Blue Eryngo (Eryngium planum)

Blue Eryngo, Blue Sea Holly, Flat Sea Holly, Sea Holly, Sea Star Thistle, Star Thistle

Blue eryngo (Eryngium planum) grows native in southeastern Europe and central Asia. It is a species of thistle that produces a blue, egg-shaped bloom in summer. It is used in landscaping for full-sun beds and borders.

I. Appearance and Characteristics 

Eryngium planum, the blue eryngo or flat sea holly, is a species of flowering plant in the family Apiaceae, native to the area that includes central and southeastern Europe and central Asia.

These plants generally grow anywhere from 18 to 36 inches (45-90 cm.) tall with a one foot (30 cm.) spread. Their green or silvery-blue stems give way to green or blue cones surrounded by spiky silver, white, green, blue or violet bracts, which bloom from summer through fall. Sea holly plants are tolerant of drought, winds, salt sprays and sandy soils. They can be used as specimen plantings, in beds and borders, or butterfly gardens. In addition, these plants make excellent dried flowers.

II. How to Grow and Care

Sunlight

A full day of sun (at least eight hours) will give you the strongest sea holly plants and the most blooms. While the plants can handle partial shade, the reduction in light may lead to weaker stems, which may make it necessary to stake the plants to keep them upright.

Temperature and Humidity

As long as it’s planted within its proper hardiness range (zones 5 to 9), sea holly does not have any additional temperature or humidity requirements. It will do better in the cooler days of spring and fall but will not die off in warmer weather.

Watering

Thanks to the long taproot, sea holly plants are very drought tolerant once established and won’t need additional water unless subjected to a prolonged drought at the peak of summer. That being said, excess surface moisture can cause the flower’s crown to rot, so sea holly plants should be somewhat segregated from other plants that require more water. If and when watering is necessary for the plant, aim the water source at the base of the plant to reduce the risk of built-up moisture.

Soil

The best planting medium for sea holly plants is well-drained, sandy, poor to moderately fertile soil. Soil that is too rich can make sea hollies sprawl. Good drainage is a must, so amend compact soil with compost. Additionally, sea holly is not particular about its soil pH—anything around the neutral range is sufficient (pH 6.1–7.8). However, the plants do need good drainage or they will be susceptible to root rot and can die off.

Fertilizing

Sea holly plants are not heavy feeders. In fact, too much fertilizer will make the plants sprawl. Plant these where you have poor, moderately fertile soil in your garden.

Planting Instructions

Choose a full sun location with well-draining soil for sea holly. They can tolerate poor soil but not wet soil. Don’t plant them next to a pathway or at the edge of a border because the flower spikes are sharp. Plant several together in a cottage garden setting or add them to a rock garden.

Sow sea holly seeds in late summer or fall in a bed amended for excellent drainage. They will germinate the following spring after a period of cold stratification but likely won’t bloom the first year.

Sea holly seeds can be started indoors, but they need to go through a cold period. The easiest way to do that is to place the seeds in the refrigerator for four weeks, beginning 10-16 weeks before the last spring frost date. Then sow them in small containers filled with sandy soil, but don’t cover them; they need light to germinate. Put them in a warm area and expect germination in 7-10 days. Allow them to grow in their containers for a few weeks until the seedlings are robust. Harden the plants off before transplanting them to the garden. Plant sea holly transplants in the spring after all danger of frost has passed. Put them in holes a few inches wider and only slightly deeper that their rootballs. Choose their permanent location carefully because sea holly doesn’t handle being transplanted well.

Pruning

Deadheading spent blooms extends the blooming season. At the end of the season, cut the plant back to a few inches from the ground, although the spent flowers are attractive enough to justify leaving the plants as is for the winter.

Propagation

Sea holly’s taproot can make it a bit difficult to propagate, but it’s possible to do it via root cuttings:

  • Carefully dig up the plants in late summer (taproot and all), then cutaway healthy sections of roots (but never cut away more than one-third of the plant’s total root mass)
  • Replant the parent clump immediately, then plant the individual root sections in a mixture of compost and vermiculite in a small pot, making sure the cut end (crown side) of the root segment is facing up. The root segment should be entirely buried, but the cut tip should be just barely under the surface.
  • Overwinter the pots in a sheltered location. In spring, the root segments should begin to generate new roots, which you will see poking out of the bottom of the pot.
  • When a network of roots is established and green shoots are emerging above the soil line, you can transplant your new sea holly plant into the garden.

