Repotting guide

When & how to repot White Onion (Allium cepa 'Sturon')

Also called Sturon onion, white onion, globe onion.

More about white onion

About White Onion

Allium cepa 'Sturon' · also called Sturon onion, white onion · edible

The bulb onion is a biennial allium grown as an annual for its swollen storage bulb. 'Sturon' is a popular, reliable globe variety usually grown from heat-treated sets, giving uniform, well-keeping, mild bulbs that resist bolting. Bulbs swell through summer as daylength lengthens, then ripen and are lifted and dried in late summer for long storage.

Mature size: Leaves 30-45 cm tall; mature bulb 7-10 cm across.

Watch for — Onion white rot: Soil fungus that rots the roots and base with fluffy white mould; plants yellow and topple. There is no cure, so rotate alliums on a long cycle and use clean ground.

How to tell white onion needs repotting

Repotting on a calendar is less reliable than reading the plant. For white onion, watch for these signs:

Flowering has tailed off year on year and the clump has become congested and overcrowded.
Lots of leaf and few flowers — a classic sign that white onion bulbs or tubers need lifting and dividing.
Bulbs visibly bursting the pot or pushing each other to the surface.
It is the natural dormancy window (foliage yellowed and died back) — the only safe time to lift and split.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot white onion

Lift and divide every 3–4 years once clumps congest. Rather than a true repot, white onion is lifted and divided once the clump congests and flowering drops off. Forms a basal swollen bulb of fleshy concentric scales topped by upright hollow blue-green leaves; a biennial grown as an annual, flowering to a globe umbel only if it bolts or is left a second year..

What size pot to step white onion up to

Pot size matters less than depth and spacing here. When you replant white onion, set the bulbs or tubers at the correct depth (a rough guide: two to three times their own height of soil over the top) and space them so they are not touching. A wide, shallow pot suits a clump better than a tall narrow one.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot white onion

The only safe window is dormancy: wait until the foliage has yellowed and died back naturally, lift and divide then, and replant before or at the start of the next growing season. Disturbing white onion in full growth or flower sets it back badly.

Step-by-step: repotting white onion

Wait for dormancy. Let white onion foliage yellow and die back completely. Lifting while it is in growth wastes the energy it is storing for next year.
Lift carefully. Loosen the soil well away from the bulbs/tubers with a fork and ease the whole clump out without spearing them.
Separate the offsets. Gently pull the clump apart into individual bulbs or tubers. Keep only firm, healthy, blemish-free ones.
Replant at the right depth. Reset them in fresh firm, fertile, well-drained loam, ph 6.5-7.0 at the correct depth and spacing — not touching — so each has room to bulk up.
Water in and rest. Water once to settle them, then keep on the dry side until growth resumes. Do not feed until leaves are actively growing.

Aftercare

After replanting white onion, keep the soil barely moist — not wet — until shoots appear; bulbs and tubers rot in cold, saturated soil. Once leaves are growing strongly, resume normal watering. Hold off feeding until the plant is in active growth again.

The right soil mix for white onion

White Onion wants firm, fertile, well-drained loam, ph 6.5-7.0. Wants a firm, fine, weed-free bed enriched with compost the previous season rather than fresh manure. Loose, stony or waterlogged soil gives misshapen bulbs and encourages rots. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting white onion — frequently asked questions

How often should you repot white onion?

Lift and divide every 3–4 years once clumps congest for white onion. White Onion is lifted and divided, not "repotted". Every 3–4 years, once the foliage has died back and it is dormant, lift the clump, separate the offsets, and replant at the correct depth in firm, fertile, well-drained loam, ph 6.5-7.0. Crowding, not pot size, is what reduces flowering over time.

What size pot does white onion need?

When is the best time of year to repot white onion?

Do you "repot" white onion, or lift and divide it?

You lift and divide it. White Onion grows from bulbs or tubers, so instead of repotting you wait for dormancy, lift the congested clump, separate the healthy offsets, and replant them at the right depth and spacing. Doing this every 3–4 years restores flowering.

Should you fertilise white onion after repotting?

Hold off feeding white onion until it is in active growth again. Fresh soil already carries enough nutrients to get it re-established, and feeding disturbed roots too soon does more harm than good.