How big does Flat sea holly grow?

About 80 cm × 50 cm after ten years (height × width). Its ultimate size is 30–90 cm × 30–60 cm.

What soil does Flat sea holly grow in?

It tolerates these soils: average garden soil, sandy, calcareous.

Is Flat sea holly hardy?

Hardy down to -29 °C (USDA zone 4, RHS H7). It gets through winter in every part of the country.

Is Flat sea holly toxic to children?

No. Reliable sources list it as non-toxic to children.

When does Flat sea holly flower?

Flowering: June–September, blue.

How much water does Flat sea holly need?

Water need: low. Drought tolerance: high.

Flat sea holly

Eryngium planum

Flat sea holly (Eryngium planum) is a perennial that reaches 80 cm × 50 cm after ten years. Light: full sun. Hardy down to -29 °C. Drought tolerance: high. Non-toxic to children.

Illustration of Flat sea holly — An illustration of the typical shape of the mature plant, not a photo.

What it's good for, and what it isn't

Flat sea holly is one of the best structural perennials for a dry, sunny garden: from a rosette of dark green leaves it sends up much-branched, blue-flushed stems in summer, covered in small steel-blue flower heads with spiky collars that are busy with bees and hoverflies. Its colour is best in lean, sandy or stony, limy soil in full sun, and it hardly needs watering, but in rich soil or shade it grows lanky and flops. It has a deep taproot and resents being moved, so plant it where it is to stay, and leave the dry flower heads for winter, as they look good in frost.

Watch out for

In heavy soil that stays wet in winter the roots rot easily, so it needs good drainage. In rich soil or too little sun the stems grow lanky and flop. Leaf spots and powdery mildew can occur, slugs and aphids may attack young plants, and voles can eat the fleshy taproot. It self-seeds readily and the deep-rooted seedlings are hard to pull out, so cut off the spent heads before the seed ripens if you do not want it to spread.

Key facts

Light: Full sun
Water need: Low
Care: Low
Size after 10 years: 80 cm × 50 cm
Hardiness: down to -29 °C
Foliage: Deciduous
Flowering: J F M A M J J A S O N D June–September, blue

Group	Perennial
Light	Full sun
Size after 10 years	80 cm × 50 cm
Ultimate size	30–90 cm × 30–60 cm
Hardiness	down to -29 °C (USDA 4, RHS H7)
Soil	Average garden soil, Sandy, Calcareous
Foliage	Deciduous
Flowering	June–September, blue
Water need	Low
Drought tolerance	High
Maintenance	Low
Root aggressiveness	Low
Suckering	no data
Spreading	Medium
Distance from buildings	no data
Distance from paving	no data
Litter	not typical
Planting distance	40 cm
For dogs	no data
For cats	no data
For children	Non-toxic
Typical price	no data

Toxicity data is for information only. If your pet or child has eaten a plant, contact a vet or a doctor.

Frequently asked questions

Data updated: October 19, 2026 · How our data is made

Appears in these lists

Perennials for full sun Perennials that flower all summer Drought-tolerant plants Drought-tolerant perennials Drought-tolerant plants for sandy soil Plants that are non-toxic to children Perennials for pollinators Low-maintenance perennials Plants for a Mediterranean garden

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