Step-by-Step Growing Guide
Cosmos thrives in poor to average, well-drained soil with a pH of 6.0–8.5 — unlike most flowering annuals, it actively declines in rich, amended soil, producing massive foliage at the expense of flowers. Do not add compost or fertilizer to the planting area. Direct sow seeds after last frost date, pressing them into the soil surface and covering with just 1/4 inch of soil. Thin to 12–18 inches apart once seedlings are 2 inches tall. Cosmos handles dry conditions remarkably well — water only when the top 2 inches of soil are completely dry, roughly every 7–10 days in summer. Overwatering is the most common mistake. No fertilizer is needed or wanted. In very long-summer climates, succession-sow every 3–4 weeks for continuous bloom. Plants grow 3–5 feet tall and may need staking in exposed, windy sites. Self-seeding is prolific — allow a few plants to set seed at season's end for a volunteer crop next year.