A diverse family of annuals and hardy perennials with a long flowering period. Loved by bees and other pollinators. Perennial forms make excellent border plants.
Botanical Name: Centaurea (kent-ow-ree-a)
Common Names: Knapweed, perennial cornflower.
Foliage: Deciduous, simple or pinnately lobed leaves.
Flowers: Open thistle-like flower heads of tube-shaped florets. Usually in shades of blue but white, pink and red forms are available.
Flowering Period: June, July, October.
Soil: Moist but well-drained, fertile soil (chalk, clay, sand or loam). Acid, alkaline or neutral pH.
Conditions: Full sun or partial shade. Can be planted in any aspect.
Habit: Bushy, upright.
Type: Herbaceous perennial.
Hardiness: Most varieties are fully hardy in the UK.
Easy to grow. Plant in a warm sunny position, in any moist but we'll drained soil. Ideal for flower beds or front/middle of the border.
Long lasting as cut flowers. Cut when partly open for best display.
Stake in exposed sites. Divide established clumps every three years or so in the spring to rejuvenate plants.
Remove faded blooms to prolong flowering period.
Cut down stems to ground level in winter.
Can be affected by powdery mildew. Usually pest free.
Sow seeds of perennials in pots in a cold frame in spring or propagate by division in spring or autumn.
For annual species see Cornflowers.
Centaurea dealbata bears pinkish purple flowers. Height to 2ft (60cm). For sale at Thompson & Morgan. Popular cultivars include:
Centaurea Jordy bears intense, near-black flowers. For sale at Crocus.
Centaurea macrocephala is a tall upright form with yellow globular flower heads. Height to 3ft (90cm) or more.
Centaurea montana bears blue flowers in May. Height to 18in (45cm). For sale at Suttons. Popular cultivars include:
Centaurea nigra (black knapweed or common knapweed) a tough wildflower bearing a whorl of brown/black bracts topped with a light purple/pink inflorescence, which sometimes can be white. For sale at Crocus.