When to plant and prune Cosmos
Cosmos flowers from mid July to early October and the dates below cover planting, flowering and the pruning window.
Planting, flowering and pruning by region
Cosmos is frost-tender, so its timings shift across the UK. Plot works them out from the Midlands and Lowland Wales baseline using Met Office average frost dates.
| Where you are | Plant | In flower | Prune |
|---|---|---|---|
| South West Coast and Cornwall | mid April to early May | mid July to early October | mid July to late September |
| South Coast and Channel | late April to mid May | mid July to early October | mid July to late September |
| Greater London (Urban Heat Island) | mid April to early May | mid July to early October | mid July to late September |
| South East and Home Counties | early May to late May | mid July to early October | mid July to late September |
| East Anglia | mid May to early June | mid July to early October | mid July to late September |
| Midlands and Lowland Wales | late May to mid June | mid July to early October | mid July to late September |
| Wales and West Coast | early May to late May | mid July to early October | mid July to late September |
| Northern England | late May to mid June | mid July to early October | mid July to late September |
| Scotland Lowlands and Central Belt | early June to late June | mid July to early October | mid July to late September |
| Scotland Highlands and Islands | late June to mid July | mid July to early October | mid July to late September |
| Northern Ireland | mid May to early June | mid July to early October | mid July to late September |
| Channel Islands and Isle of Man | mid April to early May | mid July to early October | mid July to late September |
Growing notes
Half-hardy annual - it flowers, seeds and dies in one season and does not persist, so it must be re-sown every year, and it must not go out until the last frost has passed. Flowers from late summer into autumn until frost stops it. Will not flower in shade. Single, open-centred cosmos are among the best annuals for bees and hoverflies; fully double cultivars are far less useful. Note that although cosmos tolerates lean soil, the RHS recommends moderately fertile, moist but well-drained soil rather than deliberately poor ground.
At a glance
- Spacing
- 45 cm between plants
- Eventual height
- 100 cm
- Eventual spread
- 45 cm
- Hardiness
- H3
- Sun
- Full
- Frost tender
- Yes
- Grows in a container
- Yes, 15 litres or more
- Balcony rating
- 4 out of 5
Feeding
Only container plants need feeding - a potassium-rich liquid feed every couple of weeks; do not overfeed plants in the ground.
Pruning group
Deadhead spent flowers (or cut for the vase) continuously to prolong the flowering season
What usually goes wrong
- aphids
- slugs and snails on young plants
- grey mould in wet summers
- tall plants flopping without support
- flowering delayed by over-rich soil
Related guides
Timings cross-checked against RHS guidance for cosmos.