Daylength and Onion Growth: Understanding Short, Intermediate, and Long-Day Varieties
Onions aren’t just one-size-fits-all when it comes to growing conditions—daylength plays a huge role in determining how and when bulbs form. Choosing the right variety for your latitude is essential for getting big, healthy onions that store well.
After years of trial and error (and a few tiny, disappointing bulbs), I’ve learned that knowing your onion’s daylength requirement is the key to success.
How Daylength Influences Bulb Formation
Onions start bulbing when daylight hours reach a certain threshold:
- Short-Day Onions: Begin bulbing with 10–12 hours of daylight.
- Intermediate-Day Onions: Start bulbing at 12–14 hours.
- Long-Day Onions: Require 14–16 hours before bulb formation begins.
The closer you are to the equator, the shorter your summer days—meaning you’ll need a short-day onion. Farther north, you’ll need intermediate or long-day types.
Matching Onion Types to Your Region
| Onion Type | Best For | Daylength Needed | Typical Harvest Time | Examples |
|---|---|---|---|---|
| Short-Day | Southern U.S., subtropics | 10–12 hrs | Late spring–early summer | ‘Texas 1015’, ‘Yellow Granex’ |
| Intermediate-Day | Mid-latitudes | 12–14 hrs | Early–mid summer | ‘Candy’, ‘Red Candy Apple’ |
| Long-Day | Northern U.S., Canada | 14–16 hrs | Mid–late summer | ‘Walla Walla’, ‘Copra’ |
Why Choosing the Wrong Type Hurts Yield
If you plant a long-day onion in the deep South, it will never get the daylength it needs before heat stress sets in—resulting in small bulbs. Likewise, planting short-day onions in the far north means they’ll bulb too early, before leaves have time to develop.
My Experience Tip
When I first moved from Zone 8 to Zone 5, I planted my favorite short-day onions out of habit. The result? Golf ball–sized bulbs and a big lesson learned about daylength.
Onion Growing Hub
Start here: The Ultimate Onion Growing Guide: From Seed to Harvest
🌱 Planning & Varieties
- How to Choose the Best Onion Varieties for Your Region – Match flavor, storage, and climate to your garden.
- Daylength and Onion Growth: Understanding Short, Intermediate, and Long-Day Varieties – Pick the right type for your latitude.
- Growing Bulb Onions: Pick the Right Variety for Your Garden – Choose the right variety for where you live.
- Onion Sets vs. Seedlings vs. Seeds: Which Is Best for Your Garden? – Pros and cons of each planting method.
🌿 Planting & Starting
- Step-by-Step Guide to Starting Onion Seeds Indoors Successfully – Timing and troubleshooting for strong transplants.
- Planting Onions Seeds and Sets – Here’s how to get started.
- How to Grow Onion Sets – You can grow your own onion sets from seed for next year.
- Growing Onions in Containers: Soil, Pots, and Care Tips – Small-space planting success.
- How to Grow Green Onions, Spring Onions, and Scallions – Learn how these alliums differ.
- How to Plant, Grow, and Harvest Welsh Onions – Learn to grow these mild-flavored members of the onion family.
🌞 Care & Maintenance
- Watering Onions for Optimal Growth: Techniques and Tips – How much and how often to water.
- Feeding and Fertilizing Onions: Organic and Synthetic Options – Nutrient needs for strong bulbs.
- Companion Planting with Onions: Plants to Grow With and Avoid – Beneficial pairings and plants to avoid.
- Onion Family Growing Problems: Pest, Disease, and Problem Controls – Natural control of thrips, maggots, and more; identifying and treating fungal or bacterial problems.
🧅 Harvest & Storage
- How to Harvest Onions at Peak Maturity for Best Storage – Signs onions are ready and how to cure.
- How to Harvest and Store Onions – Here are the basics for harvesting and storing onions.
- Onion Storage Techniques: Keep Your Harvest Fresh for Months – Best environments for long-term keeping.
🍳 Using Your Onions
- Cooking with Onions: Recipes and Tips to Bring Out Flavor – From fresh salads to caramelized dishes.
- How to Make Onion Soup with No Recipe – Enjoy this tasty classic!
