<em>Eryngium</em>, sea holly: planting, cultivation and care

Description and botany

Eryngiums, also called Panicauts, are perennial plants or annuals, sometimes biennial, comprising nearly 260 species. They are often spiny, display blue or silvery hues, and somewhat resemble thistles. They have a wide global distribution, although the greatest number of species is found in South America. Around twenty species occur in Europe, notably around the Mediterranean. In France, several species grow wild. They can be found in mountainous regions: Eryngium alpinum grows in the Alps (commonly called “Alpine Panicaut” or “Alpine blue thistle”), while Eryngium bourgatii occurs in the Pyrenees. Eryngium campestre is found throughout the country, whereas Eryngium maritimum grows on coastlines (Atlantic and Mediterranean). In the wild, most Eryngium species grow in dry meadows or rocky places. However, those native to South America are more often found on marshy, damp ground.

Most Eryngium have a hardiness between −15 and −25 °C. Species from mountainous regions such as the Alps or Pyrenees are fully hardy and well adapted to harsh climates. The least hardy are South American species: Eryngium pandanifolium, Eryngium proteiflorum, Eryngium agavifolium, for example.

They are prized in the garden for their originality, graphic habit and metallic tones. Botanical species are common in gardens, although there are also horticultural varieties selected by breeders.

Eryngiums belong to the family Apiaceae (over 3,200 species), formerly called Umbelliferae. These plants are characterised by often white umbels and usually highly divided, aromatic foliage. This family includes carrot, parsnip, dill, chervil, fennel, but also Astrances, Orlaya and Angelica.

Eryngium maritimum: botanical illustration

Eryngiums do not resemble other Apiaceae, but are physically closer to thistles (hence their frequent name “blue thistle”), so at first glance one might think they belong to the Asteraceae (daisies, dandelions). Their flowering resembles a dense head more than an umbel. Eryngium also bears some similarity to Echinops, although the latter belongs to the Asteraceae.

The name panicaut derives from Latin pane cardus: “bread thistle”, because of its resemblance to thistle and its edible nature. Young leaves of Eryngium campestre are edible, as are its roots after cooking. In English, Eryngium are called Sea holly (reference to holly due to prickly foliage).

In Latin, some specific names describe the leaves: Eryngium agavifolium means “with agave-like leaves” (also called Eryngium bromelifolium, “with bromeliad-like leaves”), Eryngium yuccifolium “with yucca-like leaves”, Eryngium pandanifolium “with Pandanus-like leaves” (a tropical plant similar to a palm). All these names refer to exotic, warm-climate plants.

Eryngiums can be divided into two groups:

• European species, mainly hardy and deciduous.
• Species native to South and Central America, which generally form evergreen rosettes and are tender. Examples include Eryngium pandanifolium, Eryngium eburneum, Eryngium agavifolium…

South American species are also the most striking. They are among the largest, forming spectacular clumps of leaves.

Height of Eryngiums varies considerably, from 15 cm to 1.50 m… or even up to 4 m for Eryngium pandanifolium. There are dwarf varieties, such as Eryngium ‘Tiny Jackpot’, which does not exceed 35 cm! Small Eryngiums (e.g. Eryngium bourgatii, E. variifolium, E. maritimum) are best placed at the front of borders, while the taller ones (E. pandanifolium, E. agavifolium, E. yuccifolium, E. giganteum) can be used at the back. They may require staking to keep stems upright.

Eryngiums are generally highly ramified: upright stems branch in an almost geometric, regular fashion, each branch ending in an inflorescence. Forms vary between species. Eryngium tripartitum has a disorderly habit, branches appearing to go every which way, producing numerous small inflorescences on many ramifications and creating a hazy effect. By contrast, Eryngium yuccifolium bears small white globes atop tall, straight stems, giving a graphic, elegant effect.

Eryngiums flower in summer. Depending on variety, flowering occurs between July and October. They remain decorative after flowering when holding seeds.

Inflorescences are dense and compact, unlike most Apiaceae which form airy, wide umbels. As with Astrances, Eryngiums have their umbels condensed into a domed head. Each head comprises a large number of small flowers. Eryngium inflorescences are highly original and have a dry appearance.

Flowers of Eryngiums, gathered in heads, are very small. They consist of five petals and five sepals, as well as five stamens and two styles. Close inspection shows stamens longer than petals, and many filaments or bristles protruding around the edge of the inflorescence.

The base of the head is surrounded by a collar of spiny bracts, highly developed, giving the inflorescence a striking star shape. Bracts vary by variety. They are thin and acute, very sharp on Eryngium variifolium! With its strongly spiny foliage, this species is likely very effective at deterring herbivores.

