What's the difference between horseknop and knapweed?

Horseknop

Definition:

Example Sentences:

Definition:

(n.) The black centaury (Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed.

Example Sentences:

(1) The footpaths I followed became swamped with knapweed, bramble and nettle.
(2) Repin, one of the sesquiterpene lactones found in Russian knapweed, has been shown to possess high toxicity toward chick embryo sensory neurons.
(3) Consumption of Russian knapweed (Acroptilon repens) by horses results in the necrosis of neural cells in the substantia nigra.
(4) It would be a day of spiders’ webs, drifting thistledown shredded by goldfinches , ripening berries and lethargic bumblebees clinging to the last of the knapweed flowers .
(5) As Ian prepares for the half-mile-long return journey, he points a fistful of oily cotton waste towards the purple thistle-like flowerheads of knapweed, the pink, white and blue pincushion-shaped flowers of scabious, and the vivid blue of cornflower.