Eryngium: how to grow eryngium and the best eryngium to plant
As varied in form as they are widespread, sea hollies lend instant character to a planting scheme with their sculptural bearing and shimmering colour. Photographs by Jason Ingram
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 eryngiums 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. 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.
What are eryngiums?
Eryngiums are 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. Eryngium yuccifolium lives in mid-western USA, and most evergreen species originate in South and Central America. Season? Late spring to late summer. Size? Eryngium range 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.
How to grow eryngiums
Best soil for eryngiums
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.
Where to plant eryngiums
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 lower-growing 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 as Eryngium 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, 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, Eryngium 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’.
How to grow eryngium from seed and divide
The robust biennial Eryngium 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.
Eryngium pests and diseases
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.
Which eryngiums to grow
This eryngium has 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.
Long-lived and clump-forming, native to the grass prairies of central and eastern USA. The soft, blue-green, tapered leaves of this eryngium resemble those of a yucca. Best left undisturbed as it develops a deep taproot. 80cm x 40cm. RHS H4, USDA 3a-8b.
Native to Brazil and Argentina, with evergreen basal rosettes of reflexed foliage reminiscent of a puya. This eryngiuim has 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.
Eryngium x tripartitum
An eryngium that is a 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.
Eryngium giganteum ‘Silver Ghost’
A fine form of the biennial Miss Willmott’s ghost. Native to the Caucasus and Iran. An eryngium that's vigorously self-seeding in open ground and can become dominant in soils that are to its liking. 75cm x 30cm. AGM. RHS H6.
More like this
An eryngium with 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 H4.
Eryngium pandanifolium ‘Physic Purple’
This eryngium 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.
A distinct eryngium 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.
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.
Where to see and buy eryngium
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
Dan Pearson is a British landscape designer, horticulturalist, writer and gardener. Dan has designed a number of award-winning gardens including the Best in Show Garden at Chelsea 2015.