Sea Holly (Eryngium) Plant Profile

Eryngium, Sea Holly

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Sea holly plants (Eryngium) are low-maintenance and produce striking purple-blue flowers that look like small glowing thistles. They are very similar to globe thistle (Echinops), but sea holly flowers have green or blue cones and a distinctive bract collar in silver, white, green or bluish-purple. The colors often look almost metallic and painted on and can change in the sunlight. Sea holly plants can have green or silvery-blue stems. The leaves can be long and thin, deeply lobed, or even round, depending on the variety. The plants are very tolerant of dry conditions and also handle salt spray with ease.

  • Botanical Name: Eryngium species
  • Common Names: Sea holly, Miss Wilmott's ghost, rattlesnake master, button snakeroot
  • Plant Type: Perennial flower
  • Mature Size: 6 to 18 inches
  • Sun Exposure: Full sun
  • Soil Type: Sandy
  • Soil pH: 6.5 to 7.5
  • Bloom Time: Summer to fall
  • Flower Color: Green, blue, blue-purple, silver, white
  • Hardiness Zones: 4, 5, 6, 7, 8, 9
  • Native Area: Europe and Mediterranean countries

How to Grow Sea Holly

Flowering starts in mid-summer and will continue into the fall. You will enjoy how low-maintenance they are as they don't like excessive water and fertilizer and they also don't like to be moved. Sea holly will bloom longer if you deadhead the spent flowers, but they look wonderful long into winter, so leave the fall flowers on the stems. The birds will thank you.

This is a great flower for all those spots in the garden where the hose doesn’t reach or in that infamous "hell strip" between the sidewalk and the street. But don’t confine it there. The blues and silvers blend well with just about every color, especially yellow and orange. Pair it with rudbeckia, coreopsis, zinnia, and cosmos. The tall varieties can need some support and planting them behind sturdy plants, like coneflowers, will help keep them standing.

Sea holly is popular with bees and butterflies, but not with deer and rabbits. The flowers last several days in a cut flower arrangement.

Light

A full day of sun will give you’re the strongest sea holly plants and the most blooms. They can handle a bit of partial shade, but the plants will be floppy.

Soil

Sea holly is not particular about soil pH. Anything around the neutral range would be good. However, the plants need good drainage or they will die off.

Water

Once established, sea holly is very drought tolerant and won’t need additional water unless subjected to a prolonged, hot, drought. Sea holly has a long taproot. This makes it very drought tolerant, but it also means it is getting its water from deep in the soil and surface moisture will just allow the crown to rot.

Temperature and Humidity

Most sea holly species are reliably perennial in USDA Hardiness Zones 4 to 9. Although they can be more difficult to grow in areas with cold, wet winters, if you can get them established, you should have good luck with them.

Fertilizer

Sea holly is not a heavy feeder, but you should still make sure your soil has plenty of organic matter in it. If not, you will need to feed or side dress with compost in mid-season.

Propagating Sea Holly

Because sea holly has a tap root, it does not divide easily. You can, however, take root cuttings in the spring. Sea holly transplants easily as seedlings, but it gets more difficult as the plants mature, which is why you don’t see a lot of them in plant nurseries. If you want to move volunteers (seedlings that sprout up on their own), do it early.

Varieties of Sea Holly

New introductions come out every year and claim the shelf space of older types. They’re all lovely, but some grow better in different conditions, so check to see which are successful in your area.

  • "Jade Frost" has variegated foliage and pink margins and veins.
  • "Giant Sea Holly" can grow as tall as 4 feet. Giant sea holly also goes by the name "Miss Wilmot’s Ghost", because of its shadowy coloring. It is named for an English gardener, Ellen Wilmot.
  • "Blue Glitter" has dozens of blooms on each plant, with gray-blue foliage.
  • "Sapphire Blue" is a common favorite with blue flowers and leaves
  • "Tiny Jackpot" grows to 14 inches tall and looks good at the front of a border.

Growing From Seeds

Most sea holly varieties can be started from seed. They do best if stratified first. The easiest method is to direct sow in the fall and then be patient and wait to see what sprouts in the spring. But you could start sea holly seeds indoors if you chill them for about four weeks in the refrigerator and then move them out to germinate in about two to three weeks.

Common Pests and Diseases

Problems with sea holly are usually caused by damp conditions either in the soil or air. Slugs are about the only pest to break through sea holly’s defenses. If your plants are in a dry, sunny spot, slugs should not be a major problem.

Sea holly is susceptible to root rot and powdery mildew. Rot root can be controlled if you have well-draining soil. Powdery mildew is tougher to control, but you can try a baking soda or milk spray.