Cosmos are a wonderful addition to any garden. They are quick-to-bloom, delightful in bouquets, and a welcome landing pad for pollinators. They are said to have been named by Spanish priests who used the Greek word, cosmos, which means "ordered universe" or "harmony", because of how their petals are symmetrically aligned. The cosmos flower has long been used as a symbol of order, peace, and modesty.
Cosmos begin blooming as early as 10 weeks after sowing and bloom from summer to fall frost. Cosmos will often bloom even stronger in the shorter days of fall than they do in summer.
When to Sow Outside: RECOMMENDED. 1 to 2 weeks after your average last frost date, and when soil temperature is at least 60°F.
When to Start Inside: 4 to 6 weeks before your average last frost date.
Use a lightweight seed-starting mix/medium (sterile, and lighter than potting mix), and barely cover seeds. Sow 3 seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light. Read about more indoor sowing tips.
Containers should be clean, sanitized, and have drainage holes. If you choose to sow in cell packs, you may need to up-pot seedlings once into larger containers before transplanting outside.
After hardening off, transplant seedlings 12" apart.
Sowing preparation and spacing
Cosmos grow well in average to poor soil so there usually isn't much prep needed. They perform best in full sun, but can withstand part shade.
Thin to 1 plant every 12".
Keep seedings consistently moist, but once plants are established, they are drought tolerant; avoid overwatering.
Cosmos usually do not require fertilizer. Avoid nitrogen-rich fertilizer or amendments which can result in abundance foliage and few flowers.
Keep plants well weeded. Weeds compete with crops for light, nutrients, and water, and can harbor insects and diseases.
Deadheading (removing spent blossoms) regularly encourages more blooms. Tall plants may require staking.
For longest vase life, harvest in the morning, choosing stems with flower buds that have just begun to open. Change vase water frequently.