Photographer: Joseph M. DiTomaso Affiliation: University of California - Davis Source: Bugwood.org Copyright: (CC BY-NC 3.0)
Centaurea calcitrapa is a multi-branches annual, biennial or short-lived perennial plant that grows over 3 feet tall. Below the flower heads are spine-tipped bracts and then longer spikes up to 1.2 inches in length. Upper leaves are narrow and undivided while lower leaves are deeply divided. There are about 25-40 flowers per head.
Listed as a noxious weed in 6 Western United States and is prohibited in Arizona. The prevention of seed production is mandated in those states and it is marked as a serious threat.
This plant reproduces by seed and are spread as one unit with the seed head. However, buried seeds can remain dormant for 3 years. From July until September the flowers bloom and from August to October the seeds ripen. When first growing, seedlings look like rosettes with straw-colored spines.
Accidentally through the commercial seed trade. Exact introduction year is not known but thought to have entered in the 1800s.
U.S. Habitat: Heavy, fertile and alluvial soils.
U.S. Present: AL, AZ, CA, DC, IA, IL, IN, MD, NJ, NM, NY, OR, PA, UT, VA and WA
Similar to the Yellow Star thistle (Centaurea solstitialis) but there is a color differentiation on the flowers. Also resembles other thistles from the genus Centaurea (especially the Iberian Starthistle, Centaurea iberica) and can be easily confused for them.
Timing is key for effective control of knapweeds, early detection and rapid response can stop the weeds. Management problem should be a combination of mechanical, chemical, cultural, biological controls. Fertilizer applications and poorly timed mowing can encourage survival. Rosettes are usually too low to be affected by mowing. Mowing mature flower stems disperses seed and can stimulate re-growth of stems. Burning removes current growth, but may enhance seed germination. Hand pulling 2-4 times per year or severing plants at least 2 inches below crowns can control small infestations. The Knapweed peacock fly (Chaetorellia acrolophi) has shown some progress in Montana and Oregon against several Centaurea species but is not available for general redistribution.
DiTomaso, J. M., & Healy, E. A. 2007. Weeds of California and other western states (Vol. 3488). UCANR Publications. Print.
Graebner, R. C., Callaway, R. M., & Montesinos, D. 2012. Invasive species grows faster, competes better, and shows greater evolution toward increased seed size and growth than exotic non-invasive congeners. Plant Ecology, 213(4):545-553.
Müller‐Schärer, H., & Schroeder, D. 1993. The biological control of Centaurea spp. In North America: Do insects solve the problem? Pesticide science, 37(4):343-353
Pitcairn, M. J., Young, J. A., Clements, C. D., & Balciunas, J. O. E. 2009. Purple Starthistle (Centaurea calcitrapa) Seed Germination1.