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Eryngium — sea holly

 

The genus

Eryngium — a genus of over two hundred herbaceous species, including annuals, biennials, and long-lived perennials, distributed widely around the world. They are classified in the umbellifer family (apiaceae), which they share with carrots, masterworts, lovage, and quite a few other plants in our garden. One feature shared by all members is smooth, hairless leaves.

The genus name is an ancient Greek word, which apparently alludes to the plants' prickliness. I have not been able to track down a meaning. The most common name encountered most to describe garden species is sea holly, although that name does not apply to all species – logically, it only refers to coastal species. Another common name is eryngo, which is more suitably applied to grassland and other inland species.

Over the years, I've tried growing quite a number of eryngium varieties, with greatly varying success. My first attempt was with E. planum, which has been very tenacious indeed - its offspring, greatly multiplied, still populates several areas of our garden. On the other hand, several species I've unsuccessfully tried to start from seed, while a few successful attempts yielded plants that proved to be short-lived in our garden. Still, it's one of my hobby genuses, for which I harbor mild collector tendencies. Nothing too extreme – I'm too stingy to pay much for plants, so almost all of my attempts come from seed.

I'm writing this page because eryngium seldom bubbles to the top of gardeners' plant lists (I know of no eryngium society), yet they're an interesting bunch, worth a closer look. So here goes. This page will remain rather tentative until next spring and summer, when I can take some updated photos highlighting differences between the species.

Cultivation

As mentioned above, eryngiums differ greatly in their ease of cultivation. Some, most notably E. planum and E. yuccifolium, are both long-lived as individual plants and make plenty of seedlings, requiring periodic removal. Both have strong taproots, are quite happy living in heavy clay soil, and handle dry conditions with aplomb.

On the other side of the spectrum, there are the species I can just barely keep alive. For example, I've started E. variifolium from seed several times. On several occasions, they did not come through the winter; when one finally did, it acted as a biennial, serving up flowers only once before dying. The similarly silvery species E. bourgatii is somewhat longer-lived, but still seems to fade away after a few years. As for E. agavifolium - I find they are long-lived once firmly established (as long as good drainage is provided), but can be hard to coax through their first year or two.

Notwithstanding obvious exceptions (for example, E. aquaticum), I generally presume that the eryngiums in our garden need good drainage, as well as full sun. For the most part, they don't require very fertile soil. For evergreen species, such as E. agavifolium, take care not to mulch too closely in winter.

My page only describes species that are hardy to our zone 6 garden. There are some wonderful varieties that just won't work here, but so be it. I once grew E. eburneum, which is probably hardy to zone 8. While it was still alive after the first few freezes of the year, it did not come through its first winter. Fortunately, there are still plenty of hardy kinds to try.

Propagation

Since most species grow from a single fleshy taproot, they are not easily divided (or even transplanted); those with experience with root cuttings may be successful at vegetative propagation (undertaken in late winter), but I've limited myself to sexual methods - seeds!

My records don't include much comparative data for different germination treatments: quite some time ago, I came to the conclusion that most species germinate best with extended cold conditioning before germination at room temperature, so that's my standard treatment. It works quite well for the prolific species mentioned above - often, germination already starts during the cold stage.

Some species resist most attempts to germinate. For example, E. tripartitum has yet to germinate for me (which may reflect its possible hybrid origin); I've given up trying. However, this is the one that responds well to division. As for E. agavifolium, I've collected seed from our garden many years, but only on one or two occasions were my attempts to produce offspring successful. While this may be due to sub-optimal germination conditions, I suspect that most of the problem is the low viability of the seed. In fact, seed viability is problematic for a good number of species - I've traded seed for several varieties, but they mostly appeared to be duds. Good seeds have some bulk – if you press down on them on a flat surface, they should not flatten. That's easy enough to ascertain for species with larger seeds, but not so straightforward for the smaller-seeded ones. So despite several attempts, I've yet to establish E. foetidum or E. amethystinum. For the kinds highlighted below, I've recorded detailed seed-starting records on the individual portrait pages.


