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Pacific Northwest Research Station
Blue Mountains National Resources Institute

Forestry and Range Sciences Laboratory
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BMNRI Home > Publications > Weeds > Spotted Knapweed

Publications: Noxious Weeds

Explosion in Slow Motion: A talk by Jerry Asher about noxious weeds in the Blue Mountains

Click on a weed to learn about how to find it and kill it!

Common Crupina
Diffuse Knapweed
Russian Knapweed
Spotted Knapweed
Purple Loosestrife
Perennial Pepperweed
Tansy Ragwort
Medusahead Rye
Rush Skeletonweed
Yellow Starthistle
Canada Thistle
Musk Thistle
Scotch Thistle
Dalmatian Toadflax
Mediterranean Sage
St. Johnswort
Leafy Spurge
Dyers Woad

Wanted: DEAD!

Spotted Knapweed

  • Stems: From 1 to 3 feet in height. Stiffly branching with branches more or less erect and roughly ridged.
  • Leaves: Divided feather-like into narrow segments (except the uppermost). Leaves of the basal rosette are 6 inches long.
  • Flowers: Solitary on each branch tip; purple or rarely cream-colored; the outer flowers are enlarged. Flower heads are 1/2-inch wide. Flowering period is from June to September. Flower head bracts have dark fringe at top resembling a spot.
  • Seeds: About 1/8-inch long, tan to dark brown, and tipped with plumes that fall off at maturity.

Spotted knapweed is a biennial or short-lived perennial that is generally somewhat woolly. It infests roadsides, waste areas, and rangelands; it seems to prefer more moisture than diffuse knapweed. It spreads by seeds carried by wind, animals, or people; it can produce 25,000 seeds that stay viable for 8 years.

***WARNING: All knapweeds may contain a cancer-causing agent.***

***Handle with extreme caution!***

Control: Integrated management is necessary to control this weed.

  • Mechanical: Hand pulling small infestations, and reseeding disturbed sites with desirable species. Wear gloves when pulling by hand!
  • Chemical: Herbicide application can work. Consult you local weed control office for more information.
  • Biological: Twelve insect species are cleared for use by the USDA to control spotted knapweed. Each insect attack reduces the plant's overall seed production by weakening the plant or eating the seeds.

Report all sightings to your local Weed Board

US Forest Service - Pacific Northwest Research Station, Blue Mountains National Resources Institute
Last Modified: Monday, 16 December 2013 at 14:18:43 CST

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