Sea hollies belong to the genus Eryngium and are members of the Parsley Family, Apiaceae. There are about 230 species found throughout the world, primarily in well-drained locations. Plants may be perennial, biennial or annual and range from being very cold hardy (zone 4) to subtropical. As garden ornamentals, we grow just a select few, most which hail from Europe. Their main claim to fame are their unique metallic silvery-blue, spiny thistle-like flowers which make a bold statement in the garden, last for weeks and also lend themselves admirably as both a fresh and dried cut-flower. There are a few low-growing alpines species but most of the popular species and selections are mid-height perennials reaching 75 to 120 cm. In cultivation, provide them with a well-drained, sunny site. They are not fussy about the soil pH. The hardy species are best grown from seed which require a stratification period before they will germinate. Mature plants resent transplanting so use young plants and plant them where they will remain. Like fine wine, they improve with age.
Sea holly were traditionally used in herbal medicines. The roots of E. maritimum were used as an aphrodisiac. The roots of most species are rich in iron and minerals and may be peeled and candied. They were popularly used as cough drops in the 18th century.
Several of the species that are popularly grown as garden ornamentals appear very similar. Three look-alike species include E. planum, E. amethystinum and E. tripartitum. All of these have oval-shaped, somewhat leathery basal leaves and large open clusters of somewhat rounded 2 cm silvery-blue flowerheads. They are hardy to zone 4.
Close-up of E. planum and E. amethystinum
The species E. campestre and E. bourgatii both have much-divided, spiny-tipped leaves and somewhat silvery veins. They grow a little shorter at 45 to 60 cm. Their flowers are arranged more densely and have larger bracts than the preceding species. These two may be used effectively in a larger rock garden setting. These two are hardy to zone 5.
Details of E. bourgatii
Short-lived but quite unique and showy are the silvery-foliaged E. maritimum and the robust giant E. giganteum, commonly called ‘Miss Willmot's Ghost'. The former species has silvery-blue to grey foliage that is broad with spiny-tipped serrations. Plants reach 60 cm with pale blue flowerheads subtended by broad, spiny bracts. This species is as attractive for its foliage as its blooms. Found along sand-dunes of the Mediterranean, this species is now naturalized along the east coast of the USA. The latter species may reach over 1 m. The basal leaves are broad and oval with sharp, but not too spiny, serrations. The flowers are quite large and thimble-shaped, reaching to 6 cm in length. The flowers are silvery-white and subtended by large showy broad, silvery bracts. Both are rated for zones 5 to 6 and will require seed-saving and replanting to keep them going in the garden.
Details of E. giganteum (left and center) and E. maritimum (right)
Perhaps the most impressive species is E. alpinum. The basal foliage is broadly ovate with spiny serrations. The flower stems are stout and reach to 60 cm. This species has fewer flowers per stem than many of the others but what they lack in number they make up for in size. The thimble-shaped flowerheads can reach 8 cm in length and are subtended by an impressive skirt of long, narrow spiny bracts. The best forms have quite dark silvery-blue flowers.
Details of E. alpinum
A noteworthy hybrid is E. x zabelii, a hybrid between E. alpinum and E. bourgatii. The basal foliage is most similar to E. alpinum but the stem foliage is narrow, spiny and silver-veined like those of E. bourgatii. The flowers are produced in an impressive cluster and appear like small versions of E. alpinum. Plants are compact and reach just 45 cm. A very worthwhile hybrid if obtainable. Also impressive but hard to find is the hybrid between E. alpinum and E. giganteum called E. X oliverianum, a hybrid showing the best of both parents.
Details of E. x zabellii
There are many other sea hollies hardy in zones 7 and warmer, but the focus of this article are those hardier species. One last species worth mentioning is an American species, E. yuccifolium. This is hardiest of the American (which includes South America) species, rated for zone 4. The basal foliage is, as the name would suggest, very Yucca-like, quite different from any of the European species. In late summer arises impressive stout stems to 180 cm! Only a few flowers are produced and they are silvery-white, not as showy as the previous species noted but still worthwhile as a bold statement plant and cut-flower.
Flowers and foliage of E. yuccifolium
I would like to thank the following people for the use of their pictures: beanmomma (E. yuccifolium close-up), dpacifici (E. amethystinum), Equilibrium (E. yuccifolium foliage), galanthophile (E. giganteum), Kell (E. giganteum close-up), mgh (E. X zabellii) and mike_freck (E. bourgatii flower)