Key takeaways
- Sea holly needs full sun and sharp drainage; winter wet at the root is what usually kills it
- Blue tap-rooted types such as E. planum and E. bourgatii hate being moved, so plant small and leave them be
- Make more plants from root cuttings in winter rather than by dividing
- Lean soil gives the bluest colour; skip feeding and rich mulches
- Bees and hoverflies love the flowers, and the stems dry well for winter vases
Sea holly is the plant that stops people on the path. Its metallic steel-blue heads sit on a spiky ruff, on stems washed the same silvery blue. It looks fussy, but it isn’t. Give it full sun and sharp drainage and it shrugs off drought, salty wind and stony soil. Plant it in wet clay, though, and it may vanish over its first winter. Here’s how to get it right.
What is sea holly?
Sea holly is the common name for Eryngium, a group of spiny perennials in the carrot family. Despite the name, it’s no relation to true holly. There are two main groups, and knowing which you have changes how you grow it.
Tap-rooted sea hollies are the classic steel-blue types, such as E. planum, E. bourgatii, E. alpinum and the hybrid ‘Big Blue’. They grow a deep, fleshy root, die back in winter and dislike being moved. Evergreen rosette types, such as E. agavifolium and E. pandanifolium, come from South America. They make bold clumps of sword-shaped leaves and have greenish-white or silvery flowers.
Sea holly is often confused with globe thistle, which has rounder blue balls and softer leaves. Our guide to growing echinops covers that one.
The two types of sea holly compared
| Tap-rooted (blue) | Evergreen rosette | |
|---|---|---|
| Examples | E. planum, E. bourgatii, E. alpinum, ‘Big Blue’ | E. agavifolium, E. pandanifolium |
| Leaves | Die back in winter | Evergreen, sword-shaped |
| Flowers | Steel-blue to violet | Greenish-white to silver |
| Height | 45-90cm | 1.2m and taller |
| Hardiness | Very hardy on dry soil | Less hardy, likes shelter |
| Make more by | Root cuttings in winter | Seed |
| Dislikes | Being moved, rich or wet soil | Hard frost, waterlogging |
Treat a tap-rooted sea holly as a permanent resident. Treat an evergreen type as a slightly tender feature plant that may need fleece in a cold snap.
Best eryngium varieties for UK gardens
- E. bourgatii: a compact plant about 45-60cm tall with silver-veined leaves and spiky blue heads. ‘Picos Blue’ is a popular, intensely blue form.
- E. planum: lots of small blue heads on branching stems, about 75-90cm tall. The best sea holly for cutting.
- E. alpinum: the alpine sea holly, with a soft, feathery blue ruff. It copes with slightly richer soil than the others.
- E. x zabelii ‘Big Blue’: large violet-blue heads on strong stems, about 90cm tall.
- E. agavifolium: an evergreen rosette with flower spikes up to about 1.5m. A great spiky centrepiece for a gravel bed.
All suit a gravel garden, where the stones keep the crown dry and show off the silvery colour.
E. bourgatii has some of the bluest heads of all. The leaner and sunnier the spot, the deeper the colour.
Why sea holly fails on wet clay
Sea holly usually dies on clay because its root rots in cold, soggy soil, not because of the cold itself. It comes from dry, stony places. A wet UK winter in heavy ground turns the root soft from the bottom up.
The fix is drainage. Dig in plenty of grit over a wide area and plant on a low mound so water runs away from the crown. If your soil is solid clay, grow sea holly in a raised bed or a deep pot of gritty compost instead.
Don’t feed and don’t enrich the soil. Rich ground gives soft, floppy growth, weaker colour and a root more likely to rot. Sea holly sits happily with other drought-tolerant plants that like the same lean life.
How to plant sea holly
Plant small, in spring, and only once. Young pot-grown plants settle far better than big, pot-bound ones, because the tap root hasn’t started circling.
April to early June is the best time. The soil is warming, and the plant has all summer to push its root down before winter.
- Dig a hole no deeper than the pot but twice as wide.
- On clay, mix grit into the soil and build a low mound so the crown sits slightly proud.
- Firm gently and water in once to settle the soil.
- Finish with a thin layer of gravel around the crown.
Space compact E. bourgatii about 30-40cm apart, E. planum and ‘Big Blue’ about 45-60cm apart. Leave plenty of room around evergreen types. If a sea holly is in the wrong place, don’t try to move it. Take root cuttings and start again.
Planting a young sea holly on a gritty mound. The crown sits a little high, so winter rain drains away from the root.
Watering, feeding and year-round care
Sea holly thrives on neglect. Water young plants weekly through their first summer while the root establishes. After that, they rarely need watering at all.
Skip fertiliser on normal soil. Don’t mulch over the crown with compost or manure, as it holds wet exactly where you don’t want it. A thin gravel mulch is the only top dressing sea holly needs.
Leave the stems standing into winter. The dry heads look great with frost on them and give the border some shape. Cut them back to the base in February or March, before new growth starts. Pull tatty outer leaves off evergreen types in spring. Sea holly self-seeds gently, so leave a few heads if you’d like free seedlings.
