Spotted Knapweed 
(Centaurea maculosa)

County Management Strategy: Eradication throughout the County.

Identification: A biennial or short lived perennial weed that is profusely branched and grows to 3 feet tall. Plants have a stout tap root. Rosette leaves strongly lobed or not and 3 to 6 inches long. Flowers are pink to magenta, rarely white, and occur singly on the tips of branches. Bracts below the flower petals have dark, toothed margins that look like spots.

Other names: None

Similar Species: Diffuse knapweed has a comb-like fringe on the bracts and no spots. Russian knapweed has soft, greenish-tan bracts without a fringe or spots. 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 during the rosette or early bolting stage.

Control target: Prevent seed production.

Control Methods: Mechanical control may work for small infestations, but must be repeated because shoots can arise from the tap root. Digging up the entire root is preferable but labor intensive. Tillage at the rosette stage can be effective. Herbicides are effective if applied to the rosette stage. Mature flowers should be removed and bagged to prevent seed spread. A few biocontrol agents are available, including seed and stem feeders. However, biological control is not recommended in Mesa County because eradication is the management goal for this species.

Status in Mesa County: Very small infestations exist in Mesa County on Glade Park, on Silt Divide Road, on Lands End Road, and on Highway 65 north of Powderhorn. Scattered infestations are being treated in the Bookcliffs near the Garfield County line. Often found growing with diffuse knapweed. REPORT INFESTATIONS TO THE DIVISION OF PEST MANAGEMENT!

Photos:
http://www.ppws.vt.edu/scott/weed_id/cenma.htm

Fact sheets and control:
http://www.ext.nodak.edu/extpubs/plantsci/weeds/w842w.htm
http://www.ext.colostate.edu/pubs/natres/03110.html

Additional Links:
http://www.ceris.purdue.edu/napis/pests/skw/