Diffuse Knapweed

diffuse knapweed seedhead
diffuse knapweed rosette
diffuse knapweed habit

Diffuse Knapweed

Centaurea diffusa

Information Sheet (PDF)


Colorado List B - Control required in Jefferson County


General

  • Biennial (short-lived perennial)
  • Dead plants break off near ground and tumble
  • Family: Sunflower (Asteraceae)
  • Herbaceous
  • Rosettes formed the first year and bolt early in May to June of the next year


Habitat

  • Found in disturbed areas, such as overgrazed pastures and rangelands, roadsides, right-of-ways and gravel pits
  • Intolerant of shade and flooding
  • Prefers dry, light, porous soils


Plant


Vegetation

  • Contains chemicals that may inhibit other plants and irritate skin
  • Grayish green
  • Mature plants are 1 to 2 feet tall with numerous single flower heads at the tips of shoots


Roots

  • Tap root


Flower

  • Bracts are comb-like and tipped but can be distinguished from Spotted knapweed by the long, terminal spine
  • Second season: June to September
  • Size: small - 0.13 inch wide and 0.5 to 0.67 inch long
  • White, sometimes light purple


Seed

  • 15,000-plus seeds per plant
  • Seed may remain dormant in soil for more than 6 years
  • Seed spread by plants as they break off at the base and tumble


Seedling

  • Seedlings spend their first growing season as rosettes


Reproduction

  • Seed


Control


Biological

  • Agapeta zoegana, root-boring moth
  • Cyphocleonus achates, root weevil
  • Larinus minutus, seedhead weevil
  • Sphenoptera jugoslavica, root borer


Chemical


Cultural

  • Cultivation (when plants are 3 to 6 inches tall)
  • Grass competition – promote grass growth with fertilization, irrigation and reseeding
  • Hand pulling works when populations are small and isolated
  • Prevention – maintain health of site
  • Re-vegetation of highly disturbed sites


Mechanical

  • Burning - Not effective alone.  Conditions don’t support an effective fire.  May  promote flush of weeds, which can then be treated with herbicide
  • Grazing - Sheep, goats and cattle will feed on the plant
  • Mowing- Not effective alone.  Plants will regrow from crown and produce as many seeds as plants that don’t get mowed


Use all chemicals according to the manufacturer's label. No specific recommendation or endorsement is made or implied by listing methods or products.