Why Are My Onion Bulbs Soft and Mushy?

Finding soft, squishy onion bulbs at harvest or after a few weeks in storage is one of the most disappointing results after a whole season of growing. A healthy onion bulb should be firm and solid all the way through when you squeeze it. Softness is always a sign of internal rot, and the rot can originate in the garden or develop during storage. Identifying where and how it started helps you prevent it at the correct stage next season.

Neck rot in storage

The most common cause of soft stored onions is neck rot (Botrytis allii). The fungus infects the necks of onions during the growing season but symptoms rarely appear until weeks into storage, when the inside of the onion softens from the neck downward and grey fungal growth spreads through the inner scales. The outside may look fine while the inside is completely rotten. Neck rot is almost entirely preventable by thorough curing: laying harvested onions in a single layer in a dry, airy, warm spot for two to three weeks to fully dry and harden the neck. A soft, thick neck that has not fully dried is an invitation for neck rot. Never store any onion with a soft or green neck.

White rot damage continuing into storage

Onions harvested from white rot-infected plants may look sound at harvest but have already been invaded by Sclerotinia mycelium that was not yet visibly advanced at the point of harvest. These bulbs will soften rapidly in storage. White rot-affected onions have a characteristic smell — sweet and slightly musty — and when cut show brown, water-soaked internal tissue. Any onion that came from a bed with white rot should be used within two to three weeks rather than stored through the winter.

Bacterial soft rot in wet conditions

Onions harvested in wet conditions, or stored while still damp, are prone to bacterial soft rot — a rapid, foul-smelling, wet collapse of the bulb tissue, often starting from any wound or abrasion. Harvest in dry weather when possible and allow any damp at the surface to dry off before storing. Handle bulbs gently to minimise wound surfaces. Never store damp onions in sealed or poorly ventilated containers.

Sorting and using soft onions

Check stored onions regularly — at least every three weeks — and remove anything softening before it infects its neighbours. A onion with a small soft patch at the neck can often be saved by cutting away the affected area generously and using it immediately. Any onion that is extensively soft, has a strong smell, or shows internal mould should be discarded entirely. A few rotten onions removed early saves the whole store; discovered late, they can destroy dozens of sound ones around them.