Giant Knapweed

Giant Knapweed, West Dorset

Flowerhead

Close-up of florets

Rows of papery fringed phyllaries

Lower leaves

Upper leaves and unbranched stems
Centaurea macrocephala
Bighead Knapweed, Big Yellow Centaurea, Yellow Hardhat
Asteraceae
July to September
The escape is thinly scattered throughout the UK.
For a map see the Botanical Society of Britain and Ireland –
Giant Knapweed
It is a neophyte that was introduced into the UK from the Caucasus
in the early 1800s and grown as a striking garden plant.
It has escaped into the wild and has been described as both a
noxious weed and a garden thug. However, it is not
clear that it is invasive in the UK.
It grows waste ground and relic gardens and orchards.
Giant Knapweed is a perennial herb growing up to 120cm.
The flowerheads are up to 3.5cm across and are made up of
yellow florets.
There are rows of brown, fringed phyllaries below the florets.
The leaves vary in shape and size.
Basal leaves are oval with petioles and those up the stem
are narrow and run down the stem.
The stems are unbranched.
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