Garden Cosmos
Cosmos bipinnatus
Also called: Mexican Aster
Garden Cosmos. Cosmos bipinnatus, the garden cosmos or Mexican aster, is an annual native to scrubland and grassland from Mexico to Arizona. Tall airy stems carry ferny pinnate foliage and simple daisy flowers in pink, white, magenta, or burgundy from midsummer through hard frost. Cosmos self-sows freely and is one of the most reliable cut flowers grown from seed.
Growing & care
- Sun: full sun. Plants bloom poorly in part shade and flop open from the center.
- Water: very low. Cosmos thrives on neglect; irrigated plants grow tall and leafy with few flowers.
- Soil: lean, well-drained. Fertile soil and fertilizer suppress flowering — these are plants that want to be poor.
- Hardiness: annual across USDA zones 2–11. Direct-sow after last frost when soil warms to 60°F.
- Spacing: 12–18 inches; the airy habit fills in.
- Pinch: pinch the central tip when plants reach 8–10 inches to encourage branching.
Propagation
Direct-sow seed ¼ inch deep after the soil warms; germination takes 7–10 days. Cosmos transplants poorly — the taproot resents disturbance — so start in place or in deep peat pots. Save seed by collecting mature dark seeds from finished flower heads in late summer; cosmos self-sows reliably in undisturbed beds.
Common problems
Floppy, leafless plants are almost always the result of too much water and too much fertilizer — let them be hungry and dry. Aphids can cluster on stem tips; a strong hose spray dislodges them. Powdery mildew appears in crowded plantings with poor airflow. Tall varieties topple in summer storms — stake or plant against a sturdy fence. Cosmos is non-toxic to dogs and cats per ASPCA listings.
Garden Cosmos — seeds, tools & books
Native range
Native range not recorded for this plant. Often a non-native cultivar or naturalized garden plant.