Seed for E. planum

Seed for E. yuccifolium

A bevy of leaf shapes and colors

Some eryngiums have stunning foliage, others are just OK; some are evergreen, others take a retreat in winter. The overview below showcases some of leaves, as well as the growth habits of those that call our garden home.
Flat sea holly, E. planum, has deep green, fleshy, sometimes shiny leaves, with a gentle serration. This photo, taken as the plant was declining in December, isn't very typical of its appearance the rest of the season. By late summer, the plants often flop, laying their fading flowerstalks horizontally across whatever's nearby. E. tripartitum has superficially similar leaves (it may be a hybrid involving E. planum as one of its parents), but these are packed much more tightly together, somewhat smaller, and a lighter color of bluish green. Also, the plants maintain a more upright habit, forgoing the floppiness of E. planum. E. creticum continues in the same vein with slightly serrated, leathery leaves, a bit larger and floppier than the two first ones. A quick-growing species (this photo shows a first-year plant).
E. yuccifolium's name refers to the yucca-like leaves. Indeed they are similar: bluish-green straps, quite tough and fibrous, with a bit of prickliness along the edges. They are lax, and depending on the growing conditions and gardener's attention, can give the plant a bit of a messy appearance. The common name for this species is "Rattlesnake master", which refers to the use of its root by native americans as an antidote to rattler bites. E. agavifolium is another one named for its leaf shape, again quite appropriately. This plant is evergreen in our garden; in fact, it shows off its shiny, grass-green leaves lined with stiff spines most effectively in early winter. In early spring, the plants look a bit bedraggled, until new growth kicks in. In this species, the leaves definitely outshine the flowers. E. bourgatii is the first one we acquired that has variegated foliage. The whole plant is smaller than others we've grown, including the individual leaves, which are deeply cut, prickly at the edges, and a rather pretty combination of silvery-white and dark blue-green.
Moroccan sea holly, E. variifolium, lays on the contrast a little heavier: the green parts of the leaves are a deeper hue, while the silvery veins, which in E. bourgatii blend softly into the green, are quite distinct. Very striking – I wish I could keep the plant alive. The latest addition to our Eryngium collection, E. venustum has quickly become one of my favorites, mostly because of its marvellous leaves. They are a bright bluish-green with an almost reptilian quality to their arrangement of sharp-tipped leaflets along a central rib. Being somewhat evergreen, they maintain an interest throughout the year. Swamp eryngo, E. aquaticum, is the odd one out in this bunch. As its botanical and common names imply, this plant prefers a soggy position. The photo shows a first-year plant that showed plenty of potential - fleshy red-tinged stalks holding slender, attractive bright-green leaves. For reasons unknown to us, it didn't return for a second year; we'll be trying this one again.
As an annual, E. leavenworthii doesn't have much use for a basal rosette of leaves - it carries most of its foliage along its upright stalk. One of the prickliest-leaved eryngiums we grow.

Flowers

The eryngiums that are grown for their flowers tend to be the ones showing off steely-blue flowers and/or bracts, in the same vein as globe thistles. Others have less eye-popping floral characteristics, although for several the flowers still add pizzazz. Regardless of color, each of the ones I've had the opportunity to harvest seeds from has prickly bracts! See the table below for flower details.
E. planum is a member of the steely blue clan, with fairly large flowers that are attractive to insects. E. tripartitum is similar, but its flowers are smaller and more refined, especially the bracts. This photo was taken before the blue color really came through. Like the first two, E. creticum sends up branched stalks of many flowers that proceed from green through silvery to blue - with different flowers on the same stalk undergoing the transition at different times. I don't know if it's due to the inherent growth pattern of this species or because ours are planted in richer soil, but the flowers on this one seem to come more generously.
E. yuccifolium's flowers are greenish-white pompoms. They definitely add interest, but don't poke you in the eye about it. In flower, E. agavifolium is hardly a stunner. Inflorescences are dull white, somewhat elongated, with several clustered together on each flower stalk. If it weren't for the fact that I like to collect some seed every year, I would probably just lop the flower stalks off. E. bourgatii has smaller versions of the blue flowers displayed by the first two in this table. In peak bloom, both the leaves and the flowers combine to make a little gem of a plant.
E. variifolium sticks with the silver theme: its flowers and spiny bracts don't do the blue thing, but extend the silver from the leaf veining into the floral show. The flowers of E. venustum are in the greenish white camp, but with their neat arrangement in a central leader with side stems, and the crisp (and prickly) bracts surrounding pineapple-shaped flower clusters, I find them to be more ornamental than some others in this color corner. When you're a mere annual, you've got to have some stunning power to make yourself attractive to picky gardeners. And sure enough, E. leavenworthii plays the role admirably, with royal deep purple flowers and bracts. The photo here just shows the final stage - the stepwise coloration, proceeding through various phases of green and lighter purple, is fun to watch.

Other species

Undoubtedly the most obvious omission from this page is E. giganteum, a biennial with the same style of blue flower as E. planum and tripartitum. An especially well-known cultivar is 'Miss Willmott's Ghost', which has unusually pale bracts. It's still on my list of plants to try, although its biennial nature is a negative, given that most others highlighted on this page are fine perennials.

Others on my to-try list, some of which I've already tried unsuccessfully, are E. foetidum (Mexican coriander), E. amethystinum (amethyst sea holly), E. humile (a small rock garden species), E. alpinum, and E. integrifolium (blueflower eryngo, a U.S. wildflower).

Then there's the tender ones that won't survive in our garden. Although some are tempting, I'm not good at coddling plants through the winter, especially if it involves lifting them and taking them indoors. Species in this category include E. eburneum, E. pandanifolium, E. proteiflorum, and E. ebracteatum.

Further reading

Wikipedia article


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Last modified: June 03, 2012
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