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Sea Holly (Eryngium): Planting Tips and Species Guide

Dorothy is a master gardener, former newspaper reporter, and the author of several books. Michael is a landscape and nature photographer.

Big Blue Sea Holly (Eryngium zabelii).

Big Blue Sea Holly (Eryngium zabelii).

If You Want a Blue Flower in Your Garden, Try Sea Holly

When planning your flower garden, wouldn't it be nice to have something rare and unusual to show off to your neighbors? You can do just that if you plant Sea Holly flowers, as you can see from the photographs. Some of them are the rarest color available—blue! Once you plant them, they are so easy to maintain and require very little care. The Big Blue Sea Holly (Eryngium zabelii) is my personal favorite with its steel blue prickly flowers laying atop a beautiful mound of silvery blue leaves.

Place This Plant Carefully Due to Its Tap Roots

But, when it comes to Sea Holly, the initial planning is very important because they have tap roots that, although they can grow deep into the ground and find water where other plants can't, don't like to be disturbed once planted. Wherever you plant them, that's where they need to stay. Kathy Wolfe of the Washington State University Extension Service wrote a great paper on roots called The Hidden World of Roots.

Big Blue Sea Holly (Eryngium zabelii).

Big Blue Sea Holly (Eryngium zabelii).

How to Plant and Care for Sea Holly

Sea Holly plants love sunshine and normal or sandy soil. Once established, they are extremely drought resistant. (However, in long periods of drought, you will still want to make sure they get adequate water.)

Keep in mind that they will grow to almost three feet high and have a spread of about 24 inches, so plant them where they will have adequate room and access to plenty of sunshine.

Year-Round Maintenance

You will need to cut back the old stems to ground level, preferably in late fall (although you could wait until early spring, which won't be too late should you forget in the fall). They should come back with a vengeance, then provide you with bright blooms all summer long.

Flat Sea Holly (Eryngium alpinum) bud.

Flat Sea Holly (Eryngium alpinum) bud.

Many Different Species of Sea Holly Are Available

Most nurseries should have several different species of Eryngium, which have been cultivated as garden plants; and most of the species are considered as perennials in USDA Hardiness Zones 4–9. This is a brief overview of some of the most common ones you might find available, listing both the common name and the botanical name.

  • Flat (Eryngium alpinum): This species is native to Eastern Europe, growing two to three feet tall with silvery-blueish-green flower heads which protect the plant from the negative effects of overexposure to sun radiation.
  • Alpine (Eryngium planum): This plant will grow to about two feet high; it is a native plant in Switzerland.
  • Amethyst (Eryngium amethystinum): This native European plant is one of the coldest hardies in this particular genus, but it likes a calcium-rich soil to thrive. It will grow to about a foot to a foot and a half high showing gorgeous light blue to purple flowers, thus the name.
  • Mediterranean (Eryngium bourgatii): You will need well-drained soil and plenty of sun for this attention-getting plant, which is native to the Pyrenees and Morocco. The flowers are a blue-green color, making them extremely popular in container gardens as well.
  • Common (Eryngium maritimum): Unlike most of the others, the Common Sea Holly plant is one of the smallest of the species, only growing from about six to 18 inches tall.
  • Giant (Eryngium giganteum): If you plant this one, you'll want to put it in the back of your garden, as it may require staking and it will grow from three to four feet (or even higher). This plant is also known as Miss Willmot's Ghost, having been named for Ellen Willmot, the English horticulturist who was one of the first 60 recipients of the Victoria Medal of Honor in 1897, awarded by the Royal Horticultural Society Council to those it considered deserving of special recognition.
  • Rattlesnake Master (Eryngium yuccifolium): Native Americans used this flower as an antidote to rattlesnake bites, and that's how it got its name. It is native to the eastern United States and has button-like flowers in a pale chartreuse color (somewhere between yellow and green). It will, like some of the other Eryngium species, grow from two to six feet tall, so if you choose this one, be prepared to stake it and put it in the back of the garden, as it will tower above most of your other flowers.
Big Blue Sea Holly (Eryngium zabelii).

Big Blue Sea Holly (Eryngium zabelii).

Eryngium yuccifolium (Rattlesnake Master).

Eryngium yuccifolium (Rattlesnake Master).

Eryngium alpinum (Flat Sea Holly).

Eryngium alpinum (Flat Sea Holly).

Eryngium giganteum (Giant Sea Holly).

Eryngium giganteum (Giant Sea Holly).

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: my Eryngium is supposed to be zabelli, but it has green leaves, so is it?

Answer: It could be, as zabelli tends to have more green than other varieties. If the center part is completely green as well, it probably is zabelli.

© 2017 Mike and Dorothy McKenney


Mike and Dorothy McKenney (author) from United States on August 06, 2018:

Paul, can you tell me what type of sea holly you planted. Some of them are naturally grey-green color, although I wasn't aware of them turning blue, and then turning back to the original color. Let me know and I'll do some serious research. Thanks!

PaulBrennan on August 05, 2018:

I planted a sea holly in the front garden (with lots of compost into a clay soil and lots of sun). After a few days, and as expected, the flowers and petals turned from grey-green to a beautiful shiny blue. Now, a few weeks later the blue colour has disappeared and it's back to the original grey-green...Any ideas as t what's happening???

Mike and Dorothy McKenney (author) from United States on June 25, 2017:

Intriguing is a good word for these, indeed. Thanks for stopping by.

Emily Lantry from Tennessee on June 25, 2017:

I've never heard of this plant, but I am so intrigued! I really want to find some. Thank you so much for sharing!

Mike and Dorothy McKenney (author) from United States on June 20, 2017:

Thanks Dolores. I'm glad you've had success moving them; your thumb must be greener than

Dolores Monet from East Coast, United States on June 20, 2017:

Beautiful pictures! I have 2 sea hollies in my garden. I love the luminous blue/green hue. But now that shade is taking over, I worry that they will become unhappy. I have moved sea holly before as well as other plants people say you can't move. I wait until the plant is dormant in winter and dig them up with a large clump of the surrounding soil. I dig deep as well. This has worked for several plants.