When to plant and prune Allium (Ornamental Onion)
Allium (Ornamental Onion) flowers from early May to late June and the dates below cover planting, flowering and the pruning window.
Planting, flowering and pruning by region
Allium (Ornamental Onion) is grown to the calendar rather than to the frost, so the same dates apply across the whole UK and every region below reads the same. Where a plant’s timings do follow the frost, Plot shifts them 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 September to late October | early May to late June | none after flowering |
| South Coast and Channel | mid September to late October | early May to late June | none after flowering |
| Greater London (Urban Heat Island) | mid September to late October | early May to late June | none after flowering |
| South East and Home Counties | mid September to late October | early May to late June | none after flowering |
| East Anglia | mid September to late October | early May to late June | none after flowering |
| Midlands and Lowland Wales | mid September to late October | early May to late June | none after flowering |
| Wales and West Coast | mid September to late October | early May to late June | none after flowering |
| Northern England | mid September to late October | early May to late June | none after flowering |
| Scotland Lowlands and Central Belt | mid September to late October | early May to late June | none after flowering |
| Scotland Highlands and Islands | mid September to late October | early May to late June | none after flowering |
| Northern Ireland | mid September to late October | early May to late June | none after flowering |
| Channel Islands and Isle of Man | mid September to late October | early May to late June | none after flowering |
Growing notes
Excellent architectural seedheads after flowering — leave standing through summer for structure and to feed goldfinches, rather than tidying away immediately.
At a glance
- Spacing
- 15 cm between plants
- Eventual height
- 90 cm
- Eventual spread
- 15 cm
- Hardiness
- H6
- Sun
- Full
- Frost tender
- No
- Grows in a container
- Yes, 10 litres or more
- Balcony rating
- 5 out of 5
Pruning group
No pruning; leave seedheads for architectural interest or deadhead to prevent self-seeding
What usually goes wrong
- onion white rot
- downy mildew
- allium leaf miner
- slugs and snails
Related guides
Timings cross-checked against RHS guidance for allium (ornamental onion).