Polygonum bidwelliae is an uncommon species of flowering plant in the knotweed family known by the common name Bidwell's knotweed. The plant was named for American suffragist Annie Bidwell, who at one time lived in the Bidwell Mansion in Chico, California. She is the person who collected the type specimen in Northern California.
Polygonum bidwelliae is endemic to California, where it is known from the northern Sacramento Valley and adjacent slopes of the southernmost Cascade Range in Butte, Shasta, and Tehama Counties. It grows in chaparral, woodland, and grassland habitat on volcanic soils. 
Polygonum bidwelliae is an annual herb producing an erect green, wiry, angled stem reaching 20 centimeters (8 inches) in height. The narrow, pointed leaves are oppositely arranged along the stem, mainly on the upper parts of stem branches. The leaves have relatively large stipules which form ochrea that sheath the stem, sometimes hiding the leaf bases. The sharp-pointed stipules are membranous and silvery white. Bright pink flowers occur in upper leaf axils.
- CalFlora Botanical Names
- Friends of Bidwell Park, Chico
- Watson, Sereno. 1879. Proceedings of the American Academy of Arts and Sciences 14: 294
- Biota of North America Program 2014 county distribution map
- Calflora taxon report, University of California, Polygonum bidwelliae S. Watson, Bidwell's knotweed
- Flora of North America, Polygonum bidwelliae S. Watson, 1879. Bidwell's knotweed
- Jepson Manual Treatment, University of California, Polygonum bidwelliae
- United States Department of Agriculture Plants Profile: Polygonum bidwelliae
- Polygonum bidwelliae - Calphotos Photo gallery, University of California
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