Repotting guide

When & how to repot Ailsa Craig Onion (Allium cepa)

Also called Exhibition onion, Ailsa Craig, Show onion.

More about ailsa craig onion

About Ailsa Craig Onion

Allium cepa · also called Exhibition onion, Ailsa Craig · edible

Ailsa Craig is a classic British exhibition onion, producing very large, straw-coloured globes with mild, sweet flesh — popular both in cooking and at horticultural shows. Grown from seed sown in January under glass or outdoors from March. Note: Allium species are toxic to dogs and cats, causing haemolytic anaemia.

Mature size: Foliage 50-70 cm tall; bulbs up to 20+ cm diameter on exhibition plants

How to tell ailsa craig onion needs repotting

Repotting on a calendar is less reliable than reading the plant. For ailsa craig 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 ailsa craig 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 ailsa craig onion

Lift and divide every 3–4 years once clumps congest. Rather than a true repot, ailsa craig onion is lifted and divided once the clump congests and flowering drops off. Upright-leaved annual bulb.

What size pot to step ailsa craig onion up to

Pot size matters less than depth and spacing here. When you replant ailsa craig 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 ailsa craig 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 ailsa craig onion in full growth or flower sets it back badly.

Step-by-step: repotting ailsa craig onion

Wait for dormancy. Let ailsa craig 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 fertile, well-drained, deeply dug loam 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 ailsa craig 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 ailsa craig onion

Ailsa Craig Onion wants fertile, well-drained, deeply dug loam. Prepare the bed with well-rotted compost or manure in autumn. Onions need firm, settled soil — avoid freshly manured ground. pH 6.5–7.0 is optimal; lime if necessary. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting ailsa craig onion — frequently asked questions

How often should you repot ailsa craig onion?

Lift and divide every 3–4 years once clumps congest for ailsa craig onion. Ailsa Craig 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 fertile, well-drained, deeply dug loam. Crowding, not pot size, is what reduces flowering over time.

What size pot does ailsa craig onion need?

When is the best time of year to repot ailsa craig onion?

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

You lift and divide it. Ailsa Craig 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 ailsa craig onion after repotting?

Hold off feeding ailsa craig 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.