The Cosmos plants are actually half-hardy annual plants that grow flower, set seed and die all in one year, but unlike hardy annuals, the Cosmos plants can’t withstand low temperature. The Cosmos plants are quick and easy to grow from seed, flowering in as little as twelve weeks, and can be added to borders and pots for a burst of showy color. To give your cosmos a long flowering season you can sow the cosmos seeds early, indoors, in March or April. Once they’ve sprouted and they have 2 pairs of leaves you can transplant them into individual 7cm pots and grow on under cover.
In May you can easily harden them off for a week by placing in an open cold frame or on a patio, before planting them out where they are to flower. Feed them with a liquid tomato fertilizer when the first buds appear. To read more visit https://www.flowerpeek.com/2023/03/how-to-grow-cosmos-from-seed.html