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

Description and botany

Eryngiums, also called sea hollies, 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 largest number of species occurs in South America. Around twenty species are also found in Europe, notably around Mediterranean basin. In France, several species grow wild. They occur in mountainous regions: Eryngium alpinum grows in Alps (commonly called “Alpine sea holly” or “Alpine blue thistle”), while Eryngium bourgatii occurs in Pyrenees. Eryngium campestre grows throughout country, whereas Eryngium maritimum is found on coastlines (Atlantic and Mediterranean). In wild, most Eryngium species grow in dry meadows or rocky places. However, species native to South America are more often found on marshy, wet ground.

Most Eryngiums have hardiness between –15 and –25 °C. Species from mountainous regions such as Alps or Pyrenees are clearly very hardy, well suited to cultivation under harsh climates. Least hardy species are those from South America: Eryngium pandanifolium, Eryngium proteiflorum, Eryngium agavifolium…

Eryngiums are prized in gardens for their originality, graphic habit and metallic tones. Botanical species are widespread in gardens, although horticultural varieties selected by humans also exist.

Eryngiums belong to family Apiaceae (more than 3,200 species), formerly called Umbelliferae. These are characterised by flowering often in umbels, and by generally very divided, scented foliage. Examples include carrots, parsnips, dill, chervil, fennel, but also Astrances, Orlaya and Angelicas.

Eryngium maritimum: botanical illustration

Eryngiums do not resemble other Apiaceae but look physically closer to thistles (hence common name “blue thistle”), so they may at first glance be mistaken for members of Asteraceae (daisies, dandelions). Their flowering resembles a dense head rather than an airy umbel. Eryngium also resembles Echinops, although latter belongs to Asteraceae.

Name panicaut comes from Latin pane cardus: “bread thistle”, due to resemblance to thistle and edible nature. Young leaves of Eryngium campestre are edible, as are roots after cooking. In English, Eryngiums are called sea holly (reference to holly due to spiny foliage).

In Latin, some species epithets describe leaves. For example, Eryngium agavifolium means “with agave-like leaves” (also called Eryngium bromelifolium: Bromeliad-like leaves), Eryngium yuccifolium “with yucca-like leaves”, Eryngium pandanifolium “with Pandanus-like leaves” (tropical plant resembling a palm). These names reference 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 persistent rosettes and are not very hardy. Examples: Eryngium pandanifolium, Eryngium eburneum, Eryngium agavifolium…

South American species are also the most impressive. They are among the largest, forming very handsome clumps of leaves.

Size of Eryngiums is quite variable, from 15 cm to 1.50 m in height… Even up to 4 m for Eryngium pandanifolium. Some dwarf varieties exist, 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 front of borders, while taller ones (E. pandanifolium, E. agavifolium, E. yuccifolium, E. giganteum…) suit back of borders as background. They may require staking to maintain upright habit.

Eryngiums are generally very ramified! Upright stems divide in almost geometric, regular fashion… each branching ending with an inflorescence. Eryngiums can take different forms. For example, Eryngium tripartitum appears untidy, branches spreading in all directions! It produces a multitude of small inflorescences on numerous branches, creating a vapourous effect. By contrast, Eryngium yuccifolium flowers as small white spheres at ends of tall, straight stems, producing a graphic, elegant effect.

Eryngiums flower in summer. Depending on variety, flowering occurs between July and October. Plants remain decorative after flowering when producing 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 consists of a large number of small flowers. Eryngium inflorescences are very original, quite unique, with a dry appearance.

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

Base of head is surrounded by a collar of spiny bracts, well developed, giving star-shaped form to inflorescence. Bracts vary by variety. They are thin and acute, very sharp in Eryngium variifolium! Combined with strongly spiny foliage, this species is likely very effective at deterring herbivores.

Bracts really enhance flowering, making it majestic. However, species with smaller bracts, such as Eryngium yuccifolium, are also very decorative, inflorescences appearing rounder for a graphic, almost geometric regularity. Where bracts are less developed, rounded aspect of heads stands out more.

