Centaurea cyanus 'Black Ball'

• The RHS Award of Garden Merit (AGM) helps gardeners choose the best plants for their garden.
• 
  RHS Plants for Pollinators plants

  This plant will provide nectar and pollen for bees and the many other types of pollinating insects.

  It is included in an evolving list of plants carefully researched and chosen by RHS experts. Divided into 3 groups these lists, linked below, are maintained by a team of RHS staff and are reviewed annually.

  • Garden Plants
  • Wildflowers
  • Plants of the World

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)