- Name also: Cornflower, Globe Centaurea, Yellow Hardhead, Bighead Knapweed, Giant Yellow Knapweed, Armenian Basket flower, Lemon Fluff Knapweed
- Family: Daisy Family – Compositae, subfamily Carduoideae (formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Rootstock strong.
- Height: 50–120 cm. Stem erect, branchless, hairy, bolding towards inflorescence.
- Flowering time: July–August.
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Yellow Star Thistle
Giant knapweed’s close relative, yellow star thistle (also known as golden star thistle, yellow cockspur, St. Barnaby’s thistle and Barnaby thistle) is in Finland very rare, biennial with yellow flowers, much smaller (15–70 cm) widely winged stem, narrower and shar-pointed leaves, lower leaves lobed. capitulum stalk with exceptionally long spines.
Red Star Thistle (Purple Starthistle)
Biennial red-star thistle is rare but possible species in wastelands.
A rare annual safflower resembles a bit giant knapweed, but is smaller (20–60 cm), flower colour is yellow-orange and leave margins spiny.