For optimal germination rates, initiate seed propagation indoors under controlled conditions—unless the species exhibits a high sensitivity to root disturbance. This approach allows precise regulation of temperature, humidity, and light, improving germination consistency and seedling viability. Do not skip any recommended seed treatments, as they are essential for uniform and successful germination.
Keep soil evenly moist during germination. DO NOT LET IT DRY OUT—once seeds begin absorbing water (imbibition), drying can kill the seed embryo, and even brief dryness after sprouting can kill seedlings.
Seed Treatment
- Soak seeds in warm water for 4–6 hours before planting.
When to Plant
- Outdoors: Sow after the last frost when the soil has warmed.
- Indoors: Start 4–6 weeks before your last expected frost date for earlier blooms.
How to Sow
- Spacing: Space plants 12–18 inches (30–45 cm) apart.
- Light: Cosmos seeds need light to germinate. Cover very lightly with soil or simply press into the surface.
- Use grow lights to maintain 12–16 hours of light per day.
Germination Info
- Germination Time: 7–21 days
- Optimal Temperature: Use a heat mat to maintain steady temperatures of 20–25°C (68–77°F)
- Moisture: Keep soil consistently moist but not soggy. Use a spray bottle to avoid disturbing the seeds.
Humidity: Use a humidity dome or clear cover to maintain consistent moisture during germination. Open the dome BRIEFLY once per day to allow fresh air in and reduce mold risk. Remove the dome entirely once seedlings emerge.
Post-Germination Care
- Thin seedlings to avoid overcrowding.
- Transplant outdoors after the last frost when seedlings are 3–4 inches tall and hardened off.
- Cosmos thrive in full sun and poor to moderately fertile soil. Avoid over-fertilizing—this leads to more foliage and fewer flowers.
Maintenance Tips
- Water moderately; cosmos are drought-tolerant once established.
- Deadhead spent flowers to encourage continuous blooming.
- Tall varieties may need light staking in windy areas.
- Perfect for borders, cottage gardens, and pollinator-friendly spaces.
More guides at: bloomoutlet.com/seed-guides