Short-Day vs Long-Day Onions: Find Your Type by Zone
By Ada Jones · Updated October 2026 · 7 min read
Short answer: onions bulb based on day length, not temperature or calendar date. Short-day varieties need about 10-12 hours of daylight to start bulbing, intermediate-day need 12-14 hours, and long-day need 14-16 hours. Pick the category that matches your latitude, or the wrong variety simply won’t form a proper bulb no matter how well you grow it.
Find Your Day-Length Category by Latitude
| Latitude / Region | Day-Length Category | Example States / Areas |
|---|---|---|
| Below ~35°N (southern US, similar latitudes worldwide) | Short-day | Southern Texas, Louisiana, Florida, Southern California, South Georgia |
| ~35°N to 38°N | Intermediate-day | North Carolina, Tennessee, Northern California, Kansas, Virginia |
| Above ~38°N | Long-day | Most of the Northern US, Canada, UK, Northern Europe |
These lines are approximate, not hard cutoffs — intermediate-day varieties have the widest tolerance and are a safe choice if you’re near a boundary or unsure of your exact latitude.
What Happens If You Pick the Wrong Type
- Long-day variety grown in a short-day region: days never get long enough to trigger bulbing — you’ll get mostly green tops and small or no bulbs, even with perfect care
- Short-day variety grown in a long-day region: bulbs form too early and too small, since the trigger day length is reached while the plant is still young and hasn’t built up enough leaf mass
This mismatch is one of the most common reasons new growers report “my onions never made bulbs” — and it’s a seed-selection problem, not a growing-technique problem, so no amount of fertilizer or watering adjustment fixes it.
FAQ
Can I grow a long-day onion in a short-day climate?
Not successfully for a proper storage bulb — you’d get stunted bulbs at best. Choose a variety matched to your latitude rather than trying to force a mismatched one to work.
Do day-neutral onion varieties exist?
A few “day-neutral” varieties are bred to bulb across a wider range of day lengths, which can be a safer choice if you’re right on a regional boundary, though intermediate-day varieties remain the most widely available flexible option.
Final Thoughts
Get the day-length category right before you buy seed or sets — it matters more than soil, water, or fertilizer for whether you get a proper bulb at all. Once you know your category, see our planting calendar by zone for when to put it in the ground.
How This Guide Was Checked
Based on university extension day-length/bulbing physiology research and USDA latitude data. Unsure of your category? Contact us with your location.
Related Onion-Growing Guides
- Types of Onions: Red, Yellow, White, and Sweet Compared
- When to Plant Onions by Zone (Plus Companion Planting)
- Onion Harvest Date Calculator: Estimate by Planting Date
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.
