How to Grow Cosmos
Cosmos bipinnatus
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.
Companion Planting
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Growing Conditions
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- Sow indoors
- —
- Transplant out
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- 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.
Care Guide
- Feeding
- Sparingly — too much feed produces foliage at the expense of flowers.
- Watch for
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- Aphids
- Slugs on young plants
- Grey mould
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.
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