Why Did My Onion Split Into Two Bulbs?

You harvest what looked like a single healthy onion plant and find two bulbs side by side, joined at the base, or a single bulb with a clear division in the middle creating two distinct sections. Split or double onion bulbs are edible and taste perfectly normal, but they are awkward to peel, store less well than single round bulbs (the extra surfaces and crevices trap moisture), and represent a slight reduction in yield compared to single well-formed bulbs. There are two quite distinct causes and identifying which applies to your situation helps you prevent it next season.

Multi-budded sets

Most onion sets contain a single growing point (bud) at the centre, which produces a single plant and single bulb. However, some sets — particularly large sets over 25 mm in diameter, or sets that have been stored for a long time — contain two or even three developing buds. When planted, each bud develops independently, and the result is two or three plants growing from a single set, producing two or three bulbs joined at the base. This is the most common cause of double bulbs. Inspecting sets before planting and discarding obviously large or irregular ones helps, but multi-budded sets are not always identifiable before planting. Using sets in the 14–21 mm size range significantly reduces multi-budding because smaller sets are less likely to have developed multiple buds during curing.

Irregular water during growth

A dry spell followed by heavy rain or irrigation during the bulbing phase can cause the bulb to split as the outer scales stop expanding and then re-expand in response to sudden moisture. This is the same boom-and-bust mechanism that causes cracking in other vegetables. The result is a bulb that splits its outer skin and develops two distinct sections. Consistent soil moisture throughout the growing season — keeping the soil from drying out completely between waterings during the bulbing phase — prevents this. Mulching to retain moisture is particularly helpful in free-draining soils.

Double sets accidentally planted together

Sometimes two sets are inadvertently planted very close together or on top of each other, producing two separate plants that grow and bulb side by side. At harvest these look like a split bulb but are actually two distinct plants sharing the same soil space. If your row has occasional clearly double plants but most are single, accidental double planting is likely. Spacing each set individually and pressing them in one at a time rather than pushing a handful into prepared holes prevents this.

Are split onions worth eating?

Yes — split onions taste identical to single ones. Use them promptly (within a few weeks) rather than storing them for months, as the split surfaces are more prone to dehydration and the crevices can trap moisture and develop mould. If the split is at the basal plate and there is any sign of rot, trim it away carefully; the rest of the bulb is fine.