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All ratings refer to the UK growing conditions unless otherwise stated. Minimum
temperature ranges (in degrees C) are shown in brackets
AGM plants have been through a rigorous trial and assessment programme. They are:
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.
'Alba' is a vigorous rhizomatous perennial forming a mat of undivided grey-green leaves, and woolly stems bearing white flower-heads, often tinged pink, in late spring and early summer
C. atropurpurea is a clump-forming perennial to 1m with deeply divided, grey-green leaves. The deep red flowers emerge from bronze-coloured, scaly buds on strong stems in summer
'Amethyst Dream' is a spreading, herbaceous perennial with grey-green foliage covered in silvery hairs. The bright purple, thistle-like flowers open from scaly buds in late spring and early summer
C. cyanus is an upright annual to 75cm, with simple or slightly lobed leaves and solitary deep blue flower-heads 3-4cm across in late spring and summer
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
'Amethyst on Ice' is a herbaceous perennial up to 40cm high with broadly lance-shaped green leaves and flowerheads with white ray florets and purple central discs from late spring to late summer; Centaurea flowers are attractive to bees
'John Coutts' is a vigorous herbaceous perennial, with jagged lobed leaves white-felted beneath, and showy purplish-pink flower-heads 6cm across in summer and often into early autumn
'Amethyst in Snow' is a spreading, herbaceous perennial with grey-green foliage covered in silvery hairs. The thistle-like flowers, white with purple centres, open from scaly buds in late spring and early summer
'Steenbergii' is a perennial forming a clump of pinnately lobed leaves, grey-hairy beneath, with deep pink flower-heads in summer
C. cineraria subsp. cineraria is a sprawling, evergreen perennial with silvery, blue-green foliage, woolly on both sides. It produces purple flowers in summer
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We're a UK charity established to share the best in gardening. We want to enrich everyone's life through plants, and make the UK a greener and more beautiful place.