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., seedlings emerge in 7 to 21 days. Can direct sow cosmos after all danger of frost into prepared seed beds in rows, 4-6 seeds per foot. Thin seedlings to 12 to 15 inches apart.
Grow Cosmos: Full sun. Best flower production is in soils of average fertility with moderate water. Cosmos tolerate heat, drought, and poor soil. Remove spent blooms to promote additional flowers. Cosmos will usually self-sow if some spent blossoms are kept on plants. Cut flowers typically last 5 to 9 days when blooms are cut as petals unfold. Flowers fully open in vase. Cosmos attract birds, bees and butterflies and are rarely bothered by pests or disease.
Dig Deeper: Tip to bring your cosmos plants into bloom up to 3 weeks earlier when growing under fluorescent lights: once seedlings have emerged 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.