Eryngium × olivierianum - Panicaut hybride, Chardon bleu, Panicaut d'Oliver

Eryngium × olivierianum is a hybrid sea holly remarkable for the metallic blue colouration of its stems and inflorescences adorned with long, spiny bracts. Its upright habit and long-lasting flowering make it a wonderful addition to sunny borders, scree gardens, and dry soils. Its flowers attract pollinating insects, and its stems can be cut for fresh or dried cut flower arrangements. Resistant to wind and sea spray, this sea holly is also well-suited to dry gardens.

This sea holly belongs to the Apiaceae family, like the carrot and fennel, despite its thistle-like appearance. Its official name is Eryngium × olivierianum; the name Eryngium × zabelii, still widely used in horticultural catalogues, is now treated as a synonym. It is a hybrid between the Alpine sea holly, Eryngium alpinum, and Bourgat's sea holly, Eryngium bourgatii. The former grows in montane meadows and rockeries of Europe, while the latter originates from the mountains of the Iberian Peninsula and north-west Africa. Their offspring combines the large bracts of the former with the blue colouration and drought tolerance of the latter. This herbaceous perennial forms a rosette of leathery, strongly toothed and spiny leaves in spring, which are heart-shaped or divided into lobes. Olive-green to greyish-green in colour, they are traversed by paler veins. The plant develops a deep taproot that helps it better withstand dry periods but makes transplanting difficult once established. In summer, rigid, branched stems rise to 80 or 90 cm. Initially green, they gradually take on a bluish-purple hue in the sun. Each bears several ovoid to conical flower heads, composed of a multitude of tiny flowers. These inflorescences are surrounded by a ruff of narrow, cut and prickly bracts, whose metallic blue intensifies in full sun and in poor soil. Flowering occurs from July to September. Nectariferous, the flowers are visited by bees, butterflies, and other pollinating insects. The inflorescences, 8 to 10 cm in diameter, dry on the stem without immediately losing their shape or colour, allowing them to be enjoyed in winter or cut for arrangements. The foliage and stems die back in winter, then the plant regrows from its stump in spring. It tolerates lime, poor soils, and summer drought once well-rooted. However, heavy, wet soil in winter greatly reduces its longevity and cold hardiness. It withstands frosts close to –15 °C in perfectly drained soil.

Eryngium × olivierianum can be planted in a dry border, a large rockery, or a scree garden. It pairs well with Achillea 'Moonshine', Knautia macedonica 'Mars Midget', and Artemisia 'Powis Castle', which, like it, enjoy sun and well-drained soil. The sunny yellow of the yarrow, the deep purple of the knautia, and the silvery foliage of the artemisia highlight its metallic blue inflorescences.