Sow Cosmos seed from March to May at a depth of 3mm (⅛”) in a good quality seed compost, and cover the seeds with a very fine sprinkling of vermiculite. Place the seed tray in a propagator at a temperature of 18-25C (64-77F) or seal it inside a polythene bag. Keep the soil damp but not wet. Do not exclude light as this aids germination which usually takes 7-15 days.
When seedlings are large enough to handle, transplant into 7cm (3″) pots and grow them on in cooler conditions until large enough to plant outdoors. When Cosmos plants are well grown and all risk of frost has passed, acclimatise them to outdoor conditions over 7 to 10 days. Transplant outdoors in full sun in any moist, well drained soil at a distance of 45cm (18″) apart. Pinch out the growing tip of each stem when transplanting to encourage stems to branch and produce more Cosmos flowers.