Eryngium planum

Sea holly is a perennial plant in the carrot family (Apiaceae). It grows in stiff erect clumps 1 to 3 feet tall and 1 to 2 feet wide and provides interest in the landscape with its unusual spiky flower heads that resemble a thistle. The name Eryngium is a Greek reference to the spiny nature of this genus of plants, but this species is not spiny. The species name planum means flat and refers to the flat, unarmed leaves.

Plant in the full sun in well-drained sandy or loamy soil. The more sun this plant receives, the more intensely colored the blue flowers will be. It is tolerant of a wide range of soil pH.

This plant blooms from June through September and often profusely during the summer.  It has ornamental interest in the colorful,  spiny, thick, and white-veined leaves. Steel blue flowers with a ring of spiky ornamental bracts are useful for cutting or drying. Fresh flowers smell slightly of manure. Dead head flowers to maintain a tidy appearance; cut the plant back after flowering at the end of the summer. The plant sometimes self-seeds, slowly moving around the garden, but it rarely becomes a problem.  Propagation is most easily accomplished by separating plantlets from the base of the parent plant.

Sea holly is tolerant of poor or dry soils, and its spiny leaves are resistant to browsing by rabbits and deer.  It withstands salty, coastal winds and also does well in inland gardens. Use in small groupings in a cottage garden, rock garden or as a specimen in a perennial border planting. Plant in a container or along a patio to enjoy the unusual blooms that are frequented by butterflies.

Insects, Diseases, and Other Plant Problems: Aphids, slugs, and snails may appear. Susceptible to root rot (needs dry soils and leaf spot diseases. In rich soils, plants may grow leggy and topple over.