Bracts really enhance the flowering, making it majestic. However, in species with smaller bracts, such as Eryngium yuccifolium, the blooms are also very decorative: inflorescences appear more rounded, adding a graphic, regular, almost geometric quality. When bracts are less developed, the rounded aspect of the inflorescences stands out more.

When in bloom, Eryngium tripartitum bears an infinity of small bluish globose heads. Given each head contains a very large number of tiny flowers, one can imagine the total number of flowers on a single plant! Its flowering appears as a profusion of stars that mingle into a misty effect.

Eryngium inflorescences are generally blue-silver. Eryngium amethystinum owes its name to its amethyst hue. Flowers are silvery-white on Eryngium eburneum, E. giganteum, E. yuccifolium, and rather greenish on Eryngium agavifolium.

You can harvest the inflorescences for inclusion in fresh or dried bouquets.

Eryngiums are good melliferous plants, appreciated by insects, notably butterflies and bees. Each small flower contains nectar, which insects collect while spreading pollen in return.

Flowering of Eryngium alpinum (photo Heinz Staudacher), Eryngium giganteum (photo Anntin), Eryngium maritimum (photo Svdmolen) and Eryngium serra

Besides their unusual flowers, Eryngium foliage is also very decorative. Leaves are generally spiny, helping protect plants from herbivores. But Eryngiums can also have well-developed basal leaves that are rounded, green and supple, not spiny.

Depending on species and variety, leaf shapes vary greatly. They can be deeply divided, or rounded and entire. Sometimes leaves are long and narrow, as in Eryngium pandanifolium, unfolding from an elegant basal rosette.

Panicauts have basal leaves in a rosette and cauline leaves on stems. Cauline leaves may be alternate or opposite (inserted in pairs).

Leaf length varies widely. In some species (notably European ones) leaves are under 10 cm long, while in others they can reach up to a metre. South American species have the most impressive leaves, sometimes forming majestic basal rosettes.

Eryngiums offer great diversity not only in leaf form but also in colour: green, silvery, yellow, bluish, marbled with white… Variegated-leaf Eryngiums exist, such as Eryngium planum ‘Jade Frost’! In Eryngium variifolium, leaves show very marked white veins, creating a marbled effect. Eryngium ‘Neptune’s Gold’ has bright yellow foliage, offering a striking contrast with blue flowers.

Less common in cultivation, Eryngium venustum is exceptional for finely and regularly divided foliage, each leaf seeming to bear teeth around a main midrib. A rare and very original foliage!

Foliage of Eryngium agavifolium (photo Haplochromis), Eryngium maritimum (photo Uleli), Eryngium planum ‘Neptune’s Gold’ and Eryngium bourgatii

Generally, Eryngiums have a long taproot. This helps them resist drought by drawing water from deep in the soil, but makes transplanting more difficult and increases susceptibility to rot in overly wet conditions.

After flowering, in autumn, Eryngium produces fruits and seeds. The fruit is a diachene, formed of two achenes (dry fruits that do not open at ripeness, each containing a single seed). The plant remains decorative in fruit, retaining its characteristic form as metallic-blue inflorescences are replaced by brown, dry, graphic infructescences (seedheads).

In Eryngium campestre, infructescences detach when ripe and roll along the ground, carried by the wind to disperse seeds. This gives the species the common name “rolling thistle” (hence the vernacular name Chardon Roland).

Fruits and seeds of Eryngium campestre (photo Roger Culos – Museum of Toulouse)

Main species and varieties

Most popular varieties

Our favourite varieties

Other varieties to discover

Eryngium alpinum Blue Star

This variety is derived from Alpine sea holly (Eryngium alpinum). It is prized for its inflorescences set on a collar of long, thin, very delicate bracts.

• Flowering time July, August
• Height at maturity 65 cm

See the product sheet

Eryngium giganteum

This is a large, silvery Eryngium. Its inflorescences form domed heads surrounded by a collar of fairly broad, well-developed bracts. It has been awarded the RHS Award of Garden Merit.

• Flowering time August to October
• Height at maturity 80 cm

See the product sheet

Eryngium planum

This is a fairly classic but indispensable Eryngium. It bears upright stems, nicely ramified, topped by globular inflorescences surrounded by decorative bracts. It has a lovely silvery-blue hue.

• Flowering time July to September
• Height at maturity 80 cm

See the product sheet

Eryngium zabelii Big Blue

This is a silvery-blue sea holly whose domed inflorescences are surrounded by well-developed, elegant bracts.

