Eryngium (Sea Holly) in Zone 5a & 5b
Eryngium planum
Yes, eryngium (sea holly) grows in zones 5a and 5b. Start seeds indoors Feb 8–Feb 15 and plant out May 17–May 24.
When to plant eryngium (sea holly) in zone 5b
Zones 5a and 5b share the same average frost dates, so this page covers both.
Method: Start indoors: Start indoors from seed, then transplant outside. It needs a head start most zones can't give it by direct-sowing alone.
| Start indoors | Feb 8–Feb 15 |
|---|---|
| Direct sow / transplant outside | May 17–May 24 |
| Estimated bloom | About a year after planting |
Spacing: 1 per square foot (square-foot-gardening spacing). Mature height: 2-3 ft.
Plant alongside it in zone 5b
These go outside within two weeks of eryngium (sea holly) in zone 5b, so one planting day can cover them.
- NasturtiumMay 17–May 24
- MarigoldMay 17–May 24
- Ageratum (Flossflower)May 17–May 24
- Annual Geranium (Pelargonium)May 17–May 24
- Butterfly Weed (Asclepias)May 17–May 24
- Campanula (Bellflower)May 17–May 24
Eryngium, commonly called sea holly, looks like a thistle crossed with a metal ornament, its spiny bracts taking on a blue or silvery sheen that holds up well both fresh and dried.
How eryngium (sea holly) fits zone 5's season
Zone 5b's frost-free window runs about 148 days, from May 10 to October 5. Eryngium (sea holly) takes about a year from planting to first bloom, so it spans more than one growing season.
Eryngium (Sea Holly) planting windows in every zone
Zone 5b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Bloom |
|---|---|---|---|
| 3a & 3b | Feb 28–Mar 7 | Jun 6–Jun 13 | About a year after planting |
| 4a & 4b | Feb 13–Feb 20 | May 22–May 29 | About a year after planting |
| 5a & 5b (your zone) | Feb 8–Feb 15 | May 17–May 24 | About a year after planting |
| 6a & 6b | Jan 24–Jan 31 | May 2–May 9 | About a year after planting |
| 7a & 7b | Jan 9–Jan 16 | Apr 17–Apr 24 | About a year after planting |
| 8a & 8b | Dec 24–Dec 31 | Apr 1–Apr 8 | About a year after planting |
Good companions
No well-established companions for eryngium (sea holly).
Where to buy eryngium seeds
None of the seed suppliers we list carries eryngium seeds right now. A local garden center or a specialty seed house is the place to look.
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing eryngium (sea holly)" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 5b calendar · Plan your beds in the Garden Designer →
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Growing Eryngium (Sea Holly) in USDA zone 5b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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