Most are erect, stiff-branched, thistlelike plants that bloom in summer, putting on a show of striking oval, steel-blue or amethyst flower heads surrounded by spiny blue bracts. They make long-lasting cut flowers and dry well for winter arrangements. Plants are sparsely clothed in deeply cut dark green leaves with spiny-toothed edges; upper leaves and stems are sometimes blue.
Another group of sea hollies has long, narrow, spiny-edged leaves that are quite similar to those of yucca. One of these, E. yuccifolium, is native to the U.S.
Use sea hollies in borders or fringe areas, in deep, well-drained, sandy or gritty soils that are low in fertility. They are taprooted and difficult to divide, so propagate by root cuttings or by sowing seed in place (thin seedlings to 1 ft. apart). Plants often self-sow. Resist deer.
E. alpinum. ALPINE SEA HOLLY. From southeastern Europe. To 2 ft. high, 1 ft. wide, with steel-blue, 2-in. flower heads and large, deeply cut blue bracts. Upper leaves and stems are tinged with soft blue to steel-blue. 'Blue Star' is a choice selection.
E. amethystinum. AMETHYST SEA HOLLY. From Italy and the Balkans. To 2 ft. tall, 1 ft. wide. Silvery blue stems; rich blue, inch-wide flower heads surrounded by silvery blue, 2-in. bracts.
E. giganteum. MISS WILL MOTT'S GHOST. From the Caucasus and Iran. Biennial to short-lived perennial. To 34 ft. or taller, 212 ft. wide. Oval or heart-shaped, medium green leaves to 6 in. long. Three-lobed stem leaves. In late summer, bears blue or pale green, conical flower heads surrounded by silvery, 2124-in. bracts; the plant dies after flowering ends. Reseeds well. Provide fertile soil, regular water. 'Silver Ghost' is shorter (2 ft.) with larger blooms.
E. planum. FALSE SEA HOLLY. From southeastern Europe to central Asia. To 3 ft. high, 1 ft. wide. Leathery evergreen leaves; freely borne light blue flower heads to in. across, surrounded by narrow blue-green bracts. 'Blue Glitter' is a heavy producer of glittering, steel-blue flowers.
E. 'Saphhire Blue'. One of the best sea hollies, with intense, steely blue flowers. Upper leaves and stems also infused with blue. Grows 2 ft. high and 1 ft. wide.
E. variifolium. MOROCCAN SEA HOLLY. US, MS, LS, CS; USDA 6-9. From Morocco. Grows to 16 in. tall and 10 in. wide, with inch-wide, rounded, blue-gray flower heads and bluish white bracts. Thistlelike evergreen leaves are heavily veined with white.
E. yuccifolium. RATTLESNAKE MASTER. US, MS, LS, CS; USDA 6-9. Native to eastern and central U.S. Long (to 3-ft.), narrow, spiny-edged leaves in a basal rosette to 2 ft. wide. Erect stems to 34 ft. branch toward top and carry 1-in.-wide white owers without signicant bracts.