How to Grow Cosmos
Annual
Cosmos brings airy foliage and simple daisy flowers to sunny summer borders. It is a half-hardy annual, valued for its long flowering season and easy, informal look. Single-flowered varieties are particularly useful for hoverflies, bees and other pollinators, while taller forms also make lovely cut flowers.
Planting
Sow indoors from March to April in small pots or modules at about 18 degrees C, then grow plants on in good light. Alternatively, sow outdoors in May once the risk of frost has passed and soil has warmed. Harden off indoor-raised plants for 7 to 10 days before planting 30 to 45 cm apart. In milder southern gardens this may be late April or May; wait longer in colder northern areas and Scotland.
Care
Choose an open, sunny site with reasonably free-draining soil. Water newly planted cosmos until rooted, then only during prolonged dry spells. Avoid rich feeding, which encourages leafy growth at the expense of flowers. Stake taller varieties before they lean in summer winds. Deadhead frequently, or pick stems for a vase, to keep new buds coming and prevent unwanted self-seeding.
Bloom time
Flowers usually open from July until the first autumn frosts, sometimes from June where plants were raised early under cover. Regular deadheading extends the display well into September and October in many southern gardens.
Garden uses
Plant cosmos in drifts at the back or middle of borders, among grasses and late-summer perennials, or in a sunny cutting garden. Compact selections suit large pots. Open, single flowers can support insects and feature on RHS Plants for Pollinators lists, while the feathery foliage softens more formal planting.
Tips
Pinch out the growing tip when young plants reach about 20 cm to encourage branching and more flowering stems. Do not rush plants outside in a cold spring: even a light frost can damage them. Look for varieties with an RHS Award of Garden Merit (AGM) when choosing dependable garden performers.
Growing Cosmos: frequently asked questions
How do I grow cosmos in the UK?
Sow indoors from March to April in small pots or modules at about 18 degrees C, then grow plants on in good light.
When does cosmos flower?
Flowers usually open from July until the first autumn frosts, sometimes from June where plants were raised early under cover.
How do I care for cosmos?
Choose an open, sunny site with reasonably free-draining soil.
General guidance for UK gardens. Timing and hardiness vary by region and season; the RHS or a local garden centre can advise for your area. Page updated 19 Sep 2026.