Polygonaceae
Polygonum cuspidatum

Japanese Knotweed

Dense mat with freely branched stems from spreading rhizomes. Male, female flowers on separate plants. Leaves on stem, 4–8 in. long, with shorter petioles, egg-shaped to round, often wider than long, sharply pointed tip. Inflorescence widely branched, 6 in. Flower stalks among leaves on upper stem, lengthening with age. Flowers white, 5-lobed. Grows on wet ground, disturbed places, at low to mid-elevations. Polygonum bohemicum (pictured) is a newly identified hybrid between P. cuspidatum and P. sachalinense, giant knotweed, which is erect and 6–9 ft. tall. Differences among the 3 are primarily the size and shape of leaves. All are considered noxious weeds and will crowd out native vegetation.

  • Rarity: Common
  • Flowering Time: All Summer
  • Life Cycle: Perennial
  • Height: 3--8 feet
  • Habitat: Disturbed, Bog/Fen/Wetland
  • Found In: Mt. Rainier NP, Olympic NP, N Cascades NP
  • Native: No