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Pacific Northwest Research Station
Blue Mountains National Resources Institute

Forestry and Range Sciences Laboratory
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La Grande, OR 97850

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BMNRI Home > Publications > Weeds > Diffuse 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!

Diffuse Knapweed

Alias: Tumble knapweed, bushy knapweed

  • Stem: Single with many branches, gives plant a bushy look. Up to 4 feet tall.
  • Leaves: Grayish-green. Lower leaves divided several times, uppers once divided into narrow segments. Leaves are 6 inches long at the base, they get smaller further up the stem.
  • Heads: Numerous, narrow, few-flowered.
  • Flowers: Numerous, white to rose, sometimes purplish. Flowering period is from July to September.
  • Bracts: Are yellowish-green with a light brown comb-like margin.
  • Fruits: Brown or grayish with several longitudinal stripes and usually without a terminal tuft of hair.

Diffuse knapweed infests roadsides, waste areas, and dry rangelands, and spreads into new areas quite rapidly. It forms dense stands and excludes more desirable forage species. It spreads by seeds, which are scattered by windblown tumbling mature plants.

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

***Handle with extreme caution!***

Control: Integrated management approaches are necessary to control this weed.

  • Biological: There are 12 insect species cleared by the USDA for use in controlling diffuse knapweeds. Each attack reduces the plant's overall seed production by weakening the plant or eating the seeds.
  • Chemical: Herbicide application is effective. Chemical use should be carefully coordinated with the use of biological controls.
  • Mechanical: Consistent hand pulling can be effective. The Nature Conservancy reports initial infestations can be successfully controlled with pulling three times a year for 5 years.

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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