Eryngium Planum (Sea Holly)
Eryngium Planum (Sea Holly)
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Eryngium Planum Seeds (Sea Holly)
If your borders are full of soft, fluffy daisies, Eryngium planum provides the "edge" you need. It produces tall, branching, silvery-blue stems topped with masses of thimble-sized, metallic blue flower heads. Each head is surrounded by a prickly "ruff" of bracts that gives it a starry, electric appearance.
This plant catches the light like no other, shimmering in the sun and glowing in the twilight. It creates a modern, structural quality that soft, leafy plants just can't match. It is the definition of "tough love"—thriving on neglect, loving poor soil, and looking spectacular while doing it.
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🌿 Understanding the Plant
Eryngium is a robust Hardy Perennial.
It dies back to the ground in winter and shoots up again every spring, getting bigger and better each year.
The Taproot: It puts down a deep, carrot-like taproot. This makes it incredibly drought-tolerant but means it hates being moved. Pick its spot carefully, because once it's there, it's staying!
Top Tip: Don't feed it! Sea Holly actually prefers poor soil. If you put it in rich, manured soil, it will grow floppy and lose its intense metallic blue colour.
🌱 Growing Guide: How to Sow and Grow
Eryngium seeds need light to germinate, so don't bury them deep.
Germination:
Sow indoors from February to April, or directly outdoors in May/June. Surface sow onto moist compost and do not cover (or cover very lightly with vermiculite), as they need light to trigger growth. Keep warm (20°C). Germination can be erratic (7-21 days), so be patient.
Where to Sow:
It demands full sun and excellent drainage. It thrives in gravel gardens, sandy banks, or coastal locations. It hates winter wet, so if you have heavy clay soil, add grit to the planting hole.
Care While Growing:
This is a low-maintenance plant. It needs no staking if grown in full sun. Leave the seed heads standing in winter—they look beautiful when frosted.
📋 Plant Specifications
| Latin Name | Eryngium planum |
| Common Name | Sea Holly / Blue Eryngo |
| Hardiness | H7 (Very Hardy - to -20°C) |
| Light Required | Full Sun ☀️ |
| Height | ↕️ 70cm - 90cm (Tall) |
| Spread | ↔️ 45cm |
| Spacing | 🌱 45cm apart |
| Great for | 🐝 Pollinators (High Nectar) 🌾 Dried flowers ☀️ Gravel gardens 🏛️ Architectural shape |
| Seed Count | Approx. 150 seeds per packet |
🤝 Perfect Garden Companions
The spiky, metallic texture of Eryngium needs soft, warm partners to really shine:
- 🌾 Hordeum jubatum (Foxtail Barley): The ultimate texture contrast. The soft, silky, pinkish plumes of the barley grass soften the rigid, spiky steel-blue of the Eryngium. They both love dry, sunny soil.
- 🔥 Rudbeckia 'Autumn Forest': Fire and Ice. The cool, metallic blue of the Sea Holly makes the rich, burnt orange and gold of the Rudbeckia look even warmer. A stunning late-summer combination.
📅 Sowing & Flowering Calendar
Sow in spring. Plants establish in Year 1 and flower fully from Year 2 onwards.
| Month | J | F | M | A | M | J | J | A | S | O | N | D |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sow Indoors | 🟢 | 🟢 | 🟢 | |||||||||
| Sow Outdoors | 🟢 | 🟢 | ||||||||||
| Flowers (Yr 2+) | 🌸 | 🌸 | 🌸 |
⚠️ Handling Tip
Eryngium is a thistle, so the leaves can be sharp! I strongly recommend wearing gloves when planting out or when cutting the stems for drying.
🏆 Officially Recognised Excellence
Eryngium planum is listed on the RHS Plants for Pollinators list. It is a high-nectar plant that is particularly beloved by bees in late summer.
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