Alpine sea holly

Sun

Full sun; flowering and blue colouring are strongest in an open position.

Water

Water regularly during establishment. Once established, water during prolonged dry spells; this mountain species appreciates more summer moisture than many other sea hollies, but cannot tolerate waterlogged soil.

Soil

Well-drained, moderately fertile soil, preferably neutral to alkaline. Avoid heavy, wet ground, especially in winter.

Height

2-3 ft (0.6-0.9 m)

Spread

1-2 ft (0.3-0.6 m)

Flowers

Early to midsummer, sometimes continuing into late summer; small blue flowers form elongated heads surrounded by conspicuous, feathery steel-blue to violet-blue bracts.

Foliage

Green, broad, heart-shaped basal leaves with toothed margins; upper leaves are smaller, more divided and spiny. The foliage dies back in winter.

Growth

Moderate; may take two or three growing seasons to form a substantial clump.

Care needed

Low

• Pollinators
• Deer resistant
• Drought tolerant
• Cold hardy
• Wildlife
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Easy

Toughness

Fairly tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   1.5 ft (0.45 m)

3. After 2 years

   2 ft (0.6 m)

4. After 3 years

   2.5 ft (0.75 m)

   full size

About 3 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Safe

Growth rate

Moderate

• Plant in spring in cold or winter-wet regions, keeping the crown at soil level.
• Water deeply during establishment and prolonged summer drought, allowing excess water to drain away.
• Apply a light compost dressing in spring if soil is poor; avoid heavy nitrogen feeding, which encourages floppy stems.
• Keep thick organic mulch away from the crown; a light gravel surface helps keep it dry.
• Support stems early with discreet twiggy stakes if the site is exposed or growth becomes lax.
• Avoid moving established plants unnecessarily; their deep roots resent disturbance.

Pruning. Leave the flower heads standing through autumn and winter if they remain upright, then cut dead stems to the base in late winter or early spring. Remove spent heads earlier if you want to limit self-seeding.

Late winter

Cut down weathered stems before new basal growth develops.

Spring

Plant young plants in a sunny, freely draining position and water them in well.

Spring

Check emerging growth for slugs and snails; add discreet supports before stems become tall.

Summer

Water during prolonged dry spells and cut stems for arrangements once the bracts have developed their blue colour.

Late summer

Remove fading heads to reduce seeding, or retain selected heads for seed and winter structure.

Autumn

Clear fallen leaves away from the crown and check that winter rain can drain freely.

• Plant in small groups through a sunny mixed border, where the blue bracts contrast with softer flowers.
• Combine with ornamental grasses for upright summer accents and lasting seed-head structure.
• Use in a cool-coloured border with violet salvias, catmint and white yarrow.
• Grow in a well-drained gravel border with enough soil moisture to support summer growth.
• Include in a cutting garden for fresh flowers and stems that retain their shape when dried.

Garden styles it suits: Cottage garden, New perennial (prairie), Dry gravel garden, Blues and silvers, Pollinator garden and Mountain lodge.

Plant it with: Catmint, Meadow sage, Yarrow, Feather reed grass, Hardy geranium and Lady's mantle.

Wildlife: The flowers attract bees, hoverflies and other nectar-feeding insects. Standing stems provide some shelter for small invertebrates.

• Crown and root rot in waterlogged soil: improve drainage or move young plants to a raised position; avoid burying the crown.
• Slugs and snails damaging spring shoots: inspect young growth and use hand-picking or other targeted controls.
• Weak or leaning stems in shade or rich soil: provide full sun, reduce feeding and support stems early.
• Poor performance in hot, humid summers: choose an airy site with freely draining soil; in persistently hot regions, a more heat-tolerant sea holly is a better choice.

Not known to be toxic to people, cats, dogs or horses. The spiny bracts and leaves can scratch skin or irritate the mouth if chewed; it is not a food plant.

• Flat sea holly (Eryngium planum)

  Has many smaller, rounded blue flower heads on branching stems, with much shorter and less feathery bracts.

• Amethyst sea holly (Eryngium amethystinum)

  Has more deeply divided basal leaves and narrow, stiff, star-like bracts rather than the broad, finely cut ruff of alpine sea holly.

• Globe thistle (Echinops ritro)

  Produces nearly spherical blue flower heads without a conspicuous surrounding ruff of bracts, and has lobed leaves with pale undersides.

The species name alpinum means 'of the Alps', while 'sea holly' refers to the holly-like spines shared with coastal members of the genus.

Propagation. Sow fresh seed in autumn in pots outdoors; germination can be slow and may require a period of winter cold. Seedlings may differ from named cultivars. Root cuttings taken during dormancy can preserve a cultivar; division is less suitable because the deep roots dislike disturbance.

Accepted botanical name

Eryngium alpinum

Native range

Austria, France, Italy, Switzerland and Yugoslavia.

Measured

Soil pH 7 to 7.5. Average height 73 cm.

Sources: GBIF (CC BY 4.0) and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.