Polygonum hickmanii is a rare species of flowering plant in the buckwheat family known by the common names Scotts Valley polygonum and Hickman's knotweed. It is endemic to Santa Cruz County, California, where it is known from only two sites in the Scotts Valley. It grows on coastal prairie on mudstone and sandstone substrates, in an area known for its spring wildflowers. The small plant was first noted in 1990 and described as a new species in 1995. The plant is a federally listed endangered species.