• Flowering time August to November
• Height at maturity 70 cm

See the product sheet

Eryngium maritimum

This Eryngium bears globular inflorescences surrounded by thorny bracts. Its foliage is very distinctive, leathery and thorny, silvery in colour. It is the perfect species for a seaside garden!

• Flowering time July to October
• Height at maturity 60 cm

See the product sheet

Eryngium planum Jade Frost

An Eryngium notable for its variegated foliage! Its leaves are green, marginate with cream-white which makes them quite bright, and are sometimes slightly tinged with pink-purple. It also bears silvery-blue inflorescences.

• Flowering time August to October
• Height at maturity 60 cm

See the product sheet

Eryngium variifolium

An unusual Eryngium offering foliage marked with white, whose leaves and bracts are very thorny and pointed! This species is magnificent to look at, but it's best not to touch it!

• Flowering time July to September
• Height at maturity 40 cm

See the product sheet

Eryngium bourgatii

This Eryngium bears inflorescences surrounded by very long, thin, sharp and radiating bracts. Its foliage is distinctive: very dissected and marbled with white.

• Flowering time August, September
• Height at maturity 40 cm

See the product sheet

Eryngium agavifolium

This is a surprising Eryngium, very different from other species. It has large, very long strap-like leaves, bright green. From the clump of leaves emerge long upright stems bearing green-grey heads at their tips.

• Flowering time August, September
• Height at maturity 1,20 m

See the product sheet

Eryngium tripartitum

Eryngium tripartitum is a species with a messy, spreading habit, its stems very ramified and seeming to go in every direction. It bears countless bluish heads, creating a very light, airy effect.

• Flowering time August to October
• Height at maturity 70 cm

See the product sheet

Planting

Where to plant?

Plant Eryngiums in full sun! They will appreciate a warm, well-lit spot. In sun, their colours are more vivid and pronounced, their flowering more generous, and their habit more robust and compact. In shade, Eryngiums become more lanky and slender, and may need staking.

It is important to plant them in free-draining soil, even stony. You can perfectly place them in a rock garden or a very mineral planting. Eryngiums like sandy, free-draining, rather poor soil. We recommend improving drainage by adding sand or gravel. Do not hesitate to raise the bed by planting on a mound: water must be able to drain away and not pool. Eryngium tolerates drought well but dislikes waterlogging in winter.

As Eryngium tolerates sea spray well, you can install it in a coastal garden. The species best suited to this situation is obviously Eryngium maritimum, which grows naturally on the French coast!

When siting Eryngiums, bear in mind their eventual size. Eryngium maritimum, E. bourgatii or E. variifolium are small and should be placed towards front of borders… While the large Eryngium giganteum, E. pandanifolium or E. agavifolium make a fine backdrop when planted behind lower-growing flowers.

Because of their long taproot, Eryngiums do not like being moved. It is therefore best to plant them straight into their final position.

When to plant?

You can plant Eryngium in autumn, around October, or in early spring.

How to plant?

We recommend spacing of about 50 cm if you grow medium-sized Eryngiums, such as Eryngium planum. For large varieties, spacing should be greater.

1. Dig a planting hole. Do not hesitate to add coarse sand or gravel, or to create a mound, raising the bed so water can drain away easily.
2. Remove Eryngium from its pot and plant it.
3. Replace soil around plant, then firm gently.
4. Water.

Watch young plants and continue to water if necessary. Once well established, they are fairly drought-tolerant and will no longer need regular watering.

You can also grow Eryngiums in a pot. We recommend choosing a fairly deep container (because of the taproot, which tends to go deep into the soil). Place a drainage layer at bottom, then use compost mixed with coarse sand. Plant your Eryngium, replace soil around it, then water. No need to add fertiliser. Make sure pot is placed in a sunny spot.

Eryngium amethystinum

Caring for sea holly

Eryngiums do not really require maintenance. As they grow very well in poor, low-fertility soil, it is unnecessary to give them compost or fertiliser. Also, except in the weeks following planting, it is generally not necessary to water them. These plants have the advantage of being fairly drought-resistant! On the contrary, not fertilising or over-watering Eryngiums will help them keep a compact, stocky and sturdy habit.

If you grow taller species, if soil is fertile and cool, or if plants are exposed to wind, Eryngiums can tend to flop or collapse. Consider installing staking to keep them upright.

Removing faded flowers encourages plants to prolong their flowering. However, you can also leave them in place, as faded flowers remain decorative in autumn–winter.

You can also harvest the flower stems to use in fresh or dried bouquets.

