Why Are My Onions Small?
Small onions usually result from competition, interrupted growth or poor planting material rather than one dramatic fault. They need room, light, steady moisture and a weed-free bed to make useful bulbs. Assess spacing and soil conditions now, then improve the site and planting routine for the next crop.
Measure the spacing first. Onion sets or seedlings planted too close together compete for light, water and nutrients, so they produce many small bulbs rather than fewer good ones. Thin crowded rows early, leaving the spacing recommended for your variety and intended bulb size. If you are growing spring onions, closer spacing is appropriate, but do not expect them to swell like storage onions. Replanting thinnings often checks growth, so use them in the kitchen instead. Keep rows clearly marked to make hoeing easier. On an allotment, a simple string line helps maintain even gaps and prevents accidental crowding when a second row is added later.
Remove weeds regularly while they are tiny. Onions have narrow upright leaves and shallow roots, so they lose badly to weeds that shade them or take moisture from the surface. Hoe on a dry day, keeping the blade shallow and away from onion roots. Hand-weed close to plants rather than pulling aggressively. Do not allow grass, annual weeds or self-sown flowers to establish between rows. A clean bed also improves air movement and makes pests easier to spot. If weeds repeatedly return, cover vacant ground with mulch between crops and improve your cultivation plan, but avoid piling thick material directly against onion necks. Consistent attention is more effective than one late, exhausting clearance.
Keep the bed evenly moist during dry spells, particularly while bulbs are enlarging. Water at soil level and give enough for roots to access moisture below the surface, then allow the surface to dry slightly. Frequent tiny splashes encourage shallow roots and can waste water. A light mulch of garden compost between rows can conserve moisture, but keep it away from the bulbs and leaves. Stop heavy watering as foliage begins to yellow and topple naturally, because the bulbs need to ripen for storing. In containers, check moisture more often: onions in small pots can dry out quickly. Rainfall varies widely, so use the soil feel rather than a fixed watering timetable.
Consider the quality and timing of your planting material. Soft, shrivelled or poorly stored sets may make weak growth, and sets planted into cold, saturated ground can struggle from the start. Choose firm, healthy sets or young plants from a reliable garden centre, and plant them in a sunny, well-prepared bed. In much of Britain, sets are commonly planted in March or April once soil is workable; colder northern areas may be later. Avoid planting so early that persistent cold encourages flowering rather than bulb formation. If growing from seed, start at the recommended time in module trays or a greenhouse so plants have a strong, even beginning.
Prepare soil well before planting rather than trying to correct it once onions are established. Choose an open, sunny site with soil that drains freely but holds some moisture. Incorporate well-rotted manure or garden compost in the preceding season, allowing it to settle before onion planting. Fresh manure can make growth soft and is better kept for hungry later crops. Remove large stones and break up compacted clods so roots can spread easily. If the bed is very poor, use a general-purpose fertiliser according to its label before planting, without overfeeding. Rotate onions away from previous onion-family crops where possible, and avoid plots recently used for crops with severe pest or disease trouble.
Know when to accept this season's result and use it well. Small but firm onions are perfectly useful fresh, and they often store less well than fully mature bulbs. Harvest when foliage has naturally yellowed and fallen, then dry bulbs in an airy, sheltered place before storing. Do not force leaves down, as this can create entry points for rot. Separate any soft or damaged bulbs for early use. Next season, choose a sunnier patch if available, space plants correctly from the start and keep a regular weeding and watering routine. Notes on variety, planting date and final bulb size will help you identify what works in your own soil.
Frequently asked questions
Will feeding make my onions bigger?
Feeding can help genuinely poor soil, but it cannot overcome crowding, weeds or dry roots. Prepare the bed with organic matter beforehand and use any fertiliser sparingly according to its label. Excessive feeding can encourage leafy growth instead of sound bulbs, so focus first on spacing, sunshine and steady moisture.
Can I thin onions and replant them?
You can use thinnings as spring onions, but replanting usually disturbs roots and may delay bulb development. If your aim is large storage onions, thin early and leave the remaining plants undisturbed. Sow or plant a separate close row if you want regular spring onions as well.
Why have my onions flowered?
Flowering can follow cold stress, particularly in sets that experience a prolonged chilly spell, and it can also be variety-related. A flowering onion puts energy into its stem rather than a good storing bulb. Harvest it promptly for fresh use and choose suitable, firm sets at the right planting time next year.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.