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Common knapweed

Centaurea nigra
Flowers July-Sept. Bright purple flower heads resembling thistles. Small glands on margins of leaves distinguish it from Devil’s bit scabious. Some strains have more showy flowers with the outer ring of florets developing long bases.
Soil moisture tolerances: 
It is found on sites with 10-20 weeks dry soil per year and 10-20 weeks wet soil per year.
Soil fertility tolerances: 
It is typically found on moderately fertile sites (10-25 mg P/kg) or P index 1-2.
Traditional medicinal use: 
Other names include hardheads and paintbrush. Good nectar flower for insects including butterflies such as the gatekeeper, meadow brown, painted lady, red admiral and small copper. Also good nectar source for honey bees.
Further information: 

Click here for a link to the online Atlas of the British and Irish Flora with details of the plant ecology, distribution, photos and habitats: https://www.brc.ac.uk/plantatlas/index.php?q=plant/centaurea-nigra