• AGM plants

    AGM plants have been through a rigorous trial and assessment programme. They are:

    • Excellent for ordinary use in appropriate conditions
    • Available to buy
    • Of good constitution
    • Essentially stable in form & colour
    • Reasonably resistant to pests & diseases

Centaurea macrocephala

giant knapweed

  • Other common names giant knapweed
    great-headed centaury

  • Synonyms Grossheimia macrocephala

  • Family Asteraceae

  • Genus Centaurea may be annuals, perennials or subshrubs, with simple or pinnately lobed leaves and showy thistle-like flowers-heads, often with enlarged outer florets

  • Details C. macrocephala is a large, clump-forming perennial with simple or lobed leaves and yellow flower-heads 5cm across opening in summer from brown buds

  • Plant range Caucasus

Characteristics

  • Foliage Deciduous

  • Habit Clump-forming

  • Hardiness

    Hardiness ratings

    All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

    • H1a - Under glass all year (>15C)
    • H1b - Can be grown outside in the summer (10 - 15)
    • H1c - Can be grown outside in the summer (5 - 10)
    • H2 - Tolerant of low temperatures, but not surviving being frozen (1 to 5)
    • H3 - Hardy in coastal and relatively mild parts of the UK (-5 to 1)
    • H4 - Hardy through most of the UK (-10 to -5)
    • H5 - Hardy in most places throughout the UK even in severe winters (-15 to -10)
    • H6 - Hardy in all of UK and northern Europe (-20 to -15)
    • H7 - Hardy in the severest European continental climates (< -20)

    H7

Colour

Sunlight

  • Full Sun

  • Partial Shade

  • Aspect West-facing or South-facing or North-facing or East-facing

  • Exposure Sheltered

Soil

  • Clay

    Clay

  • Chalk

    Chalk

  • Sand

    Sand

  • Loam

    Loam

  • MoistureMoist but well-drained

  • SoilClay, Chalk, Sand, Loam

  • pHNeutral, Acid, Alkaline

Size

  • Ultimate height 1-1.5 metres

  • Ultimate spread 0.1-0.5 metres

  • Time to ultimate height 2-5 years

How to grow

Cultivation Best grown in moist but well-drained soil in sun or partial shade

Propagation Propagate by seed sown in pots in a cold frame in spring or propagate by division in spring or autumn

Suggested planting locations and garden types Cut Flowers Flower borders and beds Coastal Cottage & Informal Garden Prairie Planting Wildlife Gardens

How to care

Pruning Deadhead regularly to prolong flowering

Pests Generally pest free

Diseases May be affected by powdery mildews

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