Eryngium
As varied in form as they are widespread, sea hollies lend instant character to a planting scheme with their sculptural bearing and shimmering colour
The largest-flowered form, this queen-of-the-alps has a ruff of metallic-blue flowers. Grows in sub-alpine scrub and rocky areas in Europe, but prefers deep, well-drained soil in a garden setting. 90cm x 30cm. RHS H5, USDA 2a-8b†.
PLANT PROFILE
What A diverse range of thistle-like perennials. Though not all are armoured, most have an architectural appearance.
Origins Some grow in the sand dunes of northern Europe, others in dry, rocky areas of the Mediterranean and the Near East. E. yuccifolium lives in mid-western USA, and most evergreen species originate in South and Central America.
Season Late spring to late summer.
Size Ranging in size from a modest 30cm to a domineering 3m.
Conditions Given sunshine, an open aspect and free drainage, eryngiums are a hardy and long-lived genus.
Hardiness Mainly RHS H5, USDA 3a-8b.
Native to Brazil and Argentina, with evergreen basal rosettes of reflexed foliage reminiscent of a puya. Finely spoked flower spikes hold clusters of small, white thimble-like heads free of the foliage, which mounds to 40cm x 40cm (flower spikes to 1m). RHS H4.
Native to coastlines in northern Europe, most commonly found in sand dunes. Needs perfect drainage and a lack of competition in the border. Metallic-blue thistles and aluminium-coloured foliage. Gently self-seeds. 50cm x 30cm. RHS H5.
Evergreen basal rosettes of laurel green with barbed margins that are deceptively soft to the touch. Tall flower spikes with greenish-white flowers that blacken in autumn. Long-lived and gently self-seeding. 1.5m x 60cm. RHS H4.
Eryngiums are an adaptable and distinctive tribe of perennials with as much subtlety at one end of the spectrum as drama at the other. I first became aware of them in the mid 1970s through the renowned plantswoman Beth Chatto, who was using sea hollies on her stand at the RHS Chelsea Flower Show. I had seen our native Eryngium maritimum with its galvanised foliage and electric-blue thistles growing in sand dunes on the south coast, but Beth teamed it with drought-loving perennials and grasses to demonstrate its garden-worthiness. She introduced me to many more forms through her book The Dry Garden and through her plantings in her gardens in Colchester, Essex, notably in the Gravel Garden, with its spectacular display of drought-tolerant plants. Later, when travelling in the mountains of northern Spain, I saw for myself E. variifolium, with its armoury of thorns and silvery midribs, and the imposing, intense blue E. bourgatii ‘Picos Blue’, basking in sunshine on the tussocky scree.
Eryngiums have been part of an informal palette in British gardening for longer than one might imagine, used for their ability to conjure a wild place without feeling remotely like an interloper. The horticulturist and writer William Robinson wrote vividly about them in his book The Wild Garden, published in 1870, while the great plantswoman Ellen Willmott was well known for surreptitiously distributing seed of biennial Eryngium giganteum whenever she visited a garden. Miss Willmott’s ghost, as it became known, takes easily to a place it likes and these ‘wild’ introductions soon had their influence on a generally more formal Victorian horticulture.
Today, in these more informal times, eryngiums have become an established part of the perennial palette. Appearing at the height of summer, their distinctive form and metallic sheen team well with grasses and naturalistic perennials such as Verbena macdougalii ‘Lavender Spires’ and Oenothera lindheimeri (formerly Gaura lindheimeri). Flowering over a considerable period in summer, their compound, thimble-shaped flowers are magnets to pollinating insects. A plant such as E. x zabelii ‘Big Blue’ also has considerable impact in terms of colour – blue in almost all its parts and entirely otherworldly. Eryngiums have a long season, extending the nectar-producing period into late summer, and as they go over many hold their skeletal forms well into the winter to further extend their duration of interest.
A Mediterranean sea holly from Spain and the Pyrenees, found in rubbly ground. Heavily veined, thistly rosettes of foliage, and deep-blue, metallic flowers and stems intensified by dry growing conditions. 60cm x 30cm. AGM*. RHS H5.
Long-lived and clump forming, native to the grass prairies of central and eastern USA. The soft, bluegreen, tapered leaves resemble those of a yucca. Best left undisturbed as it develops a deep taproot. 80cm x 40cm. RHS H4, USDA 3a-8b.
Prefers damp soil in warmer countries, but must not lie wet in winter if it is to avoid frost damage in cooler climates. Cut leaves to the ground every few years to reduce a build-up of old foliage. 2.5m x 1m. RHS H4.
Given sunshine and free drainage, the majority of eryngiums are reliably perennial. The deciduous forms retreat back to a tight rosette in winter which, in the first part of summer, quickly in British gardening, used for their ability to conjure a wild place H4. expands to provide great visual interest prior to flowering: marbled and laced with silver in the case of E. variifolium, or jaggedly architectural in plants such as E. agavifolium. The brilliant laurel-green of the early rosettes of this species is arguably one of its best moments, but this Argentinian has power in its root and sends up a rapid bolt of vertical growth that suspends the greenish-white flowers clear in their own air space.
Several of the herbaceous species retain an airiness in their flowering panicles. E. ebracteatum, for example, has myriad tiny thimbles that are so delicate you might at first think it was a sanguisorba. E. eburneum, meanwhile, is distinctive for the pale, silvery-white thimbles that are held rigorously on branched, arching stems, as if mapping a constellation.
