levels of dormancy. Pigweed seed can survive up to 40 years. Light may stimulate pigweed germination in freshly harvested seed. Pigweed is a C4 plant and is an efficient water user. Pigweed competes well with later planted crops, such as beans and corn, as its germination and emergence matches that of the crop. Early cultivation and seedbed preparation can promote pigweed germination and reduce the seedbank prior to planting a fall or summer planted crop. Many herbicides are effective on pigweed species. However, there are pigweed biotypes that are resistant to the triazine herbicides. Herbicides with different modes of action or from different chemical families should be utilized to prevent and control triazine resistant biotypes. Pigweed escapes from herbicide treatments should be cultivated, handweeded, or treated with another herbicide with a different mode of action to prevent them from producing seed.