Sea holly (Eryngium): where it thrives in New Zealand, and where it rots
Quick answer: Sea holly wants full sun and sharp drainage, and it will take poor, stony, lean ground happily. Ours reaches 100-120cm tall with a 30-60cm spread, flowers steel blue from summer into early autumn, and dies right back to nothing over winter. Rich ground makes it flop. Wet ground kills it. Price is $16.50 plus GST.
Run your hand up a sea holly stem in February and you will take it back off again.
The whole plant is armed. Leathery basal leaves with spines along the edges, wiry stems the colour of galvanised pipe, and at the top a cone of tiny flowers sitting inside a collar of stiff bracts, the lot of it gone metallic blue in the heat. Bees crawl all over the cones for weeks. Cut a stem and it lasts in a vase, hang it upside down in the shed and it keeps its colour into next winter.
Then July comes and there is nothing there. Bare soil, a bit of a hollow where the crown sits, and every year somebody digs the spot over or plants something on top of it.
What is sea holly?
Not a holly. Not a thistle either.
Eryngium sits in the carrot family, alongside parsley, fennel and hemlock. Look at a flower head up close and the family shows: dozens of tiny flowers packed into a cone, surrounded by a ruff of bracts that do all the visual work. The blue is in the stems and the bracts, not really in the flowers at all.
Most of it is deciduous. Ours dies back to the crown over winter and pushes up again in spring, so a pot bought in July looks empty for a reason.
The genus is big, over 250 species, spread across Europe, Asia, the Americas and Australasia. The ones sold as garden perennials are mostly the European and Mediterranean ones.
Where does it work in New Zealand, and where does it rot?
Drainage decides it.
Sea holly is a taprooted, dry-country plant. It sends a thick root straight down, finds moisture below, and asks for very little from the surface. Give it a gravelly bank, a stony strip along a driveway, the ground beside a shed where the topsoil gave out years ago, and it will do the best work it does anywhere.
Put it in ground that holds water over winter and the taproot rots. Not in a dramatic way either. It looks fine in autumn, dies back the way it should, and then never comes up again in spring.
So the map of New Zealand for this plant follows the dry ground:
Dry east coast country. Waimate, Oamaru and the light stony ground through South Canterbury suit it well. Ashburton and the Canterbury plains too, where the nor'wester dries things out faster than anything can hold moisture.
Central Otago. Hot summers, free-draining ground and hard frosts are all things it handles. This is the region where it needs least from you.
Inland Southland. Gore and Winton have plenty of heavy ground that sits damp from May through to October. That is the ground to avoid, or to raise the plant out of.
If you are a landscaper specifying for a client, get the drainage question answered before it goes on the plan rather than after. Buying for your own place? Dig a hole 300mm deep, fill it with water, and see how long it takes to drain away. If it is still sitting there the next morning, plant something else in that spot.
Does it want good soil?
No. Feed it and it falls over.
This is the part people get backwards. On lean, poor, stony ground the stems come up short, rigid and strongly coloured. On rich garden soil the plant grows soft and tall, the blue washes out to a dull grey-green, and by January the whole clump has splayed open like a dropped bunch of flowers.
Staking doesn't fix it. It just props up a plant that is in the wrong bed.
Practical version: no compost in the planting hole, no fertiliser, and keep it out of the bed you improved for the roses.
How hardy is it, and will it come back?
Frost hardy once it is established, and yes, it comes back, as long as the ground drains.
An established crown with a taproot under it handles a hard Central Otago winter without complaint. The above-ground part of the plant is gone by then anyway, which is most of the reason it copes.
The risk sits in the first winter, and it is a wet risk rather than a cold one. A plant that went in during late summer, has not got its taproot down, and is sitting in ground that stays saturated for months, is the one that does not reappear. A plant on a free-draining bank at Ranfurly, frozen solid, will be up again in October.
Two things that help a first-year plant:
Plant it in autumn or spring, not into the back end of summer. Autumn gives it a cool, damp stretch to get roots down before it has to hold on through anything.
Raise it. A hundred millimetres of extra height on a mound, or a bank rather than a flat bed, takes the plant out of the wettest part of the profile without any drainage work.
Mark where it is before winter. A label or a stake costs nothing and saves a spade going through the crown in August.