The sculptural nature of eryngiums is amplified in some of the evergreen species, with architectural foliage that can be relied upon year-round and impressive spires of summer flowers. The E. pandanifolium I grow in my Somerset garden Hillside is one around which you have to build a planting. It was given to me by the botanist James Compton, who collected it in Uruguay (although he tells me it grows more widely) where it was growing en masse on the banks of a stream. It is the exception to the rule in terms of growing conditions, as it loves damp feet and good living. Its viciously barbed, sword-like leaves erupt in a great rosette that, in high summer, sends stems of tiny, dark flowers soaring to as much as three metres in the air.
With glittering flowerheads that light up the garden and complement other colours so well, it’s easy to see how these care-free plants have become stalwarts of the summer border. And with the trend towards drier, hotter summers, they are destined to grace our gardens for years to come.
Dan Pearson is a writer and award-winning garden designer. danpearsonstudio.com
Eryngium x zabellii ‘Big Blue’ Spectacular for colour, intensely blue in all its flowering parts. Soft-green basal foliage. Not reliably long-lived in ground that lies wet in winter. 75cm x 30cm. AGM. RHS H5, USDA 5a-9b. 
Eryngium variifolium Distinct species from Morocco. Marbled foliage and spiny inflorescences with silvery midribs that make acute angles. Dramatic despite its quiet colouring. Clump-forming. 40cm x 30cm. RHS H4, USDA 5a-9b. 
Eryngium giganteum ‘Silver Ghost’ A fine form of the biennial Miss Willmott’s ghost. Native to the Caucasus and Iran. Vigorously self-seeding in open ground and can become dominant in soils that are to its liking. 75cm x 30cm. AGM. RHS H6. 
Eryngium planum Known as the flat-leaved sea holly, this robust perennial has green, rounded, basal leaves. The small flowers form in multi-headed sprays and are intensely blue. Easy to grow. 90cm x 30cm. RHS H5, USDA 5a-9b.
*Holds an Award of Garden Merit from the Royal Horticultural Society. †Hardiness ratings given where available.
WHERE TO SEE AND BUY
• Beeches Nursery Ashdon, nr Saffron Walden, Essex CB10 2HB. Tel 01799 584362, beechesnursery.co.uk
• Beth Chatto’s Nursery Clacton Road, Elmstead Market, Elmstead, Colchester, Essex CO7 7DB. Tel 01206 822007, bethchatto.co.uk
• Daisy Roots Jenningsbury, London Road, Hertford SG13 7NS. Tel 07958 563355, daisyroots.com
• Larch Cottage Nurseries Melkinthorpe, Penrith, Cumbria CA10 2DR. Tel 01931 712404, larchcottage.co.uk
• Macplants Berrybank Nursery, 5 Boggs Holdings, Pencaitland, East Lothian EH34 5BA. Tel 01875 341179, macplants.co.uk
Hybrid of unknown parentage. Reliably perennial, with a basal rosette of soft, dock-like leaves and airy sprays of small but intensely coloured electric-blue flowers. Long-lived and easy to grow. 60cm x 30cm. AGM. RHS H5.
Grass-like, evergreen, glaucous foliage and tall, airy stems with tiny, greenish-purple flowers. Happily seeds into muddy pond margins in a mild climate, and can tolerate damp ground if it drains freely in a cooler climate. 1.2m x 75cm. RHS
How to grow Eryngium
• All eryngiums need sun and the majority need friable soil and free drainage. Think of their origins in the wild, of sea holly in the open conditions of their native sand dunes and the rocky hillsides and mountainous screes of the Mediterranean where they favour open ground and a non-competitive environment. If your soil is heavy, improve drainage with the addition of grit or plant on a slope where soils tend not to sit wet.
• The herbaceous species, which die back to a basal rosette in winter, prefer not to be overshadowed, so find a position with plenty of air that allows light to reach them. Plant lowergrowing species to the front of a border so that you can see the handsome basal foliage along with other sun lovers such as Nerine bowdenii. If you are combining the taller perennial species, such asE. yuccifolium, in a naturalistic matrix planting, use them among low sub-shrubs, such as Ballota pseudodictamnus or Lavandula pedunculata, or with low perennials, such as Calamintha nepetoides and Salvia x jamensis, or short-growing grasses such as Stipa tenuissima.
• Several of the species from the Americas can tolerate more retentive ground, but, apart from the marsh rattlesnake master E. aquaticum, they will not tolerate soil that lies wet in winter. The evergreen foliage needs year-round exposure to light, so use them as stand-alone groups with ephemeral annuals such as eschscholzias, or low perennials such as Erigeron karvinskianus or Viola riviniana ‘Purpurea Group’.
• The robust biennial E. giganteum regenerates freely from seed and can become dominant where conditions suit it in open ground. If you are using it in a gravel garden, deepen the gravel mulch to 5cm where you do not want them to seed and keep the mulch thinner where you do. To prevent seedlings in a border, cut stems to the base before the seed fully ripens and leave the minimum standing.
• Eryngiums tend to have a deep root system that resents disturbance and they do not respond well to division. They are best propagated from root cuttings in late winter or seed sown when fresh. Flowering-sized plants take a year or two depending on the species.
• Plants can suffer from mildew in positions where there isn’t enough air movement. Though not common, leaf and bud eel worm can be problematic, as can root rot in poorly drained conditions.