Centaurea macrocephala

RHS Plant Profile
giant knapweed

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giant knapweed RHS 2002

Other common names

great-headed centaury

Synonyms

Grossheimia macrocephala

Plants for pollinators
Herbaceous Perennial
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A large, clump-forming perennial with simple or lobed leaves and yellow flower-heads 5cm across opening in summer from brown buds

Position

Full sun, Partial shade

Soil Types

Chalk, Clay, Loam, Sand

Max Height

1-1.5 metres

Max Spread

0.1-0.5 metres

Size

Time to Maturity
2–5 years
Max Spread
0.1-0.5 metres
Max Height
1-1.5 metres

Growing Conditions

Chalk Clay Loam Sand
Moisture
Moist but well–drained
pH
Acid or Alkaline or Neutral

Position

Full sun Partial shade
Aspect
West–facing or South–facing or North–facing or East–facing
Exposure
Exposed or Sheltered
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 & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Asteraceae
Native to GB/Ireland
No
Foliage
Deciduous
Habit
Clump forming
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
Name Status
Correct
Plant Range
Caucasus

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

  • Coastal
  • Cottage and informal garden
  • Wildflower meadow
  • Wildlife gardens
  • Cut flowers
  • Flower borders and beds

Pruning

Deadhead regularly to prolong flowering

Pests

Generally pest-free

Diseases

May be susceptible to powdery mildews