Common Knapweed - Centaurea nigra sensu lato (=nigra/debauxii)

Alternative names
Knapweed and as Black Knapweed
Description

Medium to tall hairy plant with erect stems, branched or unbranched. Thickened below the flowerhead. Flowerheads purple, 20 to 40 mm, solitary or in branched clusters, the florets all equal in length. Flower bracts with a broad, blackish triangular fringed appendage, concealing the adjacent bracts. Centaurea nigra and Centaurea debauxii are very similar and require detailed examination (see ID aids below) - there are also other quite similar species and hybrids. Unless you are able to get expert confirmation to species level, please record Common Knapweed as Centaurea nigra sensu lato.

Similar Species

Greater Knapweed Centaurea scabiosa

C. debeauxii may be present, on more calcareous soils or introduced via seed or building rubble; it was formerly considered to be C nigra subsp. nemoralis

Centaurea jacea (Brown Knapweed) has brown phyllaries and is a very rare alien species. There is only one VC55 record from Evington in 2018, but it may be introduced in wildflower mixes. Its hybrid with C. nigra (C. x gerstlaureri) has dark brown phyllaries and has not been found in  VC55.

See the Identification Aid below for help on separating the Centaurea nigra look-alikes.

Identification difficulty
Identification aids

The Centaurea species and the subspecies in the table below all have black phyllaries with long, thin teeth in the apical portion.
For photos of phyllaries see here.
Note that Centaurea jacea (Brown Knapweed) has brown phyllaries and is a very rare alien species. There is only one VC55 record from Evington in 2018, but it may be introduced in wildflower mixes. Its hybrid with C. nigra (C. x gerstlaureri) has dark brown phyllaries and has not been found in  VC55.

To identify Centaurea you need to examine the phyllaries along the mid-line of the inflorescence only.

guideguide

Notes
1.    Despite its name, C. debeauxii does not always occur on chalk.  It may be introduced in wildflower seed mixes or with building materials.
2.    This variety is often introduced in wildflower seed mixes and could be confused with Greater Knapweed, Centaurea scabiosa, but the phyllaries are different.

Geoffrey Hall
Version 2: 03 May 2022

Habitat

Roadsides, flower meadows, hedgebanks and waste ground.

When to see it

June to September.

Life History

Perennial.

UK Status

Very common throughout Britain.

VC55 Status

Very common in Leicestershire and Rutland. In the 1979 Flora survey of Leicestershire it was found in 561 of the 617 tetrads.

Leicestershire & Rutland Map

MAP KEY:

Yellow squares = NBN records (all known data)
Coloured circles = NatureSpot records: 2020+ | 2015-2019 | pre-2015

UK Map