Centaurea, also called HARDHEADS, KNAPWEED, or STAR THISTLE, is a member of the Aster family, Compositae. There are 924 species in this genus of which six or more are cultivated. Of those Centaurea dealbata, macrocephala, and montana are cultivated perennials. They grow 3-3 ½ feet tall in Zones 3-9, prefer moderate watering, and flower in summer.

The blue-flowering Centaurea montana is a cultivated perennial that can be grown from seed, grows in full sun, and can be grown in containers. It flowers in summer – in spring as well in some settings. A white-flowering variant is available.

As a member of the Compositae family it will attract a long list of butterflies to feed. In particular, the following butterflies feed on Centaurea blossoms. Perhaps other gardeners agree that there is nothing common about the sight of either Junonia coenia or Pyrgus comunis.
Hylephila phyleus, Fiery Skipper
Junonia coenia, Common Buckeye
Pyrgus communis, Common Checkered-Skipper