Centaurea nigra - Common Knapweed

Phylum: Magnoliophyta - Class: Equisetopsida - Order: Asterales - Family: Asteraceae

Common knapweed, Centaurea nigra

Equally well known as Black Knapweed, notably in the English Midlands these thistle-like wildflowers are also referred to as Hardheads, because the buds and flower heads are firm and solid.


A hairy perennial with stiff erect ribbed stems, Common Knapweed grows up to a metre in height and its upper part branches freely. Knapweeds are readily distinguished from thistles by the absence of spines and prickles.

Common knapweed - closeup of flowers

Common Knapweed leaves are dull green and finely hairy, but in other respects they vary greatly. The upper leaves are usually narrow and entire (without lobes) and untoothed, while the lower leaves are sometimes lobed and have coarse teeth.

The flower-head is hard and solid, a mass of dark-brown-fringed green bracts overlapping over each like roof tiles. The flowers, which are hermaphrodite (having both male and female reproductive organs) look rather like pinkish-purple (very occasionally white) shaving brushes, with the bracts serving as stubby handles.

Rare white form of Centaurea nigra

Above: a pinkish-purple Common Knapweed flower and the rare white form

Each 'flower' comprises many tubular florets, and generally all are of similar length; however, rayed forms of Common Knapweed do sometimes occur, when the outer florets are more like those of Greater Knapweed Centaurea scabiosa. (The bracts of these species are very different: green bracts in the case of Greater Knapweed; brown in Common Knapweed.)

Rayed form of Common Knapweed in a hay meadow, Wales


Very common throughout Most of Britain and Ireland except for the far north of Scotland, where it is an occasional find, Common Knapweed is a European native species that has been introduced to many other parts of the world including North America, where it has in places become a nuisance weed because it so easily invades places where the soil has been disturbed.


Common Knapweed grows wherever grass is not closely cropped. It is often abundant beside lakes and streams in southern Britain, especially where grazing animals have been fenced off from themargins.

Blooming Times

In Britain and Ireland Common Knapweed flowers first appear in June and continue into September.

Peacock butterfly on Common Knapweed


In 14th century Britain this wildflower was known as Matfellon, and it was eaten with pepper at the start of a meal to stimulate the appetite. Knapweed flowers are edible and can be added to salads, but the tough bracts are definitely not worth trying. In Wales, the Physicians of Myddfai included Common Knapweed with many other herbs in a potion to counteract the toxins in Adder bites. (We strongly advise against eating or using as medicines any plants without first obtaining professional advice.)

Knapweed at Borth Bog

Insects, including bees and butterflies, are very fond of these long-flowering plants. In central and southern Britain, the Six-spot Burnet Zygaena filipendulae is a common sight on the flowers of Common Knapweed Centaurea nigra and other members of the Centaurea genus.


Centaurea, the genus name, comes from the Centaur Chiron, who is reputed to have used flowers of this genus as a poultice to cover a festering wound made by an arrow dipped in Hydra's blood. The wound was cured and thereafter, so the story goes, cornflowers and knapweeds were given the name Centaurea. The specific epithet nigra means black.

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