Cosmos in Zone 10a & 10b
Cosmos bipinnatus
Yes, cosmos grows in zones 10a and 10b. Plant it outside in Mar–Jun.
When to plant cosmos in zone 10b
Zones 10a and 10b share the same average frost dates, so this page covers both.
Method: Either: Start indoors for an earlier harvest, or direct-sow outside for less fuss. Either works.
| Window | Spring window |
|---|---|
| Start indoors | Not typically started indoors |
| Direct sow / transplant outside | Mar–Jun |
| Estimated bloom | ~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.
Plant alongside it in zone 10b
These go outside within two weeks of cosmos in zone 10b, so one planting day can cover them.
- Angelonia (Summer Snapdragon)Mar–Jul
- Coreopsis (Tickseed)Mar–May
- Four O'ClockMar–Jun
- Gomphrena (Globe Amaranth)Mar–Jul
- Portulaca (Moss Rose)Mar–Jun
- Salvia (Scarlet Sage)Mar–May
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 bloom.
Cosmos planting windows in every zone
Zone 10b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Bloom |
|---|---|---|---|
| 2a & 2b | Apr 15–Apr 22 | May 27–Jun 3 | Aug 5–Aug 12 |
| 3a & 3b | Apr 25–May 2 | Jun 6–Jun 13 | Aug 15–Aug 22 |
| 4a & 4b | Apr 10–Apr 17 | May 22–May 29 | Jul 31–Aug 7 |
| 5a & 5b | Apr 5–Apr 12 | May 17–May 24 | Jul 26–Aug 2 |
| 6a & 6b | Mar 21–Mar 28 | May 2–May 9 | Jul 11–Jul 18 |
| 7a & 7b | Mar 6–Mar 13 | Apr 17–Apr 24 | Jun 26–Jul 3 |
| 8a & 8b | Feb 18–Feb 25 | Apr 1–Apr 8 | Jun 10–Jun 17 |
| 9a & 9b | Jan 11–Jan 18 | Feb 22–Mar 1 | May 3–May 10 |
| 10a & 10bSpring window (your zone) | Not usually | Mar–Jun | ~70 days after planting |
| 11a & 11b | Nov 27–Dec 4 | Jan 8–Jan 15 | Mar 19–Mar 26 |
Bloom dates are about 70 days after the sow or plant-out date, not after starting seeds indoors.
Zone 2 sows before zone 3 here, which looks wrong and is not. USDA zones rank winter cold, not season length, and zone 2 is mostly interior Alaska, where hard winters come with warm summers. More on what a zone measures.
Good companions
- Zucchini: Attracts pollinators and beneficial predatory insects nearby.
Where to buy cosmos seeds
Botanical Interests · True Leaf Market · Burpee · High Mowing Organic Seeds (each supplier's cosmos seeds)
See why we recommend these suppliers →
As an Amazon Associate we earn from qualifying purchases. Some links on this page also go to other retailers we may earn a commission from as an affiliate, at no extra cost to you. We only recommend products with a clear consensus of positive, independent reviews, or ones we bought and use ourselves. Each item on our gear page says which.
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 · Plan your beds in the Garden Designer →
Pin this for later
Growing Cosmos in USDA zone 10b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
Save to Pinterest