Garden cosmos
Also known as
Mexican aster, Cosmos, Common cosmos, Fern-leaf cosmos, Cutleaf cosmos
Keep this plant’s care guide close at hand.
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Cosmos bipinnatus, commonly called garden cosmos or Mexican aster, is a fast-growing warm-season annual in the daisy family (Asteraceae). It forms airy clumps of finely cut, bipinnate foliage that lend a delicate, ferny texture to borders and meadows. Slender, often hollow stems hold abundant, daisy-like flowers 5–10 cm across in shades of white, shell pink, rose, and carmine, each with a golden-yellow disk that draws pollinators. Blooming begins in midsummer and continues until frost, especially if plants are deadheaded. Depending on cultivar, plants range from compact 30–60 cm selections suitable for containers to towering 120–180 cm forms that may require staking in windy sites. Native to Mexico but naturalized worldwide, C. bipinnatus thrives in full sun and lean, well-drained soils, producing the most flowers under low to moderate fertility and modest moisture. It is drought-tolerant once established, deer- and rabbit-resistant, and an outstanding cut flower that also self-seeds lightly for future displays.
Taxonomy
Properties & Requirements
How to care for Garden cosmos
Water
Keep seedbeds evenly moist until germination and early establishment. Once plants are 10–15 cm tall, water deeply but infrequently, allowing the top 2–5 cm of soil to dry between soakings. In average garden soil, this often means watering every 7–10 days; provide about 2–2.5 cm (¾–1 in) per week during prolonged drought. Avoid frequent light sprinkling and prevent soggy conditions, which encourage weak, lanky growth. Containers dry faster—check daily in hot weather and ensure excellent drainage.
Light
Provide full sun: at least 6–8 hours of direct light daily for abundant blooms and sturdy stems. Cosmos will tolerate light afternoon shade, especially in very hot climates, but flowering diminishes and plants may become leggy as shade increases. Avoid deep shade. If starting indoors, use bright south-facing windows or strong grow lights, aiming for 12–14 hours of high-intensity light per day. Space plants well to maintain sun exposure to lower foliage and improve air movement.
Nutrients
Cosmos bloom best in lean soil; excessive fertility, especially nitrogen, produces lush foliage at the expense of flowers. At planting, mix in a small amount of compost or use a light application of a balanced, slow‑release fertilizer (e.g., 4‑4‑4 or 5‑5‑5) at half rate. In the ground, that’s usually sufficient for the season. For container-grown plants, supplement with a dilute liquid feed (¼–½ strength) every 3–4 weeks only if growth pales. Avoid high‑nitrogen lawn fertilizers and manure‑rich amendments.
Temperature
Frost-sensitive warm-season annual. Sow or transplant after the last frost when soil is consistently above 15°C/59°F. Ideal growing temperatures are 18–30°C (65–86°F). Plants tolerate brief heat to ~35°C/95°F if watered and mulched, but cold nights slow growth and flowering. Light frost will blacken foliage; growth ceases below ~10°C/50°F. In regions with early autumn chills, start seeds indoors 4–6 weeks before planting to ensure a full bloom period. Shelter tall plants from strong, desiccating winds.
