Eryngium planum

Description

Eryngium planum – sea holly

Botanical name: Eryngium planum
Common names: Sea holly
Family: Apiaceae (carrot family)
Plant type: Herbaceous perennial
Habit: Upright, branching, clump-forming
Pot size: 2–3 litre pot
Eventual size: Approx. 60–90cm tall × 45cm spread
Foliage: Basal green leaves; upper stems and bracts steel-blue and spiny; deciduous
Flowers: Small, cone-shaped, steel-blue flowerheads surrounded by a spiny ruff of blue bracts, on branching blue stems, in mid to late summer (July–September)
Scent: Not notably scented
Aspect / light: Full sun
Soil: Well-drained; poor, dry, sandy; drought- and salt-tolerant; dislikes wet; any pH
Hardiness: RHS H7 (very hardy, below −20°C); USDA zones 4–9
Exposure: Open, hot, dry, sunny; good coastal
Native range: Species native to Europe and Asia
Toxicity / pet & child safety: Generally considered non-toxic to people and pets; note the spiny foliage and bracts

Eryngium planum, sea holly, produces branching, steel-blue stems topped with cone-shaped blue flowerheads set in spiny ruffs, in summer. Architectural, drought-tolerant and one of the very best plants for pollinators.

GardenAdvice notes

A striking, architectural perennial, sea holly is grown for its metallic, steel-blue branching stems and its distinctive cone-shaped flowerheads, each set in a spiky ruff of blue bracts. Tough, drought- and salt-tolerant, thriving on poor, dry, sunny soils, it brings unusual blue colour and bold structure to a sunny or gravel border, is superb for cutting and drying, and is one of the very best plants of all for bees and butterflies.

Growing & planting

Plant in spring in well-drained, poor, dry soil in full sun — it thrives on sharp drainage and tolerates drought and salt, making it excellent for gravel and coastal gardens; it dislikes rich, wet, heavy ground. It has a deep taproot and resents disturbance, so plant it where it can stay. Space about 45cm apart.

Care & maintenance

Low-maintenance. Cut back after flowering, or leave the striking seedheads standing for winter structure and for the birds, then cut down in late winter. It self-seeds and has a deep taproot, so remove unwanted seedlings while young. Water only in prolonged drought. Grown lean it rarely needs staking.

Propagation

• Seed: Self-seeds readily; can be raised from seed.
• Root cuttings: Can be propagated from root cuttings in winter, as the taproot resents division.

Pests & diseases

Generally trouble-free and tough. It can flop in rich soil and self-seeds freely. Powdery mildew occasionally occurs. Otherwise little troubled by pests in a sunny, well-drained spot.

Uses in the garden

Superb in sunny, gravel, prairie-style and coastal borders, where its steel-blue stems and spiny flowerheads bring architectural drama and unusual colour; excellent as a cut and dried flower and a magnet for pollinators.

Wildlife value

One of the very best plants for pollinators: the flowerheads are absolutely alive with bees, butterflies and hoverflies in summer, and the seedheads feed birds in autumn.

Toxicity & safety

Sea holly is generally regarded as non-toxic to people and pets, though its foliage and bracts are spiny, so site it where people won’t brush against it.

GardenAdvice tip

Sea holly is a wonderful architectural plant and an absolute magnet for pollinators — its steel-blue heads hum with bees and butterflies in high summer. Grow it lean in full sun with sharp drainage (rich, wet soil makes it flop and rot), leave the seedheads for winter structure and the birds, and plant it where it can stay put, as its deep taproot dislikes being moved. Mind the spiny foliage.

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