Why Your Onions Aren’t Forming Bulbs (or Are Too Small)
Troubleshooting
Why Your Onions Aren’t Forming Bulbs (or Are Too Small)
By Ada Jones · Updated October 2026 · Figures checked against university extension sources linked in the text
Healthy green leaves with no bulb underneath, or bulbs stuck at 1 to 2 inches, almost always trace back to one of six causes: the wrong day-length type for your latitude, a late planting date, planting too deep, crowding, too much nitrogen, or too little sun. Work through them in that order and you’ll usually find the culprit.
Work Through These Causes, in Order
| Check | Why It Matters | Source |
|---|---|---|
| Day-length type | A type that doesn’t match your latitude may never get its bulbing signal at all | Illinois Extension |
| Planting date | Late planting gives small bulbs or none, in any location | Illinois Extension |
| Planting depth | Deeper than about 1 inch inhibits bulbing and can cause small, oval bulbs | Dixondale Farms, Homes & Gardens |
| Nitrogen levels | High soil nitrogen delays bulbing | Harvest to Table |
| Soil on the shoulders | Soil or mulch piled on the bulb as it forms should be pulled back | Harvest to Table, Bonnie Plants |
| Sunlight | Bulb onions need 6 to 8 hours of direct sun | Scotts |
How to Grow Bigger Onions
Bulb size depends heavily on how much healthy leaf the plant builds before bulbing starts. Dixondale Farms uses about 13 leaves at maturity as its benchmark — so anything that costs you leaf growth (weeds, pests, drought, stopping feeding too early) costs you bulb size later.
- Match the type to your latitude and plant as early as the soil allows.
- Give each plant room. Illinois Extension notes closer spacing significantly decreases bulb size — use 4 to 5 inches for large bulbs.
- Start with pencil-thick transplants if you’re buying them; Bonnie Plants notes these produce the biggest bulbs.
- Feed early, then stop once bulbs begin to swell.
- Keep water steady, and keep mulch and soil off the top two-thirds of the bulb as it swells.
- Let the tops fall over on their own. Breaking them early interrupts growth and gives smaller bulbs that store less well.
Real example: a Zone 6B grower asked a gardening community why years of planting had produced only 1 to 2 inch bulbs despite watering and fertilizing monthly. Another grower in the same zone replied that the issue was likely variety — pointing out they plant intermediate-day onions, where the original grower hadn’t specified their variety’s day-length type at all. It’s a small exchange, but it illustrates the single most overlooked cause of small bulbs: not matching the onion to the zone.
Does Epsom Salt Help Onions Grow Bigger?
Only if your soil is genuinely short of magnesium. Extension specialists note that the broadly claimed benefits of Epsom salt in home gardens aren’t backed by strong evidence unless there’s an actual deficiency, which is most likely on sandy, low-pH soils. Test your soil before adding it, and don’t spray it directly on leaves, since it can scorch them. It’s a popular question across gardening forums, but it’s rarely the fix people hope it is.
Frequently Asked Questions
Why do my onions have great leaves but no bulb?
Most often it’s a day-length type that doesn’t suit your latitude, so the plant never gets the signal to bulb, regardless of how healthy the leaves look.
What fertilizer makes onions grow big?
Steady nitrogen early while leaves are growing, then stop once bulbs begin to swell. Too much nitrogen late causes thick necks, soft bulbs and poor storage.
Does planting depth really affect bulb size?
Yes. Planting deeper than about 1 inch inhibits bulbing and can produce small, oval-shaped bulbs instead of round, full-size ones.
Does Epsom salt help onions grow?
Only on soil that’s actually deficient in magnesium. Test your soil first rather than applying it as a routine treatment.
Final Thoughts
Small or missing bulbs are rarely a mystery once you work through the six causes in order — variety, timing, depth, nitrogen, mulch, and sun. Most growers find the answer in the first two or three checks. If a flower stalk is involved instead of just a small bulb, that’s a different issue — see why onions bolt. For everything else, the main growing guide covers the full picture.
How This Guide Was Checked
Written by Ada Jones. Causes and fixes are checked against Illinois Extension, Dixondale Farms, Harvest to Table, Bonnie Plants and Scotts. Spot a mistake? Contact us and we’ll correct it.
Related Onion-Growing Guides
- Epsom Salt for Onions: Does It Really Work?
- Best Soil and pH for Growing Onions
- How to Grow Big Onions: Maximizing Bulb Size
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.
