Cosmos is an annual that can be sown from March to May. Earlier sowings are best done undercover in modules and planted out when the risk of frost has passed. Avoid any direct sowing until then. Sow in rows 30cm apart, cover with a fine layer of compost, and water. Thin out to 30cm spacings once seedlings are mature.
Ensure your seed bed is free from weeds. Sheltered sunny sites are best, as they do not like shade and can be grown at the edges of beds or the back of borders to attract beneficial insects. They prefer moist but free-draining soils, so incorporating organic matter is beneficial. Harden off your plants, then transplant in rows 30cm x 30cm apart.
Keep weed-free and watered during hot spells. You can mulch around the plants to retain moisture and prevent weeds. Deadhead to prolong flowering and pinch out to create more bushy growth.
Generally pest and disease-free, things to look out for are aphids and powdery mildew. They do not like cold weather, so if you are in a colder area, sow later for the best results.
Pick the flowers often to encourage more flowers to form; they should keep flowering well into autumn.
Only Cosmos sulphureus (orange cosmos) is considered edible.
Choose your best plants to save seeds from and don’t pick any flowers; let them mature so you can collect the seeds. If you have other cosmos growing nearby, you will need to isolate them to prevent cross-pollination.
The blooms mature at different times, so you can collect the seeds as you go or wait until most are mature and then harvest the whole plant. If you choose the second option, you will get some seeds dropping and self-seeding in situ next year. The seeds are ready when they turn brown and look a little like a spiky hedgehog. Separate seeds from chaff before storing in a cool, dry place for up to 5 years.