Cosmos in Zone 10a & 10b
Cosmos bipinnatus
When to plant cosmos in zone 10b
Zones 10a and 10b share the same average frost dates in our data, so this page covers both.
Method: Either: Start indoors for an earlier harvest, or direct-sow outside for less fuss. Cosmos tolerates either.
| Window | Spring window |
|---|---|
| Start indoors | Not typically started indoors |
| Direct sow / transplant outside | Mar–Jun |
| Estimated bloom | May 10–Sep 8 (~70 days after planting) |
Spacing: 1 per square foot (square-foot-gardening spacing). Mature height: 2-6 ft.
Zone 10b dates sourced from growinginthegarden.com.
Cosmos is one of the easiest flowers to grow from seed, and its main garden job here is pulling in pollinators and predatory insects near zucchini, a fine-textured, airy plant with feathery foliage and daisy-like pink, white, red, or orange blooms on thin stems that doesn't compete hard for space even at a full square foot per plant.
How cosmos fits zone 10's season
Zone 10b's frost-free window runs about 324 days, from January 30 to December 20. Cosmos needs about 70 days from sowing to first harvest.
Good companions
- Zucchini: Attracts pollinators and beneficial predatory insects nearby.
Where to buy cosmos seeds
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing cosmos" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 10b calendar · Full companion planting guide · Full plant profile · Plan your beds in the Garden Designer →
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Growing Cosmos in USDA zone 10b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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