When to plant
Sow Cosmos seeds directly outdoors in mid- to late spring once the risk of frost has passed. In cooler climates, you can start seeds indoors 4–6 weeks earlier and transplant once the soil has warmed. Avoid sowing too early outdoors, as cold soil delays germination.
Where to plant
Choose a sunny, open position with at least 6 hours of direct sunlight daily. Cosmos grow beautifully in borders, wildflower meadows, cutting gardens and large containers. Avoid deep shade, which leads to weak, leggy plants.
Preparing the soil
Loosen soil to 15–20 cm deep and remove weeds. Cosmos prefer light, well-drained soil that is not too rich—overly fertile soil produces lots of foliage but fewer flowers. Improve heavy clay with grit and compost; sandy soil usually needs little adjustment.
Preparing the seeds
Cosmos seeds are straightforward and require no special treatment. Keep them dry and cool until sowing. If starting indoors, use seed trays with a light, free-draining compost.
Sowing the seeds
Sow seeds thinly on the soil surface and cover lightly with a fine layer of soil (about 0.5 cm). Water gently to avoid washing seeds away. Space seeds or seedlings 25–35 cm apart for medium varieties, and 40–50 cm for taller types.
After planting
Keep soil lightly moist until seedlings establish. Thin overcrowded seedlings to ensure strong growth. Protect young plants from late frost with fleece if needed.
During growth
Water during prolonged dry spells, especially in containers. Avoid overfeeding—Cosmos flower best in lean soil. Pinch out the growing tips of young plants to encourage bushier growth. Deadhead regularly to extend the flowering season.
After flowering / end of season
Remove spent stems at the end of the season once frost has killed the foliage. If you want Cosmos to self-seed, allow some flowers to form seed heads and drop naturally. Plants will die back completely after frost.
Extra tips
Cosmos make excellent cut flowers with long vase life. Taller varieties may need light staking in windy areas. Combine with zinnias, grasses, rudbeckias and dahlias for a lively, summer-long display.
