Yellow Starthistle 
(Centaurea solstitialis)

click on the photo to enlarge

County Management Strategy: Eradication throughout the County.

Identification: A tap rooted annual weed growing to 2 feet tall. The rosettes are 6 to 8 inches across and look very similar to a dandelion rosette. Rosette leaves have a distinct triangular tip. The yellow flowers are about ½ inch wide and bloom throughout the summer. Seed production is very high. Bracts at the base of the petals are armed with stout spines up to 1½ inch long. No other part of the plant has spines. Leaves are reduced and grayish green. Plants are much branched and spindly looking.

Other names: None

Similar Species: Curlycup gumweed is much greener, rosette leaves are less lobed, and the flower bracts are not armed with spines, but have curled, soft bract tips. Buffalo bur, a native weedy species, has spines all over the leaf and stem surfaces, and has yellow bell shaped flowers. The leaves are broad and deeply lobed. Several other species in the aster family, such as wild lettuce, sowthistles, and dandelions, have yellow flowers but none are stoutly spined.

Control Timing: In the rosette to early bolting stage during spring and early summer.

Control target: Prevent seed production.

Control Methods: Mechanical control works well on small infestations but is labor intensive for large infestations. Herbicides can be applied during the rosette to early bolting stage. Repeat applications are necessary because the seeds germinate over the entire summer. A seedhead fly, accidentally introduced to California, feeds on seeds but is currently not available. However, biological control is not recommended in Mesa County because eradication is the management goal for this species. Good pasture management is necessary to keep starthistle populations from exploding.

Status in Mesa County: There is a large area of infestation south of the town of Mesa in the Coon Creek Estates area. A smaller infestation occurs on Glade Park on DS Road about 3 miles east of the state line. Mesa County is actively involved in monitoring infestations and working with landowners on eradication of these patches. REPORT INFESTATIONS TO THE DIVISION OF PEST MANAGEMENT!

Photos:
http://www.ndweeds.homestead.com/Yellowstarthistle.html

Fact sheets and control:
http://www.atasteofeldorado.com/starthistle.html
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7402.html
http://www.nwcb.wa.gov/weed_info/ystarthistle.html