Sea holly
Eryngium planum · Apiaceae
Branching, steel-blue stems carry dozens of small, thistle-like blue heads with spiny collars in midsummer. The whole plant looks metallic.
- Size
- 3 ft tall, 18 in wide
- Zones
- 5 to 9
- Light
- Full sun
- Flowers
- Early July to mid August
- Pets
- No toxicity recorded
- Job in a bed
- Accent
A bed built around sea holly
The planner designed this bed around sea holly when this page was made: a 12 × 6 ft (3.7 × 1.8 m) cottage garden in zone 6, full sun. It chose the other plants from the 715 that suit those conditions and its dry soil, and says below why sea holly stands where it does.
The plants
- 1 Sea holly × 2, blue
- 2 Candytuft × 6, white
- 3 Sedum 'Lidakense' × 15, pink
- 4 Tatarian aster 'Jindai' × 4, purple
- 5 Dwarf English boxwood × 1, dark green leaves
- 6 Foxglove beardtongue × 3, white
- 7 Tulip × 5, red
- 8 Surprise lily × 3, pink
Why this plant
- The only globes. The others are dense clusters, daisies, spires and others. Shape reads at a distance, where colours blur.
- The only blue.
Why here
- In the middle. At 3 ft it is the step between candytuft (10 in) in front and Tatarian aster 'Jindai' (4 ft) behind: each is seen over the one before.
- 2 plants in 2 places. An accent is a punctuation mark: the repeat along the bed gives it rhythm.
Why now
- In flower for 7 weeks. From early July to mid August.
- Fills a gap. For four of those weeks nothing else on the plan is in flower.
Planned as a cottage garden: 24 of the 31 plants (77%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. Tatarian aster 'Jindai' and foxglove beardtongue anchor the back corners of the bed: the tallest plants on the plan stand there. Sedum 'Lidakense' is the plant there is most of: 15 of the 39.
The planner's own design check: 13 Excellent, 4 Good. It flags one thing to watch: Nothing on this plan is highly poisonous or an aggressive spreader.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where sea holly goes in a border
At 3 ft it belongs in the middle of a bed. In the planner's ten height classes it is medium tall (2½ to 3 ft).
Its job: accent. An upright plant used singly, like a punctuation mark, and repeated at intervals to give the bed rhythm.
Blue stems as well as blue flowers: a plant for strong sun and poor, dry soil, where its colour is most intense. It has a taproot and resents moving. Leave the seed heads for winter.
- Metallic blue for hot, dry beds
- Dried flowers
- Texture contrast with soft grasses
What to plant with sea holly
Plants recorded as good partners for it, and what each pairing does:
- Yarrow 'Moonshine': blue against yellow, opposite each other on the colour wheel, in flower together for six weeks; happy in the same dry soil.
- Russian sage: in flower together for six weeks, from mid July; happy in the same dry soil.
- Blue oat grass: happy in the same dry soil.
Questions about sea holly
Does sea holly need sun or shade?
Sea holly grows in full sun (6 or more hours of direct sun a day).
It wants dry, sharply drained soil, on the lean side, with no feeding, and little watering once settled. Once settled in, it stands drought.
What zones does sea holly grow in?
USDA zones 5 to 9. It is hardy to zone 5, which means it survives winter nights down to about −10 °F (−23 °C).
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does sea holly flower in my zone?
In zone 5 it flowers early July to mid August. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 5 | Early July to mid August |
|---|---|
| Zone 6 | Early July to mid August |
| Zone 7 | Late June to early August |
| Zone 8 | Late June to early August |
| Zone 9 | Late June to early August |
Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.
How far apart should sea holly be planted?
18 in (45 cm) apart, from the middle of one plant to the middle of the next. That is about its full width, so each plant keeps its own outline.
It grows 3 ft (90 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many sea holly plants do I need?
At 18 in apart, set out in staggered rows:
- 5 to fill 10 square feet (about 1 m²)
- 7 for a single row 10 ft (3 m) long
- 17 to fill a 4 × 8 ft bed on its own
In a mixed border, plant it as one at a time, repeated along the bed at intervals; a slender one reads better as a clump of three.
In the bed above, the planner used two in two groups.
Is sea holly poisonous to dogs and cats?
No toxicity is recorded for dogs, cats, horses or people, according to NC State Extension.
"Not toxic" is not "edible": a pet that eats a lot of any plant can be sick.
Do deer eat sea holly?
Mostly not. NC State lists it as resistant to deer and rabbits. No plant is proof against a hungry deer in a hard winter, but this one is low on the menu.
What flowers after sea holly?
Sea holly is over by mid August. These start as it finishes and carry the same spot on, so it never goes flat:
- Garden chrysanthemum: from mid September to late October, 24 in tall, to stand in front of it and cover its fading stems.
- Lilyturf: from early August to mid September, 16 in tall, to stand in front of it and cover its fading stems.
- Sedum 'Autumn Joy': from mid August to mid October, 24 in tall, to stand in front of it and cover its fading stems.
What does sea holly look like in winter?
Sea holly dies down to the ground and there is nothing to see until spring. The old stems can be cut in autumn, or left until spring as shelter for overwintering insects.
Is sea holly good for bees and butterflies?
Sea holly is visited by pollinators, bees and butterflies.
More Eryngium
- Alpine sea holly Eryngium alpinum
- Mediterranean sea holly Eryngium bourgatii
- Miss Willmott's ghost Eryngium giganteum
- Rattlesnake master Eryngium yuccifolium
- Sea holly 'Big Blue' Eryngium x zabelii 'Big Blue'
- Sea holly 'Blue Hobbit' Eryngium planum 'Blue Hobbit'
In these guides
- Salt-tolerant plants for roadsides and coasts
- Deer-resistant perennials for full sun
- Rabbit-resistant plants
- Drought-tolerant perennials for full sun
- Pet-safe plants for a sunny border
- Plants for butterflies
- Plants for bees
- Plants with blue flowers
Every fact on record
- Type
- Perennial
- Size
- 3 ft (90 cm) tall, 18 in (45 cm) wide
- Plant
- 18 in (45 cm) apart
- Height class
- Medium tall (2½ to 3 ft)
- Place in the bed
- Middle
- Flowers
- Early July to mid August; globes; blue
- Leaves
- Medium texture, herbaceous; green
- Shape
- Upright, self-seeding
- Hardy in
- USDA zones 5 to 9
- Light
- Full sun
- Soil
- Lean, dry and sharply drained
- Water
- Low
- Feeding
- None needed
- Family
- Apiaceae
- Comes from
- E. Central Europe to Mongolia and W. Himalaya
- Attracts
- Pollinators, bees, butterflies
- Stands up to
- Deer, rabbits, drought, salt, poor soil
Checked against NC State Extension and VASCAN (plants of Canada). Flowering times are for zone 5: later in colder zones, earlier in warmer ones. Toxicity and spread information is a general gardening guide, not veterinary or medical advice, and invasive status varies by region. If a person or pet eats a plant, contact poison control or a vet. Check your local invasive species list before planting anything with a spread warning.