How to Grow Cosmos
Cosmos bipinnatus
Cosmos are sun-loving, half-hardy annuals that thrive in well-drained soil—they flower best on the lean side. Sow indoors in spring or direct sow after frost. Pinch young plants for branching and stake taller varieties. Deadhead to keep flowers coming until frost, or leave some seedheads for wildlife. Water during prolonged drought but avoid rich feeding, which reduces bloom. Excellent cut flowers; harvest when petals just open.
When to Start
Sow indoors 5 weeks before last frost, or direct sow outdoors after frost risk has passed and soil reaches 10°C.
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Growth Stages
Approximate days from start
Care Essentials
- Sparingly — too much feed produces foliage at the expense of flowers.
- One of the best cut flowers. Easy from seed. Tall varieties may need staking.
Growing Conditions
- Min. soil temperature
- 10°C+
- Germination temperature
- 18–25°C
- Sow indoors
- —
- Transplant out
- —
- First harvest
- —
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Spacing & Planting
| Plant spacing | 45 cm |
| Row spacing | 60 cm |
| Sow depth | 0.5 cm |
| Mature height | 120 cm |
| Mature spread | 45 cm |
For taller varieties like C. bipinnatus. Dwarf varieties like C. sulphureus can be planted closer (30 cm). Do not overcrowd, as this can lead to weak, floppy stems.
Pests & Diseases
Pest Aphids
Identification: Clusters of small green or black insects on growing tips and flower buds, causing distorted growth and sticky honeydew
- Blast off with a strong jet of water
- Spray with fatty acid-based insecticide
- Encourage ladybirds, lacewings and hoverflies
- Spray with a contact insecticide containing deltamethrin
Pest Slugs and Snails
Identification: Seedlings eaten to ground level overnight, or irregular holes in young leaves with slime trails
- Protect transplants with cloches or copper tape barriers
- Apply organic slug pellets (ferric phosphate)
- Encourage hedgehogs, frogs and ground beetles
- Ferric phosphate-based pellets
Pest Thrips
Identification: Silvery streaking or speckling on petals and leaves; tiny slender insects visible on flowers
- Spray with fatty acid-based insecticide
- Encourage predatory insects such as lacewings
- Spray with a contact insecticide if damage is severe
Disease Grey Mould (Botrytis) Botrytis cinerea
Symptoms: Fuzzy grey mould on spent flowers, stems and leaves, often spreading from dead petals into healthy tissue
Treatment: Remove and destroy affected parts promptly. Improve air circulation around plants
Prevention: Deadhead regularly, avoid overcrowding, and water at soil level rather than overhead
Disease Powdery Mildew
Symptoms: White powdery coating on leaves, causing yellowing and premature leaf drop late in the season
Treatment: Remove badly affected leaves. Rarely worth treating as it typically appears late in the season
Prevention: Space plants well for good airflow and avoid drought stress
Growing Conditions
| Min. soil temperature | 10°C |
| Germination temperature | 18–25°C |
| Base growth temperature | 8°C |
| Min. daylength to start | 10.5h |
| Photoperiod sensitivity | day neutral |
Frequently Asked Questions
- How far apart should I plant Cosmos?
- Plant Cosmos 45cm apart with 60cm between rows.
- When does Cosmos flower?
- Cosmos typically flowers approximately 60 days after transplanting.
- Is Cosmos frost hardy?
- Cosmos is classified as Half Hardy.
