How to Grow Onions: Bulb Size and Storage Life

How to Grow Onions: Bulb Size and Storage Life

To grow onions well, build leaves first and bulbs second. Establish the crop so it carries as many healthy leaves as possible before daylength triggers bulbing, place nitrogen at bulb initiation rather than at planting, lift when about 80 percent of necks have gone down, cure until the neck is dry and tight, then store cool and dry. Small bulbs almost always trace back to too few leaves at initiation. Poor storage life almost always traces back to the final six weeks.

That split is the useful reframe. Bulb size and storage life are not two versions of the same problem, and some practices that improve one will damage the other.

An onion does not choose when to start making a bulb

Bulb formation is triggered by daylength, not by plant age, calendar date, or size. FAO's crop guidance puts the critical daylength for bulbing between 11 and 16 hours depending on the cultivar — which means a variety is really a latitude specification.

Temperature moves the date, but only within limits. Lancaster and colleagues, working across two New Zealand sites and reporting in Annals of Botany in 1996, found bulbing began only when two thresholds were crossed together: about 600 degree days of thermal time, and a photoperiod of 13.75 hours. They could then predict final bulb size from bulb size at bulbing plus leaves formed after it.

When that signal arrives, the plant stops producing new leaf blades and converts leaf bases into bulb scales. The leaves already present are the material the bulb is made from.

You can measure the moment it happens. A 2019 paper in Scientific Reports uses the standard definition: bulbing has begun when the bulbing ratio — maximum bulb diameter divided by minimum neck diameter — passes 2.0. A caliper and ten plants per block will tell you where your crop is.

Why are my onion bulbs small?

Because the crop had too few leaves on the day bulbing started. Every cause below reduces to that.

The first is a cultivar-latitude mismatch, and nothing later in the season corrects it. A 2024 study in the Journal of the American Society for Horticultural Science states the two failure modes plainly: a long-day cultivar grown under short days may never bulb at all, while a short-day cultivar under long days bulbs early and produces bulbs too small to sell. Seed chosen on price rather than daylength rating produces both.

The second is a late or weak start. NC State Extension advises transplanting at about 15 cm tall and half the diameter of a pencil, with the basal plate set 2.5 to 4 cm deep. Plants set later, or checked by transplant shock, reach the daylength trigger with fewer leaves.

The third is anything that shortens leaf life. Onion thrips are the clearest example: Gill and co-authors, in the Journal of Integrated Pest Management in 2015, report New York work by Nault and Shelton showing severe thrips damage can cut bulb yield by 30 to 50 percent, almost entirely through smaller bulbs.

A worked example: nitrogen at initiation, not at planting

Nitrogen timing is where the leaf-count logic becomes an arithmetic decision.

De Jesus and colleagues at the University of Georgia's Vidalia Onion and Vegetable Research Center tracked labelled nitrogen across five growth stages, reporting in the journal Nitrogen in 2023. Fertiliser applied at transplanting was recovered at 8.9 percent in 2021 and 25.2 percent in 2022. The same amount applied at bulb initiation was recovered at 35.3 and 103 percent. Most nitrogen spread at planting is gone before the crop can use it.

Machado and Bryla measured what good timing is worth. In their 2016 field trial on intermediate-day onion in southern Portugal, plants receiving 60 kg N per hectare at 51 days after transplanting — at bulb initiation — averaged 0.28 kg per bulb and 8.2 to 8.5 cm in diameter. Plants receiving none at that point averaged 0.19 kg and 7.2 cm.

Sixty kilograms of nitrogen is about 130 kg of urea per hectare. At 200,000 plants per hectare, a 0.09 kg gain per bulb is roughly 18 tonnes per hectare, and the diameter shift moves bulbs a full grade upward in most grading systems. The response was large partly because untreated plants received only 40 kg N per hectare all season, but even a third of it repays the input. The point is placement, not rate.

