Seed Treatment
- Soak seeds in warm water for 4–6 hours before planting to speed up germination.
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
- Depth: Sow seeds about 1/4 inch (6 mm) deep.
- 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.
Germination Info
- Germination Time: 7–21 days
- Optimal Temperature: 20–25°C (68–77°F)
- Moisture: Keep soil consistently moist but not soggy. Use a spray bottle to avoid disturbing the seeds.
Post-Germination Care
- Once seedlings emerge, move them to a bright, sunny location.
- 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.