A picture of a Sea Holly

Sea Holly

Eryngium planum

Also known as

Blue Eryngo, Flat-Leaved Eryngo, Kok-Bas, Plains Eryngo, Cross Thistle, Flat Eryngo, Flat Rattlesnake Master, Plains Coyote-Thistle, Sea holly

Flat Sea Holly (Eryngium planum) (14240745500) by Bernard DUPONT (CC BY-SA 2.0)

Full Sun
Moderate care
Light watering
Frost Hardy

H5

RHS hardiness

-15°C

Minimum temperature

Expected size

Height
Spread

1m

Max

50cm

50cm

Min

30cm

2 years to reach maturity

Flowering

  • spring
  • summer
  • autumn
  • winter

This plant has no fragrance

More images of Sea Holly

Flat Sea Holly (Eryngium planum) (14240567538)
EryngiumBourgatii
Eryngium planum bgiu
A photo of Sea Holly

Sea Holly Overview

Eryngium planum is a striking herbaceous perennial plant. It is used to create decorative borders, beds, pot plants, containers and landscapes. Excellent for cut or dried flower arrangements. The blue colour intensifies with cold weather. Be aware, they can overgrow and invade areas quickly, and with every little root that stays behind in the soil growing into a new plant, they are tricky to remove from an area! This species is part of the Royal Horticultural Society “Plants for Pollinators” initiative to showcase plants which support pollinator populations by providing ample amounts of nectar and/ or pollen. A great choice for encouraging pollinating insect wildlife into your garden!

Common problems with Sea Holly

Root rot (needs dry or well-draining soils). Leaf spot diseases. Aphids, slugs and snails.

How to harvest Sea Holly

Harvest flowers when the entire flower heads and bracts turn blue.

How to propagate Sea Holly

Seed

Propagate seeds in containers in a cold frame as soon as ripe and mature.

Division

Propagate by division during spring, although they may be slow to re-establish.

Cuttings

Insert root cuttings in late winter directly in the soil.

Special features of Sea Holly

Attracts useful insects

Butterflies and beetles love the flowers.

Indoor plant

Grows well in pots, beds and containers in well-drained sandy soil under direct light.

Drought resistant

The fleshy roots help the plant to be drought tolerance.

Pot plant

Grows well in pots, beds and containers in well-drained sandy soil.

Attracts birds

Seed-eating songbirds.

Attracts bees

Attractive flowers

Other uses of Sea Holly

Borders

Medicinal

The plant is diuretic. It is also used in the treatment of whooping cough.

Edible

The roots are used for medicinal purposes. Taste is sweetish and mucilaginous, but the root has no odour.