Purity

Cosmos bipinnatus is commonly bothered by aphids, which cluster on tender stems and flower buds and can lead to distorted growth. Spray the plants with a strong jet of water to knock aphids off, then follow with insecticidal soap (especially on the undersides of leaves) and repeat every 5–7 days until new growth is clear. If you see powdery mildew (white, dusty growth on leaves), improve airflow by thinning crowded stems and avoid wetting the foliage; remove the most affected leaves early to slow spread.

For cut flowers, harvest when the outer petals are fully open and the center disk is mature (about 75 days from sowing for typical plants). Check daily as blooms open in succession—use a clean cut stem in the morning or evening when flowers are crisp. For seed, let spent flowers dry on the plant until the seed heads turn brown and the seeds feel hard, then harvest and dry them a few more days indoors.