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​Terrible 10 - Knotweed


Knotweeds (Polygonum var)

  • Four species found in Island County: Bohemian (Polygonum x bohemicum), giant (Polygonum sachalinense), Himalayan (Polygonum polystachyum), Japanese (Polygonum cuspidatum)
  • Found on open hillsides, damp ditches, prefers sun and moist soil but can adapt to drier, shady locations.
  • Height for different species varies from 4 ft to 12 ft (giant knotweed). All are clump forming, with smooth, hollow, stems swollen at the nodes like bamboo. Stems are green to reddish, often red spattered. Leaves are smooth edged, bright green, but shape varies from lance to heart shape depending on species and size varies from 4-5” long to over 12”. Small flowers appear in July and August on showy plumes ranging from green- tinted white to pink, depending on species.
  • Introduced from Asia as ornamental plants, and for erosion control; now hybridized into additional varieties.
  • Large rhizome mass provides poor erosion control resulting in bank collapse and flood hazard. Clogs waterways, destroys habitat for fish and wildlife as well as plants. Dense mats of stems and rhizomes overwhelm native species.