Eryngium amethystinum features metallic amethyst stems and flowers. Perennial, to 5 ft. high
Eryngium bourgatii features variegated leaves for the rockery. Perennial, but not long-lived here.
Eryngium giganteum features the largest flowerheads of the eryngos. An annual/biennial here in Zone 5a
Eryngium leavenworthii is nothing short of fantastic with its metallic purple stems and flowers. Note that the stamens come out as bright blue filaments. Produces mostly empty seeds away from its home range.
Eryngium maritimum features pale blue flowers and is the true sea holly. This perennial is not long-lived; to 30 inches high.