Russian Knapweed
Rhaponticum repens
Washington State Classification
Class B — Control Required (Designated)
It is an aggressive invader of pastures, non-crop areas, grain fields and other cultivated fields.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Russian knapweed is a bushy, branched perennial, reaching 1 to 3 feet tall and forming clones or colonies from vigorous, deep, spreading rhizomes.
Leaves
Leaves at the base of the stem (basal leaves) are gray-green and lobed. Upper stem leaves are smaller with toothed edges or entire (smooth) edges.
Flowers
Many flower heads, pink to purple in color. The outer bracts under the flower heads are greenish to straw colored and have a broad, papery tip.
Fruit & Seeds
Seeds (achenes) are ivory white and have a feather-like plume (pappus).
Impact
It is an aggressive invader of pastures, non-crop areas, grain fields and other cultivated fields. In addition, the plant is poisonous to horses, causing chewing disease (equine nigropallidal encephalomalacia). Livestock may avoid this species.
Ecology & Spread
Habitat
Russian knapweed is found growing in pastures, hayfields, grain fields, irrigation ditches as well as roadsides. Please click hereto see a county level distribution map of Russian knapweed in Washington.
Spread Mechanisms
Reproduction
Russian knapweed reproduces by seed and spreads laterally by its root system. Root fragments can regenerate following cultivation.
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Control Methods
Mechanical
Biological Control
- The nematode Subanguina picridis forms galls on Russian knapweed that reduce plant vigor - its effectiveness in Washington is not yet known.