Sow cosmos after the last frost, in soil that has warmed, barely covered. Do not start a tray six weeks early in a hard pot.
After frost, shallow, not an early indoor start
| Factor | Recommended | Avoid |
|---|---|---|
| When | After frost, soil about 65°F | Cold compost in March |
| Depth | About 1/8 to 1/4 inch | A deep poke that buries the seed |
| Indoor start | Optional, 4 to 6 weeks before frost, in cells | An eight-week start that winds the roots |
| Spacing | Thin to 12 to 18 inches | A turf of seedlings that mildews |
Sow outdoors after frost when the soil has warmed to about 65°F. [1] Rake seed in shallow; planting deep cuts germination. [2] Indoor start is optional, not required. [3] Keep-alive after the plants stand lives on How to Grow Cosmos. Direct sow is the usual method; a tray is only a short head start.
When should I plant cosmos seeds?
After your last spring frost, once the soil is warm. An optional indoor start is only 4 to 6 weeks before that date.
How to sow
- Rake a sunny, unfed bed. Do not dump compost or fertilizer into the row.
- Scatter or place seed and cover about 1/8 to 1/4 inch. Press so seed contacts soil. [4]
- Keep the surface moist until seedlings show, usually 7 to 21 days.
- Thin to 12 to 18 inches when plants are a few inches tall. Pinch later on How to Grow Cosmos.
How deep should I plant cosmos seeds?
Shallow — about 1/8 to 1/4 inch. Deep sowing is the usual miss.
What fails a cosmos sowing
- Cold, wet soil — seed sits and rots. Wait for warmth.
- Too deep — no emergence. Scratch and sow again shallow.
- An early indoor tray in a hard pot — leggy transplants that check and never catch the direct-sown row.
- Sowing chocolate cosmos like a pink mix — buy tubers or plants. Seed of chocolate cosmos is unreliable. Plant the tuber on How to Grow Chocolate Cosmos.
Pick height and species on Types of Cosmos. Partners live on Cosmos Companion Plants.
Sow after frost, shallow, then thin. Pinch and lean sun live on How to Grow Cosmos.
FAQ
Should I start cosmos indoors?
Only if you want a short head start: 4 to 6 weeks, in cells, then harden off after frost. Direct sow is the usual method.
How long do cosmos seeds take to germinate?
About 7 to 21 days in warm, moist soil. Cold soil stretches that window.
Can I succession sow cosmos?
Home beds usually bloom from one sowing until frost if you deadhead. Cut-flower rows can sow every few weeks.
Can I save cosmos seed?
Yes. Let heads dry on the plant, then store cool and dry. Mixes will not come true.
Can I grow chocolate cosmos from seed?
Not reliably. Buy tubers or plants. Annual garden and orange cosmos are the seed-rack plants.
Do cosmos seeds need light to germinate?
They want a shallow cover, not a deep burial. Surface contact and warmth matter more than a light/dark rule.
Related guides
Recommended plants
References
- [1] NC State Extension -
https://plants.ces.ncsu.edu/plants/cosmos-bipinnatus/ - [2] Utah State University Extension -
https://extension.usu.edu/yardandgarden/research/cosmos-in-the-garden - [3] Almanac -
https://www.almanac.com/plant/cosmos - [4] UF/IFAS Gardening Solutions -
https://gardeningsolutions.ifas.ufl.edu/plants/ornamentals/cosmos/
This note is part of Cosmos Guides.
More notes on making another plant live in Propagating Guides.
Updated: Reviewed by Bosquelle Editors