When to Plant Cosmos
Sow cosmos outside 1 to 3 weeks after your last frost.
Cosmos at a glance
- Sow outside
- 1 to 3 weeks after your last frost
- Time to bloom
- 60 to 90 days from sowing
- In bloom for
- about 16 weeks
- Spacing
- 1 plant per square foot
- Frost
- Tender: frost kills it
Growing cosmos
Cosmos grows easily from seed sown outside after the last frost, in full sun. The airy, daisy-like flowers bring bees and hoverflies to the garden and make good cut flowers from midsummer until frost. The classic pink, white and crimson kinds grow tall and willowy, up to five feet by late summer, while orange and yellow sulphur cosmos stays shorter and shrugs off heat and dry spells.
Dates by hardiness zone
Typical dates for each zone; your ZIP code gives your own.
| Zone | Sow | In bloom |
|---|---|---|
| Zone 3a | Jun 4–18 | Aug 3 – Sep 14Pick a fast variety. |
| Zone 3b | May 24 – Jun 7 | Jul 23 – Sep 25 |
| Zone 4a | May 19 – Jun 2 | Jul 18 – Sep 28 |
| Zone 4b | May 15–29 | Jul 14 – Oct 1 |
| Zone 5a | May 10–24 | Jul 9 – Oct 5 |
| Zone 5b | May 8–22 | Jul 7 – Oct 10 |
| Zone 6a | May 6–20 | Jul 5 – Oct 16 |
| Zone 6b | Apr 30 – May 14 | Jun 29 – Oct 19 |
| Zone 7a | Apr 23 – May 7 | Jun 22 – Oct 12 |
| Zone 7b | Apr 12–26 | Jun 11 – Oct 1 |
| Zone 8a | Apr 5–19 | Jun 4 – Sep 24 |
| Zone 8b | Mar 24 – Apr 7 | May 23 – Sep 12 |
| Zone 9a | Mar 9–23 | May 8 – Aug 28 |
| Zone 9b | Feb 15 – Mar 1Nights are still cool before mid-February, so this starts a little later. | Apr 16 – Aug 6 |
| Zone 10a | Feb 15 – Mar 1 | Apr 16 – Aug 6 |
| Zones 10b–11b | Frost is rare here, so cosmos can go in from late winter, and again in late summer for a fall crop. | |
Good neighbors
Questions about cosmos
Does cosmos need rich soil?
No, rather the opposite. In rich soil the plants grow tall and leafy with fewer flowers; lean soil and full sun give the best show.
How do I keep cosmos flowering?
Cut flowers often and snip off faded ones. Tall varieties may need a stake, or a neighbor to lean on in windy spots.
