Why Are My Onions Not Forming Bulbs?

Why Onions Not Forming Bulbs Comes Down to Daylength

Onion varieties are bred around three daylength categories, and knowing which one you planted matters more than soil, water, or fertilizer ever will for bulb formation specifically. Short-day varieties begin bulbing at 10-12 hours of daylight, suited to southern latitudes. Intermediate-day varieties need 12-14 hours, the middle ground for central latitudes. Long-day varieties require 14-16 hours, suited to northern latitudes. A separate category, day-neutral varieties, bulbs regardless of daylength entirely — a genuinely safer choice if you’re not sure which zone you fall into, since it sidesteps the daylength question altogether.

Plant a long-day variety in the Deep South, or a short-day variety in the northern US, and the plant simply never receives the daylength signal it needs to start bulbing — it just keeps putting energy into green top growth instead. This is different from a nutrient or watering problem, which is why the plant often looks perfectly healthy above ground while producing little to nothing below it.

The Nitrogen Trap: Why More Fertilizer Often Makes It Worse

It’s worth calling out specifically, since it’s an easy mistake to make: heavy nitrogen feeding, especially early in the season, actively delays bulb formation rather than helping it along. Nitrogen tells the plant to keep building leaves, and a plant that’s still getting a strong nitrogen signal has less reason to shift its energy toward bulbing even once the daylength trigger arrives.

This creates a genuinely misleading picture. A plant pushed too hard on nitrogen tends to put on a good show up top — full, richly colored foliage — while the part that actually matters stays small and underdeveloped underneath. It’s exactly the wrong moment to reach for more fertilizer, even though the leaves themselves would suggest otherwise. If your onions fit that description, stopping additional feeding and letting the plant respond to its daylength trigger on its own is the right move, not doubling down on fertilizer. A soil test can help confirm whether there’s a genuine nutrient gap or just a habit of overfeeding.

How to Fix Onions Not Forming Bulbs Next Season

There’s no in-season fix once the wrong variety is already growing — this is purely a planning issue for next time:

• Check your latitude against the variety type before ordering seed or sets next season — most seed catalogs label onions clearly as short-day, intermediate-day, long-day, or day-neutral.

• Roughly: south of 35°N, use short-day varieties; between 35-42°N, intermediate-day works well; north of 42°N, use long-day. If you’re near a boundary or just unsure, day-neutral varieties are the safest bet regardless of exact latitude.

• Hold off on extra nitrogen once bulbing should be starting — if tops look lush but bulbs aren’t swelling, more fertilizer isn’t the fix.

• This season’s onions are still useful. Harvest the green tops as scallions or green onions even if bulbs never form, rather than writing off the crop entirely.

What Else Affects Bulb Size, Once Daylength Is Right

Once you’ve got the right daylength type for your area, a few other factors still influence how large the resulting bulbs get. Temperature plays a real secondary role — higher temperatures during the bulbing period speed up the process, sometimes at the cost of final bulb size if the plant hasn’t had enough time to build substantial top growth first. This is part of why a short-day onion planted too far north can still technically bulb, just disappointingly small — it hits its daylength trigger before the plant has grown enough leafy top growth to support a large bulb.

Soil fertility and consistent moisture support the leafy growth phase that happens before bulbing starts, which indirectly affects final bulb size even though they don’t override the daylength trigger itself. A plant that’s been well-fed and watered going into its daylength trigger simply has more stored energy to convert into bulb growth once bulbing actually begins. Keeping weeds down while plants are small matters here too, since weeds compete directly for the same moisture and nutrients the developing plant needs during this stage.

Planting timing matters here too, and it’s worth getting right alongside variety selection. Onions generally need to build substantial leafy top growth before their daylength trigger arrives, so planting too late in the season — even with the correct daylength variety for your area — can mean the plant reaches its trigger point without enough top growth to support a decent-sized bulb. This is why short-day onions are typically planted in fall or very early spring in southern regions, giving them a long head start before the shorter daylight window of early summer arrives and triggers bulbing.

Watering needs change again once bulbs are actually finishing up. Once the foliage starts flopping over on its own — a sign the plant has essentially finished sizing up — it’s time to start cutting back rather than keeping up a regular schedule. The bulb still has curing left to do: the neck needs to dry and seal tightly, since a soft, wet neck is exactly how neck rot (Botrytis allii) gets into stored onions. Watering right through that final stretch keeps the neck too soft for that seal to form properly, which is why the guidance is to stop watering entirely, not just ease off, once tops start falling.

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Sourced from: Iowa State University Extension