Saturday, August 28, 2010

Centaurea scabiosa – Greater Knapweed

Warning: Centaurea scabiosa has a high potential of becoming an invasive pest plant. It is included here only as an aid to identification and not to promote its cultivation in any way whatsoever.

Centaurea scabiosa Linnaeus is native to Siberia and Europe (USDA, ARS 2010) and is distinguished by its deeply lobed leaves and flower heads borne singly at the ends of the stems. The flower heads are subtended by broad, usually pinkish or purplish bracts with the margins fringed at the apex. The outer florets in each flower head are spreading, enlarged, and neuter (that is, sterile and not producing seeds). The plant is a perennial with thick, woody rootstocks that form a large, conspicuous rosette of leaves early in the year and from which arise flowering stems 1–3-feet tall, with the flowers present from July to August or September. Within its natural range, it favors chalky or limestone soils and the image above was taken by John Jordan in "south-west England on the edge of Salisbury Plain, a chalky hillside which used to be grazed pasture many years ago, but is now common land trimmed once a year." Ecologically, it is notable for attracting a wide variety of nectar- and pollen-seeking insects; as the sole source of food for the caterpillars of the case-bearing moth, Coleophora didymella Chr├ętien; and, in Britain, it is usually the only host of the parasitic plant, knapweed broomrape (Orobanche elatior Sutton) (Rumsey & Jury 1991:267). Besides greater knapweed, the plant is also known as greater centaury, hardhead, and ironhead, the latter two common names being a reference to the hard, solid flower heads (Grieve 1971:456).

Greater knapweed is now well established in Canada and throughout a wide range in the northern United States. Although it is definitely weedy, it is not currently listed as ecologically invasive in North America; however, due to the tendency of Centaurea species to become invasive pest plants in the northern United States and Canada, its cultivation should not be encouraged and plants growing in the proximity of natural areas should be closely observed and eliminated if seen to be spreading at the expense of the native flora. Greater knapweed might be mistaken for spotted knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek (Centaurea maculosa auct. non Lam.), which is an invasive pest plant, since both plants have deeply lobed leaves. However, the latter has much smaller flower heads up to 1-inch wide, whereas greater knapweed has larger flower heads that are 2 inches wide or wider.

Observant readers may have noticed the red-spotted black moth at the base of the flower head. It is a burnet moth in the genus Zygaena, a group of day-flying moths that sequester cyanogenic glucosides from various members of the pea family that serve as food for the caterpillars, although George (2010) reports that these moths can also manufacture cyanogenic glucosides and do not need to obtain them from the larval food plants. The boldly patterened wings are an example of aposematic coloration, which is also known as warning coloration and serves as a visual cue to predators that the moth is distasteful and toxic.


United States Distribution Map for Centaurea scabiosa
Map courtesy of the United States Department of Agriculture Plants Database.

References:

  • George, H. 2010. Insects That Make Cyanide. suite101.com. Internet
  • Grieve, M. 1971. A Modern Herbal, Volume 2. Mineola, New York: Dover Publications. Internet
  • Rumsey, J. and S.L. Jury. 1991. An account of Orobanche L. in Britain and Ireland. Watsonia 18:257–295. Internet
  • USDA, ARS, National Genetic Resources Program. 2010. Germplasm Resources Information Network (GRIN) Online Database. National Germplasm Resources Laboratory, Beltsville, Maryland. Internet

Image © 2010 by John Jordan and used by permission. Text © 2010 by Rufino Osorio.

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