Main growing issues with Eryngiums are caused by overly wet conditions, which rot the root system or lead to the appearance of fungal diseases, such as powdery mildew or downy mildew. If these diseases appear, you can treat with a fungicide, for example a sulphur-based solution or a horsetail decoction. It is also important to grow Eryngiums in well-draining soil and avoid excess moisture.

Slugs sometimes nibble young plants when they are still tender. You can use a slug pellet such as Ferramol, or place a ring of sawdust or sand around them to create a physical barrier that will deter gastropods.

Propagation

Sea hollies are mainly propagated by sowing. Division is complicated because of their long, deep root. But it is also possible to propagate eryngium roots in late winter or early spring.

Sowing

Sowing allows multiplication of most species. It is preferable to stratify seeds by placing them in the refrigerator for about a month before sowing.

1. Prepare a pot by adding a well-draining substrate (for example a mix of potting compost and sand).
2. Scatter the seeds on the surface.
3. Cover them with a thin layer of substrate or vermiculite.
4. Water with a fine spray.
5. Place your sowing in a bright spot, out of direct sunlight.

Watch out for slugs, which like to nibble young plants.

Remember to water from time to time so substrate remains relatively cool, and prick out seedlings when they reach a size that allows handling. Young seedlings can be easily transplanted, as their roots are still not well developed. Transplanting becomes more difficult for mature plants.

It is also possible to sow seeds directly outdoors, in autumn or in spring.

Pairing Eryngiums in the garden

You can use Eryngiums to design a low‑maintenance garden where you won’t need to water, add fertiliser or compost, etc. Take the opportunity, for example, to create a mineral‑dominant garden, a scree garden (scree garden). Choose a warm, sunny spot, make a raised bed, place large stones and a free‑draining substrate. Plant Eryngiums alongside other drought‑tolerant plants. Prefer cushion‑ or mat‑forming plants (sedums, phlox, sempervivums, Cerastium…), those with silvery foliage or fleshy leaves. Include agaves, cacti, opuntias and carpobrotus.

As Eryngium tolerates sea spray well, you can plant it in a coastal garden. Choose Eryngium maritimum, found naturally on the French coast. It will charm with its silvery, stiff and thorny foliage, very architectural. Pair with Armeria maritima, cistus, Helichrysum, santolinas, Senecio cineraria, and the small grass Lagurus ovatus.

You can create a superb dry garden in full sun by planting Verbascum ‘Polarsommer’, with its downy foliage and upright yellow flower spikes, combined with Eryngium oliverianum and a few Centranthus. Add several clumps of Salvia nemorosa ‘Caradonna’ and Stipa tenuifolia to the bed to bring a natural, airy effect! You’ll achieve an original planting that requires very little maintenance. The silvery tones of Verbascum and Eryngium combine beautifully.

An idea for a pairing in a dry, sunny garden: Verbascum ‘Polarsommer’, Eryngium oliverianum, Centranthus, Salvia nemorosa ‘Caradonna’, Stipa tenuifolia (Landscape designer Robert Myers / Photo AC Nathalie Pasquel – MAP)

You can also include Eryngiums in a very natural‑style garden, pairing them with verbena Verbena bonariensis and sage ‘Mystic Spires’. You’ll get a bed with a disorderly, wild, country aspect. Add grasses (Stipa, Pennisetum…). You can also include Knautia macedonica, with beautiful light crimson flowers. Also make use of achilleas, for example the ‘Terracotta’ variety.

You can also create an alpine garden, including plants naturally found in mountain regions. Soil should be free‑draining, rocky and, if possible, sunny. Choose globe thistle of the Alps, or Eryngium alpinum, and plant gentians, edelweiss, saxifrages, androsaces alongside. Prefer mat‑forming plants and those with a cushion habit.

Finally, because they are very sculptural, Eryngiums can fit into modern or contemporary‑style gardens. Pair them with grasses, Echinops, Gaura, Veronicastrum, Eremurus, Perovskia, Verbascum…

You can include Eryngium in a naturalistic garden! Achillea ‘Terracotta’ and Eryngium oliverianum (photo Clive Nichols – MAP) / Echinacea ‘Delicious Candy’ and Eryngium ‘Picos Blue’ / Verbena bonariensis, Eryngium and Salvia ‘Mystic Spires’

→ Discover other pairings with Eryngium in our advice sheet, and in our inspiration Design an on‑trend bed “Thistles & Company”

Did you know?

• King oyster mushroom

Sometimes a fungus, the king oyster mushroom (Pleurotus eryngii), shoots on roots of Eryngium maritimum and Eryngium campestre. It is an excellent edible mushroom, sometimes cultivated (grow kits are even available commercially)!