Can you move one once it's in?
Badly, and only when it's small.
The taproot is the whole plant's strategy, and lifting it cuts the taproot. A sea holly moved after its second season usually sulks for a full year, and often does not come back at all. Treat it as a plant you site once.
That also changes how you propagate it. Dividing the clump the way you would a Phormium or a Bergenia does not work. Root cuttings do, taken in the depth of winter here, June through August, from a piece of the taproot laid in gritty mix. Seed works too and the seedlings come up readily where the ground is open.
It also self-seeds. In an open gravel garden that is a feature, seedlings turning up in the joints and the paths looking like you planned it. In a tidy border it is a weeding job every spring. If you would rather not have it spreading, cut the heads off after flowering, which costs you the dried seed heads.
Is there a native that does the same job?
Two, doing two halves of it.
Aciphylla aurea, golden speargrass, is the closest thing we grow to a sea holly in spirit. A tussock of stiff, sharp-tipped leaves out of the dry grasslands of the eastern South Island high country, sending up a golden flower spike to about 750mm in summer, the spike itself armed with spines. It wants exactly what sea holly wants: full sun, gritty lean free-draining ground, and dry feet. It is drought tolerant once away.
Two things to know before you order it. It is slow, a long-lived plant that takes years to build into a proper tussock, so a 2.5L plant is not a specimen next summer. And the leaf tips will go through a glove, at knee height. Never near a path, never anywhere a child plays.
Brachyglottis monroii does the other half, the grey-green architectural texture, without the risk of a hospital visit. A dense dome to about 60-80cm each way, leathery olive leaves with wavy crimped edges, yellow daisies in late spring and early summer. Full sun, well-drained ground, dry in summer, happy on poor and stony soil, hardy to wind and drought once established. A light prune after flowering keeps it tight.
There is a native sea holly, and it's nothing like the one you're buying
New Zealand has its own Eryngium. Eryngium vesiculosum, the native sea holly, grows on coastal gravels and sand around the edges of coastal lagoons and lakes. It is a low creeping thing, creeping on stolons and forming patches to about 300mm across, with small white flowers a couple of millimetres wide. Nothing metallic, nothing 1.2m tall.
It is also in trouble. It was assessed as Threatened, Nationally Vulnerable in 2018.
So if you searched "New Zealand sea holly" and landed on a photo of a blue border perennial, those are two different plants that share a common name and a genus. The one in the garden centre is a European garden plant. The New Zealand one is a threatened coastal native that is not a garden subject at all.
Sea holly or globe thistle?
Fair question, because they get bought for the same reason and they are not the same plant.
Echinops is the globe thistle: a round, spiky ball of a flower head, silver-white in 'Star Frost', on a taller upright stem. Sea holly gives you the cone and collar shape and the blue in the stems as well as the flowers.
The difference that matters on site is how fussy each one is. Both want sun and drainage, but globe thistle is more forgiving of an average garden soil, where sea holly gets sulky and floppy in it. If your ground is honestly just ordinary, Echinops is the safer buy. If you have a hot, stony, hopeless strip, sea holly is the plant that will do something with it.
Both dry well for indoor arrangements, and both are covered in bees for weeks.
Common questions
How far apart should I plant sea holly?
At a 30-60cm spread, 45 to 50cm centres gives a continuous drift without the plants shading each other out. Wider if you want each plant to hold its own shape in a gravel bed.
Do the flowers really stay blue when they're dried?
Yes, better than most things. Cut the stems when the colour is at its strongest, before the heads start going papery, and hang them upside down somewhere dark and airy. The blue fades over months rather than days.
Will it grow in a pot?
It will, in a deep one. The taproot needs depth more than width, so a tall pot beats a wide shallow one, and the mix wants to be gritty and free-draining rather than a standard potting mix. Water it less than you think.
Is it any good on the coast?
The name suggests it should be and the garden species mostly are.
What eats it?
Very little. The spines handle rabbits and hares, and it is not a plant slugs bother with. Nearly every sea holly failure here comes down to the ground.
Sea holly is one of the few perennials that does better the worse you treat it. Hot, stony, unimproved ground, full sun, then left alone, and it will stand there steel blue and covered in bees every summer while everything around it wants watering. Get it into damp ground or a well-fed border and you will be replacing it.
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