Japanese knotweed
Polygonum cuspidatum Sieb. & Zucc.
Buckwheat family
Key identifying traits
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Stout, hollow, reddish-brown, woody stems from 4
to 9 feet tall that die back at end of growing
season forming a heavy mat of dead material
Nodes slightly swollen with thin papery sheaths
Leaves are short petioled, 2 to 6 inches long,
about two-thirds as wide and narrowing to a point
Flower are small, greenish-white to cream, borne
in clusters at ends of stems and in leaf axils
flowering stem of knotweed
Biology and ecology
 A perennial spreading from long creeping
rhizomes, seeds & stem pieces
 Can form very dense patches, particularly in
riparian areas, excluding most other vegetation
 Introduced from Asia as an ornamental and now
found next to roadsides, streams and ditches and
in waste areas and pastures
 Giant knotweed (Polygonum sachalinense) is similar
but larger with leaves about 12 inches long and
stems growing over 12 feet tall
Control
Prevention – Learn to identify plants; know your
property and avoid introduction as an ornamental or
as fragments in fill material
Biological – None known at this time
Cultural – Healthy native plant communities help heavy grazing keeps both growth and spread in check
Mechanical – Cutting, mowing, fire, digging and
covering with barrier materials have all been used
with variable success depending on scope of
infestation and landowner persistence
Chemical – several effective at label rates if used
repeatedly and combined with a revegetation effortknotweed in the fall on Lake Roosevelt
must be careful near water
Where found – Scattered throughout Stevens county in small patches generally associated
with subirrigated or riparian areas and in yards and lots where introduced as an ornamental.
Created by Stevens County Noxious Weed Control Board, January 2002; Updated Jan 2006