At a Glance
- Annual, biennial, or short-lived perennial
- Highly branched stems that grow up to three feet tall.
- Stems and leaves have fine, cobwebby hairs when young.
- Basal rosette has a central cluster of spines.
- Stem leaves are five to eight inches long and have dots of resin on the surfaces.
- Purple flowers are subtended by green to straw-colored, spine-tipped bracts.
- Flowers June through November.
- Fruit is a white achene with brown streaks.
Habitat and Ecology
Purple starthistle (Centaurea calcitrapa) is found along the West Coast, in the Southwest, and in scattered eastern states. Native to southern Europe and northern Africa, it was introduced into California in the early 1900s. Purple starthistle is highly competitive, and because it is unpalatable, it increases on rangelands as more palatable species are consumed. Purple starthistle can colonize most soil types with a disturbed top layer, but it seldom persists in shaded areas. It is found in fields, roadsides, disturbed open sites, grasslands, overgrazed rangelands, and logged areas, from sea level to 5,500 feet in elevation.
Purple starthistle is a bushy thistle with highly branched stems and a stout taproot. It can act as an annual, a biennial, or a short-lived perennial. It first grows a basal rosette and later produces stems in late spring and summer. The stems grow up to three feet tall. Purple starthistle reproduces by seeds that can remain in the soil for three years. The seedhead falls as a unit near the parent plant, but the seeds can then disperse via vehicles, heavy machinery, water, and soil movement, and by clinging to shoes, clothes, tires, and fur and feathers of animals. Infestations can result in dense, impenetrable stands.
Before growing flowering stems, purple starthistle exists as a basal rosette with a central cluster of spines. On the stems, the leaves are alternate, with deeply divided lower leaves and narrow and undivided upper leaves. The upper stem leaves are not winged. The leaves are five to eight inches long and have dots of resin on their surfaces. Young leaves and stems have fine, cobweb-like hairs that fall off in time.
Flowers and Fruits
Purple starthistle flowers are purple and 0.6 to 1 inches in diameter. Twenty-five to 40 florets make up each flower head. Underneath the flower are spine-tipped bracts that are greenish or straw-colored. Purple starthistle flowers June through November.
The fruit is an achene (a dry fruit with a single seed and thin walls that does not open at maturity; for example, a sunflower “seed”). Purple starthistle achenes are oblong and 2.5-3.5 mm long. They are white and often streaked with brown. Usually, no bristles cap the achene.
Centaur’ea/Centaur’ium is a reference to the Centaur Chiron who supposedly discovered the medicinal uses of a plant in Greece that came to be called Centaury. Calcitra’pa comes from the word caltrop, which was a four-pointed weapon usually positioned on the ground to impede enemy movements.
Purple starthistle can be confused with other invasive Centaurea species, like spotted knapweed (Centaurea maculosa), diffuse knapweed (Centaurea diffusa), squarrose knapweed (Centaurea squarrosa), and Iberian starthistle (Centaurea iberica). See https://www.cdfa.ca.gov/plant/ipc/encycloweedia/weedinfo/centaurea.htm for comparisons among these species.
Possible control methods are explained at these websites:
California Department of Food and Agriculture. No date. Centaurea in Enycyloweedia. Available at https://www.cdfa.ca.gov/plant/ipc/encycloweedia/weedinfo/centaurea.htm.
Charters, M. L. 2009. California plant names: Latin and Greek meanings and derivations. Available at http://www.calflora.net/botanicalnames (accessed 9 March 2010).
Keil, D. J., and J. Ochsmann. 1997. Centaurea. In: Flora of North America Editorial Committee, editors. 1993+. Flora of North America North of Mexico. 15+ vols. New York and Oxford. Vol. 19, 20, and 21, pp. 191.
Nevada Cooperative Extension. Wanted—dead, not alive! Purple starthistle. University of Nevada, Reno, NV.
San Francisco Peaks Weed Management Area. No date. Purple starthistle, Centaurea calcitrapa. Flagstaff, AZ.
Prepared by Kelly Reeves, Southern Colorado Plateau Network Inventory and Monitoring Program, 2010.
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.
Rejmánek, M. 2007. Weeds of California and Other Western States. Madroño, 54(4), 361-363. Reviewed by JM DiTomaso and EA Healy.