Sea holly care calendar
| Month | What to do |
|---|---|
| January | Enjoy the frosted seedheads. Check no water is pooling around crowns |
| February | Cut old stems to the base before new growth starts. Take root cuttings |
| March | Tidy evergreen types. Top up gravel mulch |
| April to May | Plant young pot-grown plants. Water new plants weekly |
| June | Buds form. Hoverflies arrive on the first heads |
| July | Peak flowering. Cut stems for fresh or dried arrangements |
| August | Keep cutting. Watch for powdery mildew in hot, dry spells |
| September to October | Colour fades to silver. Collect seed if you want it |
| November | Leave stems standing for winter interest |
| December | Take root cuttings from established plants |
How to take sea holly root cuttings
Root cuttings are the reliable way to make more blue sea hollies, as they don’t like being divided. Take them while the plant is dormant, from December to February.
- Carefully scrape soil away from one side of an established plant, or lift a plant you’re happy to sacrifice.
- Choose roots about the thickness of a pencil and cut a few off.
- Cut them into pieces about 5cm long. Cut the top end straight and the bottom end slanted, so you know which way up they go.
- Push them upright into a pot of gritty compost, flat end level with the surface.
- Cover with a thin layer of grit and keep somewhere cold but frost-free, such as a cold frame.
New shoots usually appear in spring. Pot them on once they have a few leaves, and plant out the following spring.
Root cuttings are pencil-thick pieces about 5cm long. Straight cut at the top, slanted at the bottom, and they’ll know which way to grow.
Cutting and drying sea holly
Sea holly is brilliant for both fresh and dried arrangements. E. planum, with lots of small heads on branching stems, is the top choice.
For fresh flowers, cut stems when the heads are fully coloured. Do it in the cool of the morning, strip the lower leaves and stand them in deep water for a few hours before arranging. Expect them to last well over a week in a vase.
For drying, cut at the same stage, tie small bunches together and hang them upside down somewhere dry, dark and airy. After two or three weeks they’ll keep their colour for months. It’s a lovely partner for other best flowers for cutting in a summer bunch.
E. planum gives armfuls of small blue heads on branching stems. Cut in the morning and stand in deep water.
Hang bunches upside down for a few weeks and sea holly keeps its silvery blue right through winter.
Sea holly for bees and seaside gardens
Sea holly is a bee and hoverfly magnet. Each domed head is packed with tiny flowers that insects can reach easily, and it blooms through July and August. A group of E. planum in a sunny border is a busy place on a warm afternoon.
As the name suggests, it’s also a good choice by the sea. It copes with wind, salt spray and sandy, free-draining soil. Our guide to coastal gardening and salt-tolerant plants has more plants that thrive in the same conditions.
A honeybee and a hoverfly sharing one flower head. Sea holly’s open little flowers make for easy nectar.
Evergreen rosette types up close
Evergreen sea hollies give a very different look. E. agavifolium and E. pandanifolium keep their spiny, sword-like leaves all year, so they hold their shape in winter when the blue types have died back. The flowers are greenish-white or silvery, on tall branching stems.
They’re less hardy than the blue types and do best in milder gardens or a sheltered, sunny corner. They grow easily from seed and may self-sow where happy. Use one as a spiky full stop at the end of a gravel bed, perhaps alongside other bold shapes such as red hot pokers.
E. agavifolium keeps its spiky rosette all winter. Give it a sheltered, sunny spot in mild gardens.
Common mistakes with sea holly
- Planting in wet clay. The root rots over winter. Add plenty of grit and plant on a mound, or use a pot.
- Feeding and enriching the soil. Rich ground gives floppy stems and dull colour. Keep it lean.
- Moving or dividing it. Blue tap-rooted types rarely survive. Take root cuttings instead.
- Buying a big pot-bound plant. Small, young plants settle in much better.
- Growing it in shade. You’ll get leggy stems, dull colour and more mildew. Give it full sun.
Love the look of sea holly? Our guide to creating a Beth Chatto-style dry garden shows how to build a whole bed around plants like it. More ideas are in our flowers and plants hub.
Frequently asked questions
Where should I plant sea holly?
Plant sea holly in full sun on free-draining, fairly poor soil. It suits gravel gardens, sunny banks and coastal plots. Avoid shade and any spot where water sits in winter, because the roots rot in cold, wet ground.
Why is my eryngium dying or rotting?
Winter wet at the root is almost always the cause. Sea holly comes from dry, stony places and can’t cope with soggy soil. On clay, dig in plenty of grit, plant on a low mound and never overwater.
Can you divide sea holly?
No, dividing usually kills the blue tap-rooted types. They hate root disturbance and rarely recover. Take root cuttings in winter or sow fresh seed instead. Buy small plants so the root settles in without a check.
How do you take sea holly root cuttings?
Take pencil-thick root pieces between December and February. Cut them into 5cm lengths, straight at the top and slanted at the bottom. Push them upright into gritty compost, cover lightly with grit and keep them somewhere cold but frost-free.
Is sea holly good for bees?
Yes, bees and hoverflies love it. The domed heads are packed with tiny nectar-rich flowers that insects can reach easily. It flowers in July and August, right in the middle of summer.
Can you dry sea holly for arrangements?
Yes, sea holly dries beautifully. Cut stems when the heads are fully coloured, strip the lower leaves and hang small bunches upside down somewhere dry and airy. After two or three weeks they’ll keep their colour for months.
How tall does eryngium grow?
Most blue sea hollies grow 45-90cm tall. Compact E. bourgatii is at the low end, while E. planum and ‘Big Blue’ reach around 75-90cm. Evergreen types such as E. agavifolium send flower spikes up to about 1.5m.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
