Eryngium planum Seeds | Blue Eryngo
LATIN NAME: Eryngium planum
COMMON NAME: Blue Eryngo, Flat Sea Holly, Blue Thistle
Plant Profile
Sea holly is a stiff, architectural perennial with a metallic blue-green cast and a habit that looks more like a thistle than a member of the carrot family. Plants reach 60-90 cm, with branching stems, spiny-margined leaves and small, cone-shaped flower heads surrounded by a ruff of spiny bracts.
The colour is strongest in early summer and fades to a dusty blue-green by autumn, when the structure remains and is at its most useful in a winter border. Every part of the plant is spiny, which is why it is deer-proof and why it needs handling with gloves.
Eryngium planum comes from dry grassland in central and south-eastern Europe, and it grows a long taproot to find water. That taproot is the reason the plant resents being moved and the reason it survives drought when neighbouring perennials collapse.
Science & Biotechnology
The blue colour is not a pigment in the usual sense but a structural effect: a layer of cells in the epidermis scatters light in a way that produces a blue-green sheen, and the same mechanism appears in some blueberries and in the wings of certain insects. That is why the colour does not fade like anthocyanin and does not dissolve in water.
Eryngium belongs to the Apiaceae, and like many members of the family it contains essential oils and coumarins. The genus has a long medicinal history in Europe, mostly as a diuretic and a wound herb, though the evidence base is thin and the plant is not used in modern practice.
The spiny bracts are a defence against grazing, and their stiffness comes from lignified tissue that persists after the flower has set seed. That persistence is what makes the plant a good dried flower and a poor cut flower for a mixed vase, where the spines damage softer stems.
Uses & Benefits
Sea holly is a structure plant: it belongs at the front of a gravel garden, in a dry border or against a wall, where its shape can be seen. It combines well with grasses, with lavender and with the taller sedums, and its blue holds the border together when the summer flowers are over.
The flower heads dry well and keep their colour for a year if cut before the stems harden. They are a mainstay of dried arrangements and of winter wreaths. Bees and hoverflies work the small flowers, which are otherwise inconspicuous. Commercially the plant is sold as a cut flower under the name blue thistle.
Growing from Seed
Sow in autumn or early spring at 18-20°C; germination is slow and uneven, often four to eight weeks, and a period of cold improves it. The seedlings grow a taproot early, so prick them out into deep pots as soon as they can be handled.
Plant out at 40 cm apart in full sun and sharply drained soil, ideally on a slope or in gravel. Do not enrich the ground. Water through the first summer only, and stake nothing: the stems are stiff enough. Leave the heads standing through winter for structure, and cut them to the base in early spring. Do not attempt to divide the plant, which will not survive it.
Our Take
A plant for the dry, sharp-drained corner where colour is hard to hold, and one that still looks good in February.
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