Cosmos bipinnatus

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.