When flowering, Eryngium tripartitum bears countless small globose, bluish heads. Given each head contains a very large number of tiny flowers… one can hardly imagine total number of flowers on one plant of this species! Its flowering appears as a profusion of stars blending into a mist.

Inflorescences of Eryngiums are generally blue-silver. Eryngium amethystinum owes its name to amethyst hue! Flowers are silvery-white in Eryngium eburneum, E. giganteum, E. yuccifolium… They are greenish, poorly coloured, in Eryngium agavifolium.

You can cut inflorescences for fresh or dried bouquets.

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

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

Besides original flowering, Eryngium foliage is also highly decorative. Leaves are generally spiny, protecting plants from herbivores. But Eryngiums can also have well-developed basal leaves, fairly rounded, green and supple, not spiny.

Depending on species and varieties, leaves may take very different forms. They can be very divided, deeply lobed, or rounded and entire. Sometimes leaves are very long and narrow, as in Eryngium pandanifolium. These unfold from an elegant basal rosette.

Panicauts possess basal leaves in a rosette and cauline leaves on stems. Latter may be alternate, placed one after another, or opposite (inserted two by two).

Leaf length varies greatly. In some species (notably European) leaves measure less than 10 cm, but can reach up to one metre in others. Leaves of South American species are most impressive, sometimes forming majestic basal rosettes.

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

Less common in cultivation, Eryngium venustum is exceptional for finely divided, regular, pointed foliage. Each leaf seems toothed around a main midrib. Rare, 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 allows good drought resistance (they seek water deep in soil), but makes transplantation more difficult and increases sensitivity to moulds (roots may rot if excess moisture).

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

In Eryngium campestre, infructescences have the peculiarity of detaching when ripe and rolling along ground, carried by wind, dispersing seeds. This behaviour gives species name “rolling thistle” (by deformation becoming “Chardon Roland”).

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

Main species and varieties

Most popular varieties

Our favourite varieties

Other varieties to discover

Eryngium alpinum Blue Star

Variety derived from blue alpine eryngo (Eryngium alpinum). Appreciated for inflorescences set on a collar of long, fine, very delicate bracts.

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

See the product sheet

Eryngium giganteum

Large Eryngium with silvery hue. Inflorescences formed as domed heads surrounded by a collar of fairly broad, well-developed bracts. Awarded RHS Award of Garden Merit (AGM).

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

See the product sheet

Eryngium planum

Fairly classic but essential Eryngium! Bears upright, nicely ramified stems topped by globular inflorescences surrounded by decorative bracts. Lovely silvery-blue hue.

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

See the product sheet

Eryngium zabelii Big Blue

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 spiny bracts. Foliage very distinctive, leathery and spiny, silvery in colour. Perfect species for a coastal garden!

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

See the product sheet

Eryngium planum Jade Frost

Eryngium notable for variegated foliage! Leaves green, marginate with cream-white, making them quite bright, sometimes slightly tinged with pink-purple. Also bears silvery-blue inflorescences.

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

See the product sheet

Eryngium variifolium

Unusual Eryngium with foliage marked in white, leaves and bracts very spiny and pointed. Species magnificent to look at, but best not touched!

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

See the product sheet

Eryngium bourgatii

This Eryngium bears inflorescences surrounded by very long, fine, sharp, radiating bracts. Foliage unusual: deeply cut and marbled with white.

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

See the product sheet

Eryngium agavifolium

Surprising Eryngium, very different from other species. Bears large, very long, strap-like, bright green leaves. From the clump 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, stems very ramified and appearing to go in all directions. Bears countless bluish heads, creating a vapoury, very light 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 position with good light. In sun, their colours are more beautiful, more pronounced, their flowering more generous, and their habit more robust, compact and sturdy. In shade, Eryngiums become more lax and lanky, and may require staking.

It is important to plant them in well-draining soil, even stony. You can easily place them in a rock garden or in 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, planting on a mound: water must be able to run off and must not stagnate. Eryngium tolerates drought well but dislikes stagnant moisture in winter.

As Eryngium tolerates sea spray well, you can plant it in a seaside garden. Species most suited to this situation is obviously Eryngium maritimum, which naturally grows on French coastline!

