Giant Onion

How to grow Giant Onion

Written by the GardenDecoded editorial team · Last reviewed 21 July 2026 · Written for UK gardens · How we research our guidance

Planting

Plant the bulbs in autumn, about 20 cm deep and 25–30 cm apart, in the sunniest and best-drained spot you have. Plant them behind lower perennials, because the strappy leaves start to yellow and collapse just as the flower opens and want hiding.

Pruning

Never remove the leaves while they are still green, even though they look untidy — the bulb is refilling itself. Leave the seed heads standing after flowering; they hold their shape into autumn.

Propagation

Lift and separate offsets when the clump is dormant in late summer. Seed is possible but takes several years to reach flowering size.

Pests and diseases

Allium rust shows as orange pustules on the leaves; allium leaf miner and downy mildew also occur. Bulbs rot in wet, heavy soil over winter.

Value to wildlife

Bees work the drumstick heads intensively during the few weeks they are open.

Notes

It often flowers magnificently in the first year from a bought bulb and then dwindles in later years, particularly on heavy or damp soil. Many gardeners treat it as a short-term planting and add new bulbs each autumn.

This guidance was researched and written by GardenDecoded — see our editorial policy for how guides are checked and reviewed.

Other names for this plant

These names have all been used for Allium giganteum but are no longer the accepted name. You may still meet them on old labels, in books, or in nursery catalogues.

Others in Allium