Knotweed Complex - Japanese, Giant,
and Himalayan, not Prostrate Knotweed
(Polygonum cuspidatum, sachalinense & polystachyum)
Photo from Weeds of the West, revised edition.
Photos from Montana Weed Control Association
Growth Habit: Perennial, 4 to 9 feet in height
Leaves: Leaves are short-petioled, broadly ovate, 2 to 6 inches long and about two-thirds as wide, narrowed to a point.
Stem: Stems are stout, woody, reddish-brown, die back at the end of the season.
Flower: Flowers are greenish white to cream, borne in large plume-like clusters at ends of stems and in leaf axils.
Roots: Creeping Rhizomes, roots can spread 20 feet from the plant and go down as deep as 7 feet into the soil.
Seeds: Fruit is 3-sided, black and shiny.
Reproduction: By rhizomes and seed.
Habitat: Escaped ornamental, found along roadsides,waste areas, ditch banks, and pastures.
Herbicides: Glyphosate, Trichlopyr, Imazapyr