When to plant
Most deciduous erygniums set viable seed. This should be sown in late-summer and autumn and will germinate in the following spring. Plant out young in the following autumn or spring. Seedlings may vary.
Where to plant
Plant eryngiums where there is bright light, poor soil and good drainage in order to develop a strong, rigid framework and steely patina. If grown on damp, heavy soil (or in wetter parts of the country) most eryngiums stems tend to flop and become a dull, grey-green.
However, eryngiums are very diverse: there are over 240 species worldwide. If you really want to grow them, try one or two in the driest hot spots you have. Some even do well on clay.
How to plant
Prick sea holly seedlings out when the true leaves appear into individual pots of gritty compost.
The easiest are the perennial eryngiums that appear year after year. The shorter E. bourgatti 'Picos Blue' has crinkled, variegated foliage and deep-blue thimbles. This is easier to place than amethyst-flowered forms.
Miss Willmott’s Ghost, Eryngium giganteum, will seed down readily in gardens. This tap-rooted, drought-resistant plant produces silver ruffs and bee-pleasing thimbles on silver stems.
Start off by buying two ready-grown plants in one year and then two in the following year. Leave the seed heads to allow the seeds to fall and germinate. This way you will have flowering eryngiums every year, not every other year.
When it blooms
Summer-flowering sea hollies provide a long-lasting flowerhead that eventually browns and fades, giving months of value. For summer flowering, these sea hollies die down completely in winter and, given good drainage, will return every year.
Root cuttings taken in early spring are the best way to propagate the rosette formers vegetatively to reproduce the same plant. Lay lengths of root on the compost and cover lightly.
Top sea holly varieties
Summer-flowering sea hollies
Eryngium x oliverianum AGM
This hybrid sea holly, originally grown from seeds collected in the wild in 1731, may be a hybrid between E. planum and E. alpinum. The simple blue thimble is surrounded by roughly nine to fifteen long, violet-blue feathered bracts that splay straight out. Foliage is not spectacular with dark-green three lobed leaves (90 cm 3ft). Can be divided in spring.
Eryngium x zabelii
Spiny leaves and dark-blue stems topped with a branching stem containing oval blue thimbles ringed in upward-facing bracts. A graceful habit - a hybrid betweem E.planum and E. bourgatii. Can be divided in spring. Good forms include 'Jos Eijking' (a vigorous violet-blue) and the greyer-blue 'Donard Variety' (60cm - 90cm/ up to 3ft).
A small-flowered willowy eryngium which develops bright blue stems in good summers. Short blue bracts surround the flowers. Often used as a cut flower (90 cm/3ft). The most commonly grown form, 'Blaukappe', is rather short and stumpy.
Eryngium alpinum 'Superbum'
This sea holly has the largest flowers and the most feathery bracts of all: they almost form a complete frill. However its amethyst flowers are more difficult to place as they can look very washed out in full sun.
Eryngium x tripartitum AGM
A superior hybrid from E. planum with sterile flowers, so therefore does not seed. There is a variegated form 'Jade Frost', but this tends to revert back to green.
Winter-flowering sea hollies
Some eryngiums from South America have evergreen rosettes and tiny button-like heads in shades of brown and red. These evergreens need moister soil, but they are also less hardy.
I grow several in the Cotswolds and, although they do re-appear here following harsh winters, they struggle to flower as the rosette normally disappears completely in cold weather. However, these rosette formers do much better in southern England because their foliage persists throughout winter, although it can get ragged. These evergreens are most often used in prairie planting to add an extra silhouette in winter.
They rarely set viable seed in Britain and are mostly propagated from root cuttings. They are generally propagated by root cuttings and only certain specialist nurseries sell them. Phoenix Perennials (01420 560695) have some interesting eryngiums including 'Physic Purple'. They include:
Probably the most interesting and impressive with lots of tiny thimbles later in the year. Some forms have dark-purplish heads that age to dark-maroon. 'Physic Purple' (grown from seed collected at The Chelsea Physic Garden) is the best form. Usually 5 ft/1.5m - but can reach a triffid-like 12ft/3-4 m in the right place.
Greenish white flowers, which tend to be oval, with sword-shaped foliage (up to 2m/6ft when happy).
From Argentina, with rich-green toothed leaves and upright green stems topped by roughly 20 green thimbles (90-120 cm/3-4 ft).