Sow Cosmos seed from March to May in seed compost and place the seed tray 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 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 18″ apart. Pinch out the growing tip of each stem when transplanting to encourage stems to branch and produce more flowers. Deadheading will encourage new flowers. Information from T&M. These seeds can be sown outside directly into the soil about May time when the ground had warmed up a little.
Today, 11th April 2017, I have sown seeds of this elegant looking Cosmos, Purity, onto some damp compost. I have also sprinkled a few seeds directly here and there in the garden so I am hopeful of getting some good results this year. Update 21st February 2018. – I got no flowers from these seeds at all last year. I had four seeds left in the packet and as I have been planting the Dahlias up today I have just put them into the compost and hoped for the best. I will buy more seed as I do Like the flowers and will give them another try by sowing direct during May.