Cosmos bipinnatus aka Cosmos

Cosmos bipinnatus is an annual in the daisy family. Garden cosmos and Mexican aster are its common names. Kew lists Mexico as its native home. Other sources extend its range into the southern United States. Gardeners grow it for its flowers and finely cut leaves. It grows fast and forms a tall, bushy clump.

Height reports differ between sources. NC State describes plants reaching about 4 feet [1.2 metres]. RHS gives a garden range of 4.9–8.2 feet [1.5–2.5 metres]. RHS lists a spread of 0.3–1.6 feet [0.1–0.5 metres]. The green stems stand upright or lean upward. Their smooth surface has no hairs. Opposite leaves divide deeply into fine, threadlike parts. This gives the foliage a light, lacy look. The leaves measure 0.25–0.5 feet [0.08–0.15 metres] long.

The flower heads look like broad, open daisies. Outer ray florets surround a tight group of yellow disc florets. Flower colours include white, pink, red, lavender, and maroon. NC State reports blooms about 2–4 inches across. Missouri Botanical Garden gives a flowering season from June until frost. RHS places flowering in late summer and autumn. Removing spent heads helps the plant keep making flowers. Leave some heads to ripen when self-seeding is wanted. NC State describes the fruit as a capsule. Plants can seed themselves freely and return in later seasons.

Grow garden cosmos in full sun with good soil drainage. RHS recommends moist soil with a moderate level of fertility. It accepts clay, loam, sand, and chalk. It also grows across acid, neutral, and alkaline soils. Missouri Botanical Garden lists medium water needs and low upkeep. NC State notes tolerance of drought, heat, and poor soil. Sow outside once frost risk has passed and soil has warmed. Keep the seedbed moist while seeds start to grow. Tall plants may need stakes where storms and wind are frequent. RHS assigns H3; Missouri Botanical Garden lists USDA zones 2–11. These ratings do not change its annual life cycle.

Use it in mixed borders, cottage gardens, and cutting beds. It also suits containers, meadow plantings, and wildlife gardens. Its flowers draw bees, butterflies, and other pollinators. Birds also visit the plants. Serious problems may be uncommon, but several pests can occur. These include aphids, slugs, aster leafhoppers, and gray looper moths. Disease reports include stem canker, powdery mildew, and gray mold.