These are best described as perennial ‘Cornflowers’, though the wild ‘Knapweed’ is in fact a Centaurea too. They arein any soil as well as being long lived, and respond easily to division. C. steenbergii has been popular for several years as the best form of C. dealbata. It has deep pink in June and July, on very robust growth with greyish foliage, but the newer variety C. ‘John Coutts’ has even larger flowers of a more delicate shade of pink. C. macrocephala, is a giant with large green and stout 6 feet , carrying fluffy yellow flower heads from June to August. The species C. ruthenica is also tall but of more graceful appearance with dark green, deeply cut leaves and carrying lemon yellow flowers for a long season.
CENTAUREA dealbata steenbergii
C. pulchra major is handsome for its fine silver grey foliage and erect 3 feet spikes of large tufty pink flowers in June-July. Dwarf Centaureas have their place and nothing looks prettier than the 15 inches. C. hypoleuca, with its silver foliage and pink flowers and a much longer season than the May-June flowering C. montana varieties in pink, violet and purple. Other dwarfs, with good silver grey foliage are the 18 inches. C. rigi-difolia and the 9 inches. C. simplicicaule. The latter does not flower very freely, but makes quite attractive ground cover. Both these have pink flowers in June-July and are tidier growing than the rather vigorous C. montana which is rather floppy and making a good deal of spread for all its short flowering season. Generally, Centaureas are sun loving plants and as is often the case with subjects having grey or silver foliage, prefer to be in dry rather than damp, ill drained soil. Another generalisation applies to Centaureas, which is that those that flower early are best divided or planted in autumn, whereas later flowering kinds are best in spring, but nursery grown plants can be moved at either season.