How to grow Cosmos from seed in South Africa
Applies to: Afternoon White, Casanova Mix, Picotee, Pink Picotee, Red Illusion, Sea Shells Red, Sensation Mix, Sonata F1 Mix, Sonata F1 Pink and Sonata F1 White. These are Cosmos bipinnatus selections.
Cosmos is a sun-loving, frost-tender annual. It flowers most generously in full sun and free-draining soil that is not overfed. Rich soil and too much nitrogen produce tall leafy plants with fewer flowers.
Quick sowing guide
| Field |
Recommendation |
| Seed depth |
Cover lightly with 5–6 mm of fine soil; never deeper than 10 mm |
| Germination |
Usually 7–14 days at about 20–22 °C |
| Light |
Full sun, ideally 6 or more hours daily |
| Tall varieties |
30–45 cm between plants; stake or net in wind |
| Compact varieties |
Sonata 25–30 cm; Casanova 35–45 cm |
| First flowers |
Commonly 8–12 weeks from sowing in suitable warmth |
Choose the correct growth type
Tall border and cut-flower types
Afternoon White, Picotee, Pink Picotee, Red Illusion, Sea Shells Red and Sensation Mix. Allow 30–45 cm and add support early in exposed gardens.
Compact Cosmos
Sonata F1 Mix, Pink and White are about 60 cm tall and suit borders and larger containers. Casanova Mix is genetically compact but broad, approximately 40 cm high and 35–45 cm wide; space it 35–45 cm apart.
Sowing and planting
- Sow where plants are to flower, or sow one seed per small cell or pot. Cosmos dislikes unnecessary root disturbance.
- Cover lightly, water with a fine rose and keep evenly moist—not waterlogged—until emergence.
- Give seedlings bright light immediately. Thin or transplant when they have two to three true leaves.
- Harden tray-grown seedlings for about a week before planting out after danger of hard frost.
- Pinch tall types once at 15–20 cm if bushier, more branched plants are wanted. Do not pinch if the goal is the earliest single stems.
Ongoing care
- Water deeply when the upper soil begins to dry, then let excess water drain. Established plants tolerate short dry spells, but repeated drought shortens stems and flowering.
- Avoid high-nitrogen fertiliser. In poor soil, a light balanced feed at planting is enough.
- Deadhead often, or cut stems regularly, to prevent seed formation and extend flowering.
- In windy sites, install support while plants are small. Tall plants are difficult to rescue once flattened.
- Space for airflow in humid districts and water at soil level to limit leaf disease.
When to sow in your region
Winter-rainfall Western Cape: Sow from early spring into early summer once cold, saturated soil has passed. Choose a sheltered sunny site, irrigate through the dry season and support tall plants against southeasterly winds. An early-autumn sowing is only reliable in a mild, frost-free, well-drained position.
Frost-prone interior: Sow in spring after the local hard-frost period; start trays under protection a few weeks earlier if needed. Make a second sowing in early summer. Do not rely on autumn-sown plants to survive hard winter frost.
Humid coast and KZN Midlands: Favour the less humid shoulder seasons—spring or late summer into autumn—while maintaining strong airflow. On the coast, avoid prolonged saturated soil; in the Midlands, wait until hard-frost danger has passed.
Hot-dry or subtropical interior: Sow in late winter to spring where frost permits, or again in late summer as extreme heat eases. Mulch lightly, water deeply and provide afternoon relief only where reflected heat is severe; ordinary shade is not a substitute for full sun.
Flower and harvest outcome
For the longest vase life, cut in the cool of morning when petals are just opening and have not yet flattened fully. Strip foliage that would sit below water and place stems in clean water immediately. For garden display, remove faded heads every few days. Leave a few final heads only if seed is wanted; named hybrids and mixes may not reproduce true to type.
