• 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 nigra

common knapweed

  • Other common names common knapweed
    hardheads

    bell weed
    black centaury
    black knapweed
    bowl weed
    bull weed
    button weed
    churl's head
    club weed
    crop weed
    hardhead
    hard weed
    horse knops
    horse knots
    iron hard
    iron heads
    iron weed
    knob weed
    knot weed
    lesser knapweed
    loggerheads
    nob weed
    Spanish buttons
    sweeps
    tassel

    see more
  • 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. nigra is an upright, branching perennial to 75cm, with simple or pinnately lobed leaves and thistle-like purple flowerheads to 4cm across, with conspicuous blackish involucres

  • Plant range NW to C Europe

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)

    H6

Colour

Sunlight

  • Full Sun

  • Partial Shade

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

  • Exposure Exposed or Sheltered

Soil

  • Chalk

    Chalk

  • Clay

    Clay

  • Sand

    Sand

  • Loam

    Loam

  • MoistureMoist but well-drained

  • SoilChalk, Clay, Sand, Loam

  • pHAcid, Alkaline, Neutral

Size

  • Ultimate height 0.5-1 metres

  • Ultimate spread 0.1-0.5 metres

  • Time to ultimate height 2-5 years

How to grow

Cultivation Prefers a well-drained fertile soil in a sunny position. Tolerant of dry, alkaline soils and established plants are tolerant of considerable neglect. Self-seeds readily and can become invasive

Propagation Propagate by seed

Suggested planting locations and garden types Low Maintenance Wildflower meadow Wildlife Gardens

How to care

Pruning No pruning required

Pests Generally pest free. Resistant to browsing by deer

Diseases May be affected by powdery mildews

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