GIANT KNAPWEED

Centaurea macrocephala

Daisy & Dandelion Family [Asteraceae]

month8jul month8july month8aug month8sep month8sept

status
statusZneophyte
flower
flower8yellow
petals
petalsZmany
petals
petalsZ5
stem
stem8round
stem
stem8ribbed

7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George


12th July 2016, a garden, Shebdon, London. Photo: © George
Up to 1m tall, it is a neophyte not native to the UK but rather from the Caucasus.


11th July 2016, a garden, Shebdon, London. Photo: © George


12th July 2016, a garden, Shebdon, London. Photo: © George
At the base the leaves are quite long and taper to a point fairly abruptly at ~70° near the end.


11th July 2016, a garden, Shebdon, London. Photo: © George
Although it can be quite tall it is the flower-head which is 'giant' in this case, hence the specific epithet 'macrocephela' (big head) in the scientific name. The round to angular stem thickens gradually but considerably towards the flowerhead whereas the leaves get shorter and narrower. Leaves lanceolate and only infrequently toothed. the phyllaries (seen best in the spent specimen below) are large, pale brown and papery which conceals the whole of the capitulum.


7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George


11th July 2016, a garden, Shebdon, London. Photo: © George
Most unusual for a knapweed, the flowerhead is yellow (rather than purple). Stem slightly white-hairy. Leaves peel off the stem at intervals.


11th July 2016, a garden, Shebdon, London. Photo: © George
The flowers consist mostly of disc-florets, each with their 5 petals.


7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George
The styles are the longest parts of the flowers, extending way beyond the opening of the tubular disc floret with its 5 splayed-out petals. At the summit, occupying perhaps 1/10th of its total length a stigma perches on a discoidal part and splays out splitting into two near the top.


7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George
Each disc floret has 5 'petals', but in the case of Giant Knapweed, these 'petals' are very long, as is the tube which bears them.


7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George
When finished flowering it aquires a Boris Johnson flop-over mop hairstyle before it enters the fruiting stage. It does not seem to have any parachute hairs for the individual seeds to take to the air.


11th July 2016, a garden, Shebdon, London. Photo: © George
Leaves lanceolate, pale green, with even paler mid-rib. The fruiting stage after flowering.


7th July 2019, a canalside garden, Parbold, Lancs. Photo: © George
The curled up, long-toothed, brown phyllaries are all that you can now see - like an old-style hair-do.


Photo: © George
The curled over phyllaries.


Not to be semantically confused with : Greater Knapweed (Centaurea × gerstiaueri) [another Knapweed, but one with a more normal purple inflorescence]

Some similarities to : Yellow Thistle (Cirsium eristhales) but that has several yellow thistle-like flowers clustered at the top and prickly leaves. Also to Yellow Star-thistle (Centaurea solstitialis), but although this is indeed another knapweed and has only one inflorescence per main-stem, it has strong yellow spines which protrude from the phyllaries which again are underneath the thistle-like but yellow inflorescence atop.

Slight resemblance to : Safflower (Carthamus tinctorius) which also has a yellow inflorescence, but that all emerges from a what could be described as a narrow-necked globular container shaped rather like a gourd.

Uniquely identifiable characteristics

Distinguishing Feature :

It is an introduced but naturalised garden species, rare on waste or rough ground in England, but has, since 1985, occupied an abandoned orchard in East Suffolk.


  Centaurea macrocephala  ⇐ Global Aspect ⇒ Asteraceae  

Distribution
 family8Daisy & Dandelion family8Asteraceae
 BSBI maps
genus8Centaurea
Centaurea
(Knapweeds)

GIANT KNAPWEED

Centaurea macrocephala

Daisy & Dandelion Family [Asteraceae]