Why Is My Onion Bulb So Small?

You harvest your onion crop and find golf-ball sized — or even smaller — bulbs where you expected large, satisfying ones. Small onion bulbs are a common disappointment but they almost always have a clear cause. Onions are day-length sensitive plants that go through distinct growth phases, and if anything interrupts or limits growth during the bulbing phase, the final size pays the price. Here are the most common reasons for undersized onion bulbs and what to do about them.

Overcrowding

Onion spacing directly determines maximum bulb size. Each plant needs adequate space to develop a full-sized bulb, and when sets or plants are too close together, they compete for water, nutrients, and light — and the bulbs stay small. The standard spacing for onions is 10 cm between plants in the row, with 25–30 cm between rows. At this spacing, most varieties produce good-sized bulbs. Closer spacing produces more onions per metre but all of them will be smaller. If you want large onions, 15 cm in-row spacing and consistent feeding gives each plant the room it needs. Check your spacing — it is the most commonly misjudged factor.

Planting too late

Onions grown from sets need adequate time in the ground before the longest day triggers bulbing. Sets planted in late May or June simply do not have enough weeks of vegetative growth to build a large plant before the signal to start bulbing arrives. The larger the plant at the time of bulbing, the larger the final bulb — it is a direct relationship. Plant sets as early as the soil allows in spring, typically March to mid-April, to maximise the vegetative growth phase. Autumn-planted sets (overwintering onions) get around this problem entirely by having the whole winter and early spring to establish before the bulbing trigger.

Bolting

An onion plant that sends up a flower stalk diverts its energy from bulb development to reproduction. The resulting bulb is small, often hollow around the flower stalk, and does not store well. Bolting is triggered by cold temperatures experienced by the young plant (vernalisation), which is why cold spells in early spring after planting, or planting large sets that are more bolt-prone, leads to more bolting. Use heat-treated sets (sold specifically to reduce bolting) and smaller sets in the 14–21 mm size range, which are significantly less likely to bolt than large sets over 25 mm.

Nutrient and water limitations

Poor, infertile soil and dry conditions during the bulbing phase — roughly from the longest day until harvest — directly limit bulb size. Onions are not as hungry as potatoes but they do benefit from good soil preparation and consistent watering during bulking. Incorporating well-rotted compost before planting and applying a high-potassium feed (tomato fertiliser works well) two or three times during the bulbing phase gives plants the resources to fill out the bulb fully. A dry spell during this critical phase without supplementary watering is a reliable cause of small, tight bulbs.