Cosmos sulphureus Polidor - Sulfur Cosmos

Sow Cosmos seeds from March to May, at a depth of 3mm (0.1in), in fine soil and cover the seeds with a very light sprinkle of vermiculite. Place the seed tray in a mini-greenhouse at a temperature of 18-25°C (64.4-77°F) or wrap it in a polyethylene bag. Keep the soil moist but not waterlogged.

Light promotes germination, which occurs 7 to 15 days after sowing. When the seedlings are large enough to handle, transplant them into individual pots with a diameter of 7cm (3in).

Cultivate them in cooler conditions until the plants are sufficiently developed to be planted outdoors. When all risk of frost has passed, gradually acclimatise the plants to outdoor conditions for 7 to 10 days.

Plant them in full sun, in moist, well-drained soil, spaced 45cm (18in) apart. Pinch the tips of the shoots on each stem during transplantation to encourage branching and the production of numerous flowers.

Cosmos prefer a soil pH between 6 and 8.5, corresponding to the soil in their native habitat, the alkaline regions of Central America. They prefer full sun but tolerate partial shade. These plants are drought-tolerant once established, they are not prone to diseases but can be targeted by aphids. Spontaneous seedlings are common but not always true to type, without becoming invasive.