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We have combined these two powerful search tools into a single Find a Plant service searching over 250,000 plant records.
Virtually all of the features of the old searches are still available and in addition we have added several new features to create a more comprehensive and user friendly search experience.
When using this search
Search by plant name, key attributes or both to find plant details and a list of
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.
© RHS 2002
the RHS today and get 12 months for the price of 9
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
Centaurea may be annuals, perennials or subshrubs, with simple or pinnately lobed leaves and showy thistle-like flowers-heads, often with enlarged outer florets
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
NW to C Europe
All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets
Aspect South-facing or North-facing or West-facing or East-facing
Exposure Exposed or Sheltered
MoistureMoist but well-drained
SoilChalk, Clay, Sand, Loam
pHAcid, Alkaline, Neutral
Time to ultimate height
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
Propagate by seed
Suggested planting locations and garden types
Wildflower meadow Wildlife Gardens
Pruning No pruning required
Pests Generally pest free. Resistant to browsing by deer
May be affected by powdery mildews
RHS members can get exclusive individual advice from the RHS Gardening Advice team.
Buy from £6.99 at the RHS Plants Shop
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.