When siting eryngiums, take their adult size into account. Eryngium maritimum, E. bourgatii or E. variifolium are small and should be planted towards front of borders, while larger Eryngium giganteum, E. pandanifolium or E. agavifolium make a fine backdrop when placed behind lower-growing flower varieties.

Because of their long root (taproot), Eryngiums do not like to be moved. It is therefore preferable to plant them directly in 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, planting distance should be greater.

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

Keep an eye on young plants and continue to water them if necessary. Once well established, they will be fairly drought-tolerant and will no longer need watering.

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

Eryngium amethystinum

Caring for sea hollies

Eryngiums do not really require maintenance. As they grow very well in poor, low-fertility soils, it is unnecessary to add compost or fertiliser. Moreover, except during weeks following planting, there is generally no need to water them. These plants are fairly drought-tolerant! On the contrary, withholding fertiliser and avoiding overwatering will help Eryngiums maintain a compact, stocky habit.

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

Removing spent flowers encourages plant to extend its flowering. However, you may also choose to leave them in place, as spent flowers remain decorative in autumn–winter.

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

Main cultivation problems with Eryngiums are caused by overly wet conditions, which rot the root system or lead to 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 free-draining soil and avoid excess moisture.

Slugs sometimes nibble young plants when still tender. You can use a slug-control product such as Ferramol, or scatter sawdust or sand around them to create a physical barrier that will deter gastropods.

Multiplication

Eryngiums are mostly propagated by sowing. Division is complicated because of their long taproot. But it is also possible to propagate roots of Eryngium in late winter or early spring.

Sowing

Sowing allows propagation of most species. It is advisable to stratify seeds by placing them in the fridge for about a month before sowing.

1. Prepare a pot by filling it with a free‑draining growing medium (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 occasionally so the substrate remains relatively moist, and pot on seedlings once they are large enough to handle. Young seedlings can be transplanted easily, as their roots are still underdeveloped. Transplanting becomes more difficult with mature plants.

It is also possible to sow seeds directly in open ground in autumn or spring.

Combining eryngiums in the garden

You can use Eryngiums to design a no‑maintenance garden where you won’t need to water, add fertiliser or compost, etc. Use this opportunity, for example, to create a mineral‑dominant garden, a scree garden (gravel garden). Choose a warm, sunny spot, build a raised bed, place large stones and a free‑draining substrate. Plant Eryngiums alongside other drought‑tolerant plants. Favour cushion‑forming or mat‑forming plants (sedums, phlox, sempervivums, Cerastium…), those with silver foliage or succulent leaves. Add agaves, cacti, opuntias and Carpobrotus.

As Eryngium tolerates sea spray well, you can plant it in a seaside garden. Choose Eryngium maritimum, which occurs naturally on French coasts. It will charm you with its silver, stiff and thorny foliage, highly graphic. Pair it with Armeria maritima, rockroses, Helichrysum, santolinas, Senecio cineraria, and the small grass Lagurus ovatus.

Create a superb dry garden, in full sun, by planting Verbascum ‘Polarsommer’, with its downy foliage and tall yellow flowering spikes, paired with Eryngium oliverianum and some Centranthus. Include several clumps of Salvia nemorosa ‘Caradonna’ and Stipa tenuifolia in the bed to bring a natural, airy effect! You will obtain an original border that requires very little maintenance. Silver 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 will obtain a bed with a wild, untidy, country look. Add grasses (Stipa, Pennisetum…). You can also include Knautia macedonica, with its beautiful light crimson flowers. Also make use of yarrows, for example variety ‘Terracotta’.

You can also create an alpine garden, by including plants that occur naturally in mountain regions. Soil should be free‑draining, stony and, if possible, sunny. Choose the Alpine blue thistle, or Eryngium alpinum, and plant gentians, edelweiss, saxifrages, androsaces alongside. Favour mat‑forming plants and those with a cushion habit.

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

You can include Eryngiums 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 care sheet, and in our inspiration Create a trendy “thistle and company” planting

Did you know?

• King oyster mushroom

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