A Blue-Green Surprise Among the Marram

Walk onto a sand dune in midsummer and the colours are almost entirely predictable: dull gold sand, straw-coloured marram grass, a wash of pale lichen. Then a patch of something metallic catches your eye. Stems of pewter-blue, leaves like stiff holly studded with spines, and flower heads that look like small thistles wrapped in a collar of jagged bracts. That is sea holly, Eryngium maritimum, and once you know it you will spot it on dunes all around the British coast.

It is not a thistle, despite appearances, and not a holly either. It belongs to the carrot family, which explains the umbel-like arrangement of its flower heads, if you look closely enough through all that armour.

Built for Salt, Wind and Shifting Sand

Everything about sea holly is a solution to the problem of living at the edge of the sea. The leaves are thick and leathery, with a waxy bloom that limits water loss and shrugs off salt spray. Those spines are not just for show: they make the plant unpalatable to grazing rabbits and livestock, which otherwise mow dune vegetation flat.

Below ground it is more impressive still. Sea holly sends down a deep, fleshy taproot that can reach well over a metre into the sand, anchoring the plant while hunting for moisture far below the dry surface. That root is also why the plant is so hard to move, and why digging one up usually kills it.

The flowers appear from June into September, sometimes later in the south-west. They start greenish and mature to a soft amethyst blue, sometimes with a purple or pinkish cast. It is the stems and bracts, rather than petals, that carry most of the colour. On a dull day they seem to glow.

Where to Look on a UK Beach

Sea holly is a plant of the upper beach and mobile dunes, not of the strandline where you find your driftwood and mermaid's purses. Scan the transition zone where flat sand begins to rise into the first low dunes. A few practical pointers:

• Best on the seaward face of foredunes, where sand is still moving and marram grass is thin.
• Look for clumps rather than carpets. It spreads by seed more than by root, so it tends to appear scattered.
• Coastal counties with good populations include Norfolk, Lincolnshire, Anglesey, Pembrokeshire, Cornwall, Lancashire and the Scottish east coast.
• Empty seed heads stay standing into winter and are easy to spot once the stems have dried to a pale bronze.

Its seeds are buoyant and corky, and they travel by sea. That is how the plant colonises new dunes, and why you occasionally find a single sea holly washed up and sprouting on a beach far from any dune.

Why It Matters for Dune Health

A dune with sea holly on it is generally a dune that is working properly. The plant needs bare, mobile sand and crucially it needs disturbance: wind, wave action and shifting sediment that keep the dune open and prevent it from scrubbing over with coarse grass, bramble and willow.

Healthy mobile dunes are one of the rarest habitats in Europe, and the UK holds a significant share of them. Where dunes become stabilised and overgrown, sea holly disappears along with a whole community of specialist insects. Its flowers are a late-summer nectar source for bees, hoverflies and beetles, and several dune moths depend on it. If you see sea holly in bloom, you are looking at a functioning coastal ecosystem rather than a static landscape.

Sadly it is in decline in many places, squeezed between sea defences on one side and dune encroachment on the other. In some counties it is now a scarce plant.

Beachcombing with Care

Sea holly is beautiful enough that people want to pick it, dig it or take it home for the garden. Please resist. Uprooting any wild plant without the landowner's permission is against the law in the UK, and sea holly rarely survives transplanting anyway because of that deep taproot. Picking the flower heads prevents seed set and removes the winter structure that shelters small invertebrates.

Photograph it instead, and beachcomb around it rather than through it. A few habits worth keeping:

• Stay on established paths across dunes where they exist; trampling compacts sand and breaks the root systems of dune plants.
• Keep dogs from digging in dune slacks and foredunes, particularly in spring and summer.
• Take only what the sea has already discarded: cuttlefish bone, sea glass, mermaid's purses, whelk egg cases. Leave growing plants alone.
• Report unusual finds or damage to dune vegetation to the local wildlife trust or the site's ranger.

If you want sea holly at home, buy seed or plants from a reputable nursery that propagates rather than collects. It grows well in a sunny, sharply drained bed or a large pot of gritty compost, and once established it will tolerate drought and salt wind better than almost anything else in the border. Fresh seed sown in autumn gives the best results.

A Plant with a Past

Sea holly has been useful to people for centuries. Its roots were candied and sold in London as a sweetmeat called eryngo, thought to be an aphrodisiac, and the plant was once harvested in quantity along the east coast. Sailors ate the young shoots, and herbalists used the root for all manner of complaints.

That commercial pressure is long gone, but the plant remains a marker of a particular kind of coast: windswept, sandy, a little bit wild. Next time you walk a dune system, give the blue-grey clumps a second look. They are telling you something good about the place you are standing in.