Diffuse Knapweed (State designated weed)


Diffuse Knapweed (State designated weed)

  • Centaurea diffusa
  • Asteraceae (Sunflower family)

Diffuse knapweed was introduced to the U.S. from the Mediterranean region of Eurasia. It infests roadsides, waste areas and dry rangelands, and threatens to exclude many desirable species. It is a tough competitor on dry sites and rapidly invades and dominates disturbed areas.

Growth Habit: Annual or short-lived perennial, bushy, up to 2 feet tall. Rosette formed first year, flowering stalk elongates second year.

Leaves: Grayish-green, alternate, basal leaves whorled, upper leaves much reduced. Covered with fine hair.

Stem: Upright, single main stem from a rootstock, branched near or above the base.

Flowers: Solitary, usually white, sometimes pink, rose or lavender; papery seed head bracts end as sharp rigid spines. Diffuse knapweed can resemble spotted knapweed, which has black-tipped bracts that appear spotted. Flowering occurs from July to September.

Root: Elongated taproot.

Other: Reproduces only by seed. Each plant can produce about 18,000 seeds that are mostly spread as the plant breaks off at the base becoming a “tumbleweed.”

Status: State Designated Noxious Weed, Category 2 Fremont County. Localized, no tolerance of new infestations and aggressive control on all known infestations.

Control: Pulling or digging works well. Chemical controls can be effective. FCWP offers a 40 percent Cost Share on chemicals for controlling this species. Contact your local weed and pest office for further treatment recommendations.

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Lander, WY 82520


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