When to Plant Cosmos

Cosmos is a tender warm-season flower. Frost damages it, so it goes out only after the last spring frost.

Start seeds indoors 2 to 4 weeks before the last spring frost, and plant out 1 to 3 weeks after the last frost. Or sow it where it will grow, 1 to 3 weeks after the last spring frost. The first flowers open about 60 to 90 days after sowing.

Example dates

The same rules in dates for three climates, using typical frost dates. Enter your ZIP code above for the dates where you live.

Cosmos: dates by last spring and first fall frost
TaskLast frost Mar 15first Nov 15Last frost Apr 15first Oct 25Last frost May 15first Sep 30
Start seeds indoorsFeb 15 – Mar 1Mar 18 – Apr 1Apr 17 – May 1
Harden offMar 15 – 21Apr 15 – 21May 15 – 21
Plant outMar 22 – Apr 5Apr 22 – May 6May 22 – Jun 5
In flowerApr 16 – Jul 9May 17 – Aug 9Jun 16 – Sep 8
Direct-sown: Sow outdoorsMar 22 – Apr 5Apr 22 – May 6May 22 – Jun 5
Direct-sown: In flowerMay 21 – Aug 13Jun 21 – Sep 13Jul 21 – Oct 13

Growing cosmos

Cosmos flowers more in poor soil; deadhead or cut regularly, and expect it to reseed itself.

  • Spacing: 18 in apart, in rows 24 in apart. In a raised bed, 1 per 2 sq ft at square-foot spacing.
  • Days to first flowers: 60 to 90 from sowing, then about 12 weeks of flowers.

Plan your beds with these dates

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Questions

When do I start cosmos indoors?

2 to 4 weeks before the last spring frost: with a last frost of April 15, that's Mar 18 – Apr 1. Enter your ZIP code above for your own dates.

Can cosmos go in before the last frost?

No. Cosmos is damaged by frost: plant it out 1 to 3 weeks after the last spring frost.

How long does cosmos take to grow?

About 60 to 90 days from sowing to the first flowers, which then lasts about 12 weeks. Varieties differ: check the seed packet.

More planting dates

Sources

  1. NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 (annual/seasonal, by station: freeze dates at 32 °F)
  2. Iowa State: Germination Requirements for Annuals and Vegetables
  3. Purdue University Extension: Starting Seeds Indoors (HO-14-W)
  4. Colorado State University Extension: Growing Annual Plants from Seed
  5. Utah State University Extension: Cosmos in the Garden
  6. UF/IFAS Extension: FP149: Cosmos bipinnatus 'Sonata White'