Flat sea holly – info, planting, care and tips

Flat sea holly (Eryngium planum)
Flat sea holly (Eryngium planum) - by Bogdan

The flat sea holly inspires with its blue flowers, which are surrounded by split bracts in the form of a collar. Here are tips on planting and care.

Profile of flat sea holly:

Scientific name: Eryngium planum

Plant family: umbellifer family (Apiaceae)

Other names: blue eryngo

Sowing time: spring

Planting time: spring or autumn

Flowering period: July to September

Location: sunny

Soil quality: Soil type

sandy to loamy, calcipholous, nutrient rich, low in humus

These information are for temperate climate!

Use in: flower beds, bouquets, borders, flower garden, natural garden, rock garden

Winter hardiness: hardy, USDA Plant Hardiness Zones: 4 (-32 °C / -25 °F)

Bee and insect friendly: Yes

Plant characteristics and classification of flat sea holly

Plant order, origin and occurrence of flat sea holly

Eryngium planum is sold under the names flat sea holly or blue eryngo. The plants from the umbellifer family (Apiaceae) are often referred to as a thistle because they look like thistles. This species is found in nature in Central Europe as well as in the Middle East and Central Asia. It prefers to settle in dry and poor, sunny and warm places, i.e. often on sandy soils. Eryngium planum was first mentioned in writing in the middle of the 18th century. At that time, sea holly was also used medicinally, for example as a remedy for gastrointestinal diseases or as an aphrodisiac.

Characteristics of flat sea holly


The perennial, herbaceous flat sea holly is one of the hardy perennials. Depending on the location, it is 40 to 80 centimeters (16 to 32 in) high, grows upright and forms lush clusters over time. The gray-green stems are smooth or only slightly covered with thorns. In the subsoil, fleshy roots form, which used to be candied in England and France and eaten as sweets.


The gray-green, heart-shaped foliage of blue eryngo is arranged alternately on the shoots. The lower, basal leaves are elongated and stalked, the upper ones sit directly on the stem. The leaf margins are serrated and denticulated with thorns. In autumn the leaves dry up, but are still an ornament.


The flowers of flat sea holly are actually pseudanthiums that stand together in a 1 to 1.5 centimeters (0.4 to 0.6 in) large, spherical, blue metallic shimmering inflorescence. At the base they are surrounded by strikingly long and narrow, strongly divided and blue-colored bracts that protrude like a collar. The flowering period of flat sea holly extends from July to August or September, sometimes even to October. The plant is a valuable pasture for bees and butterflies.


The seeds of the blue eryngo are formed by insect and self-pollination. The wild species like to seed themselves.

Flat sea holly – cultivation and care


A warm, sunny place in the garden is ideal for blue eryngo.


The soil should be well-drained and dry to fresh. In dry locations, the stems and bracts become more intense. Eryngium planum also needs a calcareous and quite nutrient-rich subsurface.


The flat sea holly comes into its own individually or in small groups of three to five pieces. A planting distance of around 40 centimeters (16 in) is ideal.


If you want to prevent self-sowing, cut off what has faded in autumn. However, do not cap the whole plant, as it is a beautiful sight even in winter, with hoar frost or snow.


The wild species of flat sea holly is most easily propagated by sowing; the offspring are always slightly different in height and blue color. Varieties are therefore propagated by root cuttings. To do this, take the flat sea holly out of the ground between December and February, separate a few of the fleshy roots, cut them into pieces about 5 centimeters (2 in) long and press them vertically and, most importantly, the right way up into pots filled with potting compost. Then they are watered abundantly and set up at around 12 °C / 54 °F. After four to six weeks, the root pieces sprout.

Diseases and pests

The robust blue eryngo has no problems with plant diseases or pests.


Blue eryngo is hardy down to -32 °C / -25 °F. There are no special measures for winter protection necessary.

Use in the garden

Eryngium planum is a great for structure formation in steppe, gravel and other permeable beds. The perennial is suitable as a cut flower as well as for drying; the stems are hung upside down for this. You can combine the flat sea holly, for example, with stemless inula (Inula ensifolia), viper’s bugloss (Echium vulgare), yarrow (Achillea filipendulina), coneflower (Echinacea pallida), large-flowered mullein (Verbascum densiflorum), crimson pincushion flower (Knautia macedonica), ox-eye daisy (Buphthalmum salicifolium) or yellow succory (Catananche lutea). Grasses such as blue oat grass (Helictotrichon) or quaking grass (Briza media) are also wonderful.


There are some types of Eryngium planum whose inflorescence is even more intensely blue in color, such as “Blue Cap” (70 centimeters high) or “Blue Glitter” (80 centimeters high). The varieties ‘Silver Salentino’ (120 centimeters high) and ‘White Glitter’ (80 centimeters high) bloom in pure white. The two compact dwarf types ‘Blue Hobbit’ (intense steel blue) and ‘Blue dwarf’ (deep blue) are only around 30 centimeters high.

Be the first to comment

Leave a Reply

Your email address will not be published.