How to grow Cosmos

At a glance

🌱 Grow your Cosmos with GardenDex

• Reminders to water it, timed to your weather and whether it's in a pot
• Frost and disease alerts for your garden, the night before
• Follow it from seed to harvest, and see what your garden grew in $
• Point your camera at any plant or bug to identify it

🗓️ When to plant

Sow seeds directly into the garden after the last frost when soil has warmed, or start indoors 4–6 weeks before the last frost for earlier blooms.

🌰 How to plant

1. Scatter seeds on well-prepared, loose soil and press lightly into place (they need light to germinate).
2. Keep soil consistently moist but not waterlogged until seedlings emerge in 7–10 days.
3. Thin seedlings to final spacing once they're 5–8 cm (2–3 in) tall.
4. For pot-grown seedlings, harden off gradually before transplanting after the last frost.

💧 Care

• Deadhead spent flowers regularly to encourage continuous blooming throughout the season.
• Pinch out growing tips when plants are 15 cm (6 in) tall to promote bushier, fuller growth.
• Stake tall varieties in windy locations to prevent flopping.
• Avoid over-feeding with nitrogen, which promotes foliage at the expense of flowers.

🧺 Harvest

Cut flowers in the early morning when they're fully open, or simply enjoy them in the garden and let pollinators visit. Deadheading extends the flowering season until the first hard frost arrives.

🐛 Common problems

Powdery mildew: Improve air circulation by spacing plants adequately and avoid wetting foliage; spray with a sulphur or fungicidal solution if needed.

Spider mites: Increase humidity and spray affected plants with insecticidal soap or a strong water spray to dislodge mites.

Slugs and snails: Remove by hand in early morning, use beer traps, or apply organic slug pellets around plant bases.

🤝 Companion plants

💚 Good neighbours: Zinnias, Sunflowers, Marigolds, Salvias

⚠️ Keep apart from: Fennel, Dill

More flowers guides