Sea holly: planting, care & winter hardiness

Sea holly, also known as noble thistle, is a real feast for the eyes due to its shape and colour. In addition, the unique perennial is quite easy to care for and robust.

Sea holly (Eryngium) is even suitable for novice gardeners – after finding the right location, many species can be enjoyed for several years. Find out all you need to know about planting and caring for the Eryngium genus in this article.

Sea holly: origin and characteristics

The Eryngium genus belongs to the umbelliferae family (Apiaceae), which includes some well-known crops such as the carrot (Daucus carota subsp. sativus) and fennel (Foeniculum vulgare), but also ornamental plants such as masterwort (Astrantia). It is said that sea holly was used as an aphrodisiac. Due to its thistle-like appearance, sea holly is also called noble thistle. Overall, the Eryngium genus is very rich in form and can thus be described differently from species to species. Common features of all varieties, however, are the taproot, the decorative, spiny leaves and the mostly spherical or cylindrically domed flowers. Botanically, they are double umbels. The striking inflorescences are usually surrounded by strongly divided, grey-blue coloured bracts. Sea holly grows to a height of 30 to 120 cm and flowers from July to October, depending on the variety.

Eryngium occurs in temperate and warm regions. The highest species diversity is found in Brazil, Argentina and Mexico. Find out more about the different types of sea holly, including Eryngium planum, in our article on sea holly species and varieties. In Germany, this unique noble thistle can be used in gravel and dry beds, where it offers bees, bumblebees and butterflies a rich supply of nectar. The usually perennial sea holly is also often used in floristry as a cut flower. The unusual umbels and leaves are also suitable as dried flowers.

Tip: although sea holly is very popular with insects, snails are fortunately not interested in it.

Planting sea holly: location and method

Sea holly prefers a full-sun and wind-protected spot, although a spot without direct sunlight is equally tolerated. The sunnier the location, the more intense the colour of the plant parts. The soil conditions are important when selecting a site – the substrate should be dry to moist and very permeable. Selected species even require moist to damp soil. Sea holly does not tolerate waterlogging. In its natural habitat, the soil is usually nutrient-poor to moderately nutrient-rich. Very heavy soil can be leached with sand and a drainage layer of gravel or grit is recommended in any case.

Young plants and seeds can be purchased in shops. Plants in pots should be planted in spring with a distance of about 60 cm. In general, the mighty sea holly is best planted alone or in small groups with 1 to 3 other plants. Plant the perennial deep enough that the base of the plant and roots are flush with the soil surface. Water the sea holly well after planting.

Alternatively, sea holly can also be sown. Since most Eryngium species are so-called cold germinators, care must be taken to ensure the optimum germination temperature when sowing. They should be kept warm – at about 20 °C – and moist for the first 2 to 4 weeks. Afterwards, the cultivars should be exposed to cool temperatures of 0 to 3 °C for 4 to 6 weeks. The seeds can therefore be sown either directly after maturing in the ground or outdoors in pots. Likewise, the seeds can be grown in autumn or early spring on a warm windowsill or in a greenhouse.

• Fill the pot with compost. These soils have a low nutrient content, which stimulates the root growth of the seedlings. Our Plantura Organic Herb & Seedling Compost provides a loose and permeable structure in addition to the optimal nutrient composition.
• Place around 3 seeds and press down on the substrate lightly.
• The growing pot can be covered with foil to keep the seedlings moist but ventilate every 2 days to avoid mould.
• After the cold period, the seedlings should not be directly exposed to high temperatures (5 – 12 °C is optimal).
• After about 8 weeks, the seedlings can be transplanted to larger pots and, after another 2 weeks, planted outdoors or in containers.

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Sea holly is excellent as a container plant. Make sure there is enough space – 30 cm from the edge of the pot is ideal. In addition, it should be taller than it is wide because sea holly develops a taproot. To avoid waterlogging, create a drainage hole and add a drainage layer made of expanded clay. Compost for balcony plants is suitable as a substrate.

Combining sea holly:this striking thistle can be wonderfully combined with low or medium-high growing grass species. However, flowering perennials also get along very well with sea holly:

• Coneflower (Echinaceae spec.)
• Lady’s Mantle (Alchemilla spec.)
• Red hot poker (Kniphofia spec.)
• Yarrow (Achillea)
• Spurge (Euphorbia spec.)
• Asters (Astereae spec.)
• Flame flower (phlox spec.)
• Fat hen (Sedum spec.)
• Ornamental onion (Allium spec.)
• Sage (Salvia spec.)

Sea holly care

Sea holly is a low-maintenance perennial provided the location is suitable. Water sea holly moderately – the soil should not necessarily dry out completely but must also never be too wet. Short dry spells are well tolerated. One application of fertiliser in summer during the year of planting is sufficient for the next 2 to 3 years. Use a primarily organic fertiliser here such as our Plantura All Purpose Plant Food. This provides the plant with the necessary nutrients such as nitrogen for healthy growth and phosphorus, which is needed for abundant flower production, among other things.

Pruning is not absolutely necessary unless you want to avoid self-seeding. To do this, cut off the withered cones of the sea holly in late summer before the seeds ripen. Snow-covered sea holly is not only a beautiful sight in winter; the foliage also provides a hibernation site for insects and should therefore be left alone.

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Is sea holly hardy?

The winter hardiness is highly dependent on the Eryngium species in question. It is best to carefully read up on the specific winter hardiness to be on the safe side. Some species can easily withstand temperatures as low as – 28 °C while others are best covered with brushwood or similar. Under no circumstances will it tolerate winter dampness. Container plants should ideally be overwintered on the house wall. If necessary, these can be wrapped with fleece or a jute bag.

Propagation

Some sea holly species self-pollinate, especially in open areas without strong competition. However, this does not happen in a pure variety as they depend on cross-pollination by other, genetically non-identical sea holly plants. If you want to propagate sea holly yourself, the seeds can be harvested in late summer after they have ripened. If direct sowing is not possible, the seeds should be stored in a cool, dry and dark place. Proceed with sowing as described above.

Depending on the species, it is also possible to propagate pure sea holly via root cuttings. The optimal time for this is in autumn. To do this, expose the root ball of the mother plant and cut off a healthy root from it. The root cuttings should be 5 to 10 cm long and placed horizontally on a growing medium. Cover the cuttings with a 1 cm layer of substrate and store in a bright place at a temperature of about 15 °C. Successful propagation can be recognised in spring by the first new shoots.

Drying sea holly: instructions

Sea holly is perfectly suitable as a cut flower: it even retains its colour and shape when dried.

• Cut the shoots of the sea holly when they are in bud, i.e. before the individual flowers open.
• The individual stems can then be tied together and dried upside down.
• Choose a slightly airy, dry and dark place for this.
• After 2 – 4 weeks, the flower shoots will be dry and can be used as desired.

Is sea holly poisonous?

Sea holly is not considered poisonous to humans or pets. Some species contain so-called saponins, which only have a toxic effect in larger quantities. Unlike its relatives, sea holly is not a crop and so it is not advisable to eat it. However, the thorns on the leaves reliably prevent this anyway.

The low-maintenance safflower (Carthamus tinctorius) also prefers a warm and sunny location. We show you how to sow the orange flowering plant and what oil can be extracted from it.