Cosmos: Sowing, planting and harvest calendar
How to sow cosmos successfully outdoors?
This outdoor sowing method mainly suits annual cosmos, such as Cosmos bipinnatus and Cosmos sulphureus. Check species and variety: cosmos do not all share the same life cycle or height. Wait until frost risk has passed and soil has genuinely warmed, around 16°C or above, rather than following a fixed April date.
Choose sunshine and well-drained soil. Remove competing weeds, break up clods and prepare a fine surface. Moisten if necessary, then distribute seeds thinly and cover lightly. A stated thirty-centimetre distance generally relates more to the final space for a plant. Adjust it to the variety, especially tall or strongly branched forms.
Water gently to avoid moving seeds and maintain even moisture until emergence. Watch for slugs and weed without pulling out young cosmos. Thin once plants are easy to recognise, allowing room for growth. Label rows and keep the packet. A late sowing in a short growing season may flower later or for less time. Flowers throughout the whole summer are not guaranteed merely by sowing the seeds.
When do cosmos flower and how do you support flowering?
How to care for this plant
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Remove faded flowers
To encourage further flowering, remove faded flowers with their short stalks without damaging buds. Leave any heads you want to set seed or keep for wildlife.
