Diffuse Knapweed (Centaurea diffusa)

County Management Strategy: Eradication throughout the County.

Identification: An annual or short lived perennial weed that is profusely branched and grows to 2 feet tall. Rosette and stem leaves are deeply lobed. Produces numerous white to pale lavender flowers that bloom in early to mid summer. Bracts below the petals are spine tipped, with comb-like spines and are prickly to the touch. Plants break off at the base and become tumbleweeds, facilitating seed spread.

Other names: None

Similar Species: Russian knapweed stem leaves are not lobed and its flowers are darker purple. Spotted knapweed has spots on the bracts below the flowers. There is some evidence that diffuse and spotted knapweed can interbreed and both species can be found growing in the same patch.

Control Timing: In the spring and early summer during the rosette or very early bolting stages.

Control target: Prevent seed production.

Control Methods: Mechanical removal is effective on rosettes and plants in the early bolting stage. Herbicides are effective tools if applied before flowering. Once the plants have flowered, they should be removed manually and bagged to prevent seed spread. Biological control is not recommended in Mesa County because eradication is the management goal for this species.

Status in Mesa County: An infestation exists at the Garfield/Mesa County line on I-70 and frontage roads and south-southwest of Debeque. It has been found on private property on Silt Divide Road (48.5 Road). Scattered plants may exist along I-70 throughout Mesa County. Infestations exist in the Bookcliffs on Bureau of Land Management land; it is being treated. A large infestation was found in 1998 on property owned by Public Service Company (Excel Energy) located east of 34 Road on C Road. This patch is being controlled with herbicides and digging and is being monitored by the Mesa County Division of Pest Management. REPORT INFESTATIONS TO THE DIVISION OF PEST MANAGEMENT!

Photos:
http://www.invasive.org/species/subject.cfm?sub=4472

Fact sheets and control:
http://www.ext.colostate.edu/pubs/natres/03110.html

Additional Links:
http://www.cnr.uidaho.edu/rx-grazing/Forbs/Diffuse_Knapweed.htm