Centaurea cyanus 'Blue Boy'

RHS Plant Profile
cornflower 'Boy Blue'

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Annual Biennial

An annual cornflower, to around 80cm high in flower, with lance-shaped, grey-green leaves. Showy flower-heads 3-4cm across, with vivid blue florets crowded around the outside and dark purple stamens in the centre, are produced all through the summer

Position

Full sun

Soil Types

Chalk, Loam, Sand

Max Height

0.5-1 metres

Max Spread

0.1-0.5 metres

Size

Time to Maturity
1 year
Max Spread
0.1-0.5 metres
Max Height
0.5-1 metres

Growing Conditions

Chalk Loam Sand
Moisture
Well–drained
pH
Acid or Alkaline or Neutral

Position

Full sun
Aspect
South–facing or West–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)
H6
Drought Resistance
Yes

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
Accepted

How to Grow

Cultivation

Grow in almost any well-drained soil in full sun. Thrives in poor soils but will not tolerate waterlogging

Propagation

Propagate by seed (sown outdoors) in early spring. Sow successionally rather than all at once for a longer show of flowers

Suggested planting locations and garden types

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

Pruning

Deadhead to encourage more flowers, unless seed is required

Pests

Generally pest-free

Diseases

May be susceptible to downy mildews, powdery mildews and rust diseases