Star thistle

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The star thistles, cornflowers and knapweeds are a group of thistle-like plants in the Genus Centaurea of the Family Asteraceae. Some 350 species of herbaceous flowering plants belong to Centaurea, most native to the Old World. Some species are cultivated as garden ornamentals.

Species in this genus include:

Centaurea nigra – Black knapweed or Common knapweed
Centaurea scabiosa – Greater knapweed
Centaurea cyanus – Cornflower
Centaurea calcitrapa L. – The Purple star thistle or Caltrop is a native of Europe. The Caltrop has spiny, pink-purple flower heads. It can be annual or biennial and has been naturalized in America. The common name "Caltrop" comes from the resemblance of the spiny flower head to the caltrop, an ancient instrument of war.
Centaurea iberica Sprengle – A spiny Mediterranean species thought to be the "thistle" mentioned in Genesis

Yellow star thistle

Centaurea solstitialis L. – The Yellow star thistle, shown in the picture at right, is an annual that grows to a height of 1 to 2.5 feet. This plant is widely naturalized outside of Europe where it is native.
Centaurea repens L. – Russian knapweed (also Turkestan thistle) is a perennial, native to southern Russia and Asia Minor to Altai and Afghanistan. It is a pernicious weed within parts of its native range as well as in places where it has been accidentally naturalized. The flower head is lilac in color and not armed with spines.

Centaurea are copious nectar producers, especially on high lime soils, and are major honey plants for beekeepers. Star thistle varietal honey is light and slightly tangy. It is one of the finest honeys produced in the US, but as it is abundant, some of it is fraudulently relabeled and sold as the scarce, but high priced sourwood honey of the Appalachian Mountains.

Bumblebee on Star Thistle
Chenango Bridge, New York

Bibliography

Mabberley, D.J. 1987. The Plant Book. A portable dictionary of the higher plants. Cambridge University Press, Cambridge. 706 p. ISBN 0 521 34060 8.
Robbins, W.W., M. K. Bellue, and W. S. Ball. 1970. Weeds of California. State of California, Dept. of Agriculture. 547 p.