Plant density decides which bulbs you get, not just how many tonnes

Density and bulb size move in opposite directions. A Polish trial series reported through the International Society for Horticultural Science found average marketable yield rising from 20.5 t/ha at 20 plants per square metre to 32.8 t/ha at 80 plants per square metre over 1991 to 1993.

Large-bulb yield went the other way, peaking at 20 to 40 plants per square metre. Density also advanced maturity: at 140 plants per square metre, tops fell over 8 to 10 days earlier than at 20. Choose density for the grade your buyer pays for.

What causes poor onion storage life?

Four things, in order: harvest timing, late-season water and nitrogen, curing, and store conditions.

Harvest too late and you trade keeping quality for weight. Wall and Corgan at New Mexico State University compared five harvest dates across four short-day cultivars and reported in 1994 that bulb weight rose and firmness fell as harvest was delayed, with disease incidence rising in one of two years. They identified 80 percent neck maturity as the optimum.

Late nitrogen delays maturity and widens necks, and a wide neck is an open wound. The Georgia team notes that delaying nitrogen to bulb maturation reduces yield, flavour and storage life — while Machado and Bryla found nitrogen at initiation left neck diameter, soluble solids and water content unchanged. The same nutrient is safe at one stage and damaging three weeks later.

Cutting irrigation early has a limit that is easy to overshoot. Rattin and co-authors in Argentina tested this directly, publishing in Scientia Horticulturae in 2011: water deficit during bulb filling produced smaller bulbs that sprouted sooner and lost weight faster over six months at 15 °C, in both glasshouse and field experiments. Dry the crop down at maturity, not during filling.

Curing is an endpoint, not a duration. The UC Davis Postharvest Research and Extension Center defines field-dry maturity as a neck completely dry to the touch and not slippery, typically reached at 5 to 8 percent weight loss after harvest. Field curing needs air of at least 24 °C (75 °F); forced air achieves the same in about 12 hours at 30 to 45 °C (86 to 113 °F).

Store conditions decide the rest. UC Davis puts whole-onion respiration at 3 to 4 mL of carbon dioxide per kg per hour at 0 to 5 °C, against 27 to 29 mL at 25 to 27 °C — roughly eight times faster. Anything between 5 and 25 °C encourages sprouting. Target 65 to 70 percent relative humidity with air movement of about 1 cubic metre per minute for every cubic metre of onions. Pungent cultivars hold 6 to 9 months at 0 °C; mild ones give you 2 to 4 weeks.

Cooling is not the only lever. ICAR's Directorate of Onion and Garlic Research in Pune, India, measured 54 percent total loss over four months in an unregulated ventilated store, against 7.14 percent in a purpose-designed structure held at 27 ± 2 °C and 60 to 65 percent humidity with forced air circulation, and 5 percent in cold storage. The gap between 54 and 7 percent is airflow and humidity control, not refrigeration.

Frequently asked questions

Why did my onions bolt instead of forming bulbs?

Bolting is vernalisation: the plant read a cold period as a completed winter and switched to flowering. It only responds once the stem passes roughly pencil thickness. NC State Extension notes bolted plants produce small bulbs with thick necks that cure poorly. Plant smaller transplants, or shift the planting date so seedlings meet cold weather while still thin.

Should I stop irrigating onions early to improve storage?

Only at maturity, not during bulb filling. Rattin and colleagues found in 2011 that water deficit during filling produced bulbs that sprouted earlier and lost weight faster across six months of storage. Withhold water once bulbs have sized and tops begin to go down, which dries necks and outer scales without stressing a filling crop.

What is the best storage temperature for onions?

For long-term storage, 0 °C (32 °F) at 65 to 70 percent relative humidity with steady air movement. UC Davis data show respiration is about eight times slower at 0 to 5 °C than at 25 to 27 °C. Temperatures between 5 and 25 °C are the worst option, because that band actively encourages sprouting.

Every decision above depends on something local: daylength at your latitude on your planting date, the thermal time your site accumulates, the conditions in your store. Valora Earth's advisor answers those questions for a specific field, in the farmer's own language, using local weather and soil data.