Spotted Knapweed
Centaurea stoebe
Washington State Classification
Class B — Control Required (Designated)
Spotted knapweed is a very aggressive species that can quickly infest large areas.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
It’s a biennial or perennial growing to 5 feet with a stout tap root. It is hairy and rough, with a somewhat woolly appearance. Plants start as a rosette (cluster of radiating leaves) of deeply lobed leaves the first year and then produce flowering stems.
Leaves
Leaves vary from unlobed on the upper part of the plant to lobed on the lower part of the plant. Lobed leaves have narrow lobes on each side of the center leaf vein. Leaves are blue-gray in color and may have glands in small pits.
Flowers
Flowerheads are solitary. Bracts at the base of heads are egg-shaped with black veins and a brown/black triangular tip with comb-like fringe along its edge. Bract tip's color gives the flower head base a spotted look. Flowers are pink purple or white.
Fruit & Seeds
Seeds are 0.1 inch (2.5 mm) long, black or brown and oval. Seeds have short bristles (pappus) on one end.
Impact
Spotted knapweed is a very aggressive species that can quickly infest large areas. Knapweed infestations increase production costs for ranchers, degrade wildlife habitat, decrease plant diversity, increase soil erosion rate and pose wildfire hazards.
Ecology & Spread
Habitat
It is found in dry meadows, pastures, rocky areas, gravel mines, roadsides, railroads, airports, vacant lots, hayfields, forest clearings and on the sandy or gravelly floodplains of streams and rivers. Please click here to see a county level distribution map of spotted knapweed in Washington.
Spread Mechanisms
Reproduction
Spotted knapweeds reproduce by seeds with each plant producing 1,000 seeds on average. Seeds may be viable for up to eight years.
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Control Methods
Mechanical
- Seasonal mowing and tillage
Biological Control
- Many biological control agents are used on spotted knapweed. The sulfur knapweed moth (Agapeta zoegana) larvae feed within the plant's roots. The knapweed peacock fly (Chaetorellia acrolophi) larvae feed on the plant's seeds. The green clearwing fly (Terellia virens) larvae feed on seeds within flowerheads of spotted knapweed. The knapweed root weevil (Cyphocleonus achates) larvae feed within spotted knapweed roots. Larinus minutus, a seedhead weevil, larvae destroy spotted knapweed seed in the seedheads. Larinus obtusus, blunt knapweed flower weevil, larvae feed on seeds within the seedheads and adults feed on leaves. Urophora affinis and Urophora quadrifasciata are seedhead gall flies that are also used as biocontrol agents for spotted knapweeds.For more information about these biological control agents of spotted knapweed, please visit WSU Extension Integrated Weed Control Project.