Pests and Diseases

Common Pests & Plant Diseases

In dry soils, sea holly is generally free of nearly all pests and diseases. In damp soil, it can be susceptible to fungal leaf spot diseases and root rot. Powdery mildew can also be a problem; base watering and good air circulation will minimize mildew infection. Moist conditions may encourage aphids, slugs, and snails to feed on the foliage.

Fungicides can help control serious leaf spot disease. Aphids can usually be easily controlled with horticultural oil (or simply ignored, since the damage is rarely serious). Slugs and snails can be removed by hand or controlled with baits.

Common Problems 

Sea holly is largely free of complaints, but you may find that the plants become overly leggy and sprawling if it is planted in soil that is too fertile and rich. Thus, it’s best to reserve this plant for those parts of the garden with dry, sandy soil where other plants won’t grow. Such sprawling may also occur if sea holly doesn’t get enough sun, or if you overwater it.

Potting and Repotting 

Sea holly is an excellent container plant. Choose a container with good drainage and fill it with sandy soil. Add a plant and provide it with at least six hours of full sun daily, but don’t water it often. If you combine it with other plants in a container, choose companion plants that prefer the same poor soil and dry environment sea holly prefers. Sea holly doesn’t transplant well, so repotting may not be successful.

Overwintering

Provide some protection against winter cold to allow the rosettes to overwinter successfully, or the plant will use all of its energy making new foliage in the spring. Lay evergreen branches over the plants or mulch with pine needles. Withhold water in fall, as it does not care for damp winter soil. After flowering is complete, cut the flower stalks and foliage back to just above ground level. You can also leave the flowers for winter interest.

How to Get to Bloom

When grown in properly dry soil and sunny conditions, there’s rarely any problem getting sea holly to bloom in its typical summer-to-fall flowering period. Failure to bloom can be the result of too little sun, but it can also be the result of soil that is too rich and fertile.

III. Types of Sea Holly

Many of the newer types of sea holly boast dwarf habits, which is beneficial as some can become quite large—up to 6 feet tall. Other varieties feature richer blues, and a few have golden foliage that creates a stunning look with the steel blue flowers.

  • ‘Blue Cap’ Sea Holly

Eryngium planum ‘Blue Cap’ is a clump-forming perennial that features small steel-blue flowers with spiky collars of narrow bracts that are standout cut flowers. The plant stands 2-3 feet tall in zones 4-8.

  • ‘Blue Hobbit’ Sea Holly

Tiny Eryngium planum ‘Blue Hobbit’ stands only 8-12 inches tall, but it produces a profusion of purplish-blue, egg-shaped flowers. Zones 4-8

  • ‘Blue Glitter’ Sea Holly

The flowers of Eryngium planum ‘Blue Glitter’ stand well above the basal rosette of leaves. The plant produces many tightly packed, glittering steel-blue flowers throughout the summer growing season. It matures quickly and stands 2-3 feet tall with a 1-2 foot spread. Zones 4-8

IV. Uses and Benefits 

  • Ornamental uses

Blue eryngo is worth considering for garden sites with heavy sun exposure and can be planted in naturalized border areas. This plant’s compact nature also makes it ideal for small garden sites, as a specimen plant, and for narrow borders when planted in groups. The flowers can also be cut and dried for floral arrangements.

  • Medicinal uses

Eryngium planum is used in European folk medicine as a diuretic, a stimulant, and an appetizer owing to its essential oils, and bioactive compounds, and in this usage it may be known as Eryngii plani herba or Eryngii plani radix.

Blue Eryngo (Eryngium planum) Details

Common name Blue Eryngo, Blue Sea Holly, Flat Sea Holly, Sea Holly, Sea Star Thistle, Star Thistle
Botanical name Eryngium planum
Plant type Perennial
Hardiness zone 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b
Height 2 ft. 0 in. - 3 ft. 0 in.
Width 2 ft. 0 in. - 3 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition Loam (Silt)
Flower color Blue
Leaf color Green