Flat sea holly

Sun

Full sun; shade reduces flowering and makes stems more likely to flop.

Water

Water regularly during establishment, then mainly during prolonged drought. Avoid persistently wet soil, especially in winter.

Soil

Well-drained, light or sandy soil, including gravelly and relatively poor ground. Tolerates a range of soil pH but struggles in waterlogged clay.

Height

2-3 ft (0.6-0.9 m)

Spread

1-2 ft (0.3-0.6 m)

Flowers

Summer into early autumn; small blue flower heads with narrow, spiny blue bracts on blue-tinged stems.

Foliage

Green, leathery, oval to heart-shaped basal leaves with toothed edges; smaller upper leaves are more deeply divided and spiny.

Growth

Moderate; usually takes two or three growing seasons to form a substantial clump.

Care needed

Low

• Plant in a sunny, freely drained position where the deep roots can remain undisturbed.
• Water young plants deeply when the soil dries; reduce watering once established.
• Avoid rich feeding and heavy manure applications, which encourage weak, floppy growth.
• Keep damp organic mulch away from the crown; use a light gravel mulch if needed.
• Support stems early with discreet twiggy stakes if they lean in exposed positions.
• Remove unwanted seedlings while small, before their taproots develop.

Pruning. Leave dry flower heads standing for winter structure, then cut dead stems to just above the basal growth in late winter or early spring. To limit self-seeding, remove flower heads before the seeds ripen instead.

Early spring

Cut back old stems before new shoots become tall.

Spring

Plant young plants in well-drained soil and water them in thoroughly.

Late spring

Add discreet supports where needed and remove unwanted seedlings.

Summer

Water newly planted clumps during dry spells; cut flowering stems for fresh or dried arrangements.

Late summer

Remove seed heads if you do not want seedlings, or leave them for autumn and winter structure.

Autumn

Check that fallen leaves and wet mulch are not collecting around the crown.

• Repeat small groups through a sunny perennial border to carry blue colour between softer flowering plants.
• Combine with ornamental grasses for contrasting spiky flower heads and fine, moving foliage.
• Use in gravel gardens with lavender, yarrow and stonecrop where drainage is sharp.
• Plant behind low-growing border plants so the branching stems rise above them without forming a dense screen.
• Include in a cutting garden for fresh bouquets and dried flower arrangements.

Garden styles it suits: New perennial (prairie), Dry gravel garden, Cottage garden, Pollinator garden, Mediterranean and Winter interest.

Plant it with: Lavender, Yarrow, Russian sage, Feather reed grass, Purple coneflower and Autumn stonecrop.

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

• Crown or root rot in wet soil: improve drainage or move young plants to a raised, freely drained position; avoid winter irrigation.
• Flopping stems in shade or rich soil: provide full sun, stop high-nitrogen feeding and add light supports before flowering.
• Slugs and snails damaging spring shoots: inspect new growth and use hand-picking or suitable barriers.
• Poor recovery after moving or division: leave established clumps undisturbed and replace them with young seed-grown plants rather than repeatedly transplanting them.

Not known to be toxic to people, cats, dogs or horses. The pointed bracts and spiny leaf edges can scratch skin or irritate the mouth if chewed; do not treat it as an edible plant.

• Mediterranean sea holly (Eryngium bourgatii)

  Its basal leaves are deeply divided and prominently marked with pale veins; flat sea holly has broader, mostly undivided basal leaves.

• Miss Willmott's ghost (Eryngium giganteum)

  It has much larger, pale silvery flower structures with broad, conspicuous bracts and is usually biennial or short-lived.

• Globe thistle (Echinops ritro)

  It bears rounded, ball-shaped blue flower heads without the distinct collar of pointed bracts found around sea holly heads.

The epithet planum means flat, referring to the relatively flat basal leaves, while sea holly describes the holly-like prickliness shared with coastal members of the genus.

Propagation. Sow fresh seed in autumn in pots kept outdoors in a sheltered position; winter chilling can help germination, which may be slow and uneven. Named cultivars may not come true from seed. Root cuttings taken during dormancy are another option; division is difficult because of the deep root system.

Accepted botanical name

Eryngium planum

In the US and Canada

Introduced in Canada and the lower 48 states.

Native range

Altay, Austria, Central European Rus, Czechoslovakia, East European Russia, Germany, Hungary, Kazakhstan, Krym, Mongolia, North Caucasus and Northwest European R.

Measured

Average height 60 cm.

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