Dr. Allan Armitage discusses Cosmos bipinnatus.
How to Plant and Grow Cosmos
Sow Cosmos: Sow cosmos seeds in cell packs or flats, 4 to 6 weeks before transplanting into the garden. Kept at 70 to 85° F., germination is in 7 to 21 days. Can direct sow cosmos into prepared seed beds in rows, after all danger of frost. Thin seedlings to 12 to 18 inches apart. A less efficient but easier method of sowing cosmos: scatter seeds. Rake seeds into soil, keep area moist until germination.
Tip for bringing your cosmos plants into bloom up to 3 weeks earlier, growing under fluorescent lights: once seeds have germinated provide 11 hours per day of artificial light. Keep seedlings in complete darkness for the remaining 13 hours. After 2 weeks, keep the lights on for 14 or more hours per day.
Grow Cosmos: Full sun. Plants tolerate heat, drought, and poor soil. Most blooms produced with moderate water in soils of average fertility. Remove spent blossoms to promote additional flowering. Will usually self-sow if some spent blossoms are kept on plants. Excellent cut flowers, last 7 to 10 days when blooms are cut as petals unfold. Flowers fully open in vase. Rarely bothered by pests or disease. Cosmos attract birds, bees and butterflies.