Polygonum spergulariiforme is a North American species of flowering plants in the buckwheat family known by the common name Spurry knotweed or fall knotweed . It grows in western Canada (British Columbia and Saskatchewan) and the western United States (primarily Washington, Oregon, and northern and central California but with a few isolated populations in Idaho, Wyoming and Montana).Polygonum spergulariiforme is a green, branching herb up to 50 cm (20 in) tall.