Eryngium, the Sea Holly

Erygium (sea holly) being visited by a bettle, e. de raedt, july
Francois De Heel/Photolibrary/Getty Images

If you’ve given up on finding a showy flower to grow in that sun-baked part of the landscape, perhaps in the hellstrip between the sidewalk and street or along the driveway, look at the merits of the sea holly. All this low-maintenance plant asks is a full day of sun and good drainage. In return, you’ll get otherworldly blue flowers and foliage that are showing up in trendy floral arrangements from wedding bouquets to hotel lobbies.

Get to Know Sea Holly

The genus Eryngium belongs to the Apiaceae plant family. This family contains edible plants like sweet fennel as well as poisonous plants like the water hemlock. The common denominator of the Apiaceae family is that the plants have hollow stems, and that they produce compound umbels, which are small flowers that radiate from a single stem. 

You might hear the sea holly referred to as Miss Wilmott's ghost, after Ellen Wilmott, an English horticulturist who like to sprinkle Erngium seeds in other people's gardens. Other common names of rattlesnake master and button snakeroot refer to the use of this plant as an old-fashioned snake bite remedy. 

Sea holly plants grow in zones 4-9, and bloom from July through September. Flowers appear as blue or green cones with a spiky blue, grey, white, or green bract. Foliage is usually long and narrow, although in Europe gardeners prefer plants with lobed leaves. A full day of sun is necessary for best color and form. The plants look dull and tend to flop over in partial shade. As the name indicates, sea holly plants don’t mind some wind and sea spray in coastal gardens.

Start Sea Holly From Seed

Resourceful gardeners can slash their landscaping expenses by growing easy perennials like the sea holly from seed, filling up large areas of the flowerbed for a few dollars. Sea holly seeds aren't difficult to germinate, provided you follow two rules to mimic their natural growing conditions. First, ensure that the seeds are only pressed into the soil enough to make contact, not buried beneath it. Sea holly seeds need light to trigger germination. Second, stratify the seeds to trick them into thinking they've experienced a period of winter dormancy. You can do this by refrigerating the pots or cell packs you've planted for two weeks. Following these steps, keep the seedbeds moist, and you should see germination in about a month. 

Sea Holly Planting Tips

You can plant eryngiums in dry or moist soils, but good drainage is critical. One of the keys to the drought tolerance of the eryngium is the taproot, which plunges deep into the earth to seek out moisture during dry spells. This characteristic makes the eryngium resistant to transplanting, which is why you’ll only find young plants for sale at the nursery.

Sometimes eryngium plants fail to emerge in the spring, and gardeners mistakenly think the plants weren’t hardy in their growing zone. In fact, wet winter soil can cause the plants to rot, so if you experience long periods of soggy soil in early spring, install your sea holly plants in a raised bed.

Sea Holly Care

Water your sea holly plants deeply, but infrequently during the first season of growth. This will encourage the tap root to grow deep into the soil, a key to sea holly survival during droughts. Otherwise, the sea holly thrives on benign neglect. An excessively fertile soil makes plants lanky, so withhold the fertilizer. Deadheading will only leave you with scratches from the thorns, and won’t yield additional blooms. In fact, the faded flowers add winter interest long after the first frost occurs. If you don’t want any sea holly volunteers, sheer the top of the plants down to the foliage level with a pair of hedge trimmers to avoid coming into contact with the spines.

Garden Design With Sea Holly

The true blue flowers of eryngium blend pleasingly with pink or purple flowers in a cottage garden, and also pop against green, orange or yellow flowers. With its branching habit, one stem of eryngium combined with a few other zinnias or celosia make an easy bouquet. Plant eryngium with flowers that have textural as well as color contrast, such as ‘Moonshine’ yarrow.

As with many blue flowers, the eryngium attracts butterflies. If you use this in a coastal garden, consider planting your eryngiums near a fence or building to provide a butterfly-friendly windbreak.

Sea Holly Varieties to Try

  • Big Blue: Four-inch blooms are larger than other varieties
  • Blue Glitter: Expect several dozen blooms per plant; foliage is grayish-blue as well
  • Blue Hobbit: Mounding plants one foot tall brighten up the rock garden
  • Jade Frost: A variegated cultivar, featuring leaves with a pink margin that eventually turns to white
  • Sapphire Blue: Introduced in 1999, a widespread favorite with blue flowers and leaves
  • Tiny Jackpot: If you’re looking for a sea holly for the front of the border, this variety only grows 14 inches tall