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How daylength and latitude control onion bulbing
Above: an onion variety trial.
Where short-, intermediate- and long-day onions grow best, and the traits of each
Daylength is what tells onions to make bulbs. Light is the main signal for bulbing, though temperature and fertility also play a part. When day length passes a certain point, which differs by variety, the plant stops putting its energy into leaves and starts building a bulb. Onions are sorted into three main groups:
Below we list the distinguishing characteristics for each group. Look up your latitude, find your USDA hardiness zone, and check our onion comparison charts to pick the varieties that best fit your region and needs.
*Zone boundaries on the map are approximate and shift somewhat from region to region.
How long-day, intermediate-day and short-day onions differ
The daylength groups overlap instead of ending at sharp lines: certain long-day varieties bulb farther south than others, and certain intermediate-day varieties bulb farther north.
Bulbing also responds to other conditions such as heat and soil moisture, so even the right variety for your latitude needs good growing conditions. Big, healthy tops produce the best bulb size and quality, which means the plants need enough time to grow before bulbing begins. Days to maturity vary a lot with variety and growing practices. In short-season areas you can gain that time by setting out transplants instead of, or alongside, direct seeding. Hensley’s sells many varieties as field-grown plants ready to transplant.
Use the full-size onion comparison chart to check the daylength group of every full-size onion variety Hensley’s carries and compare the traits that set them apart.
Long-Day Onions
- Grow best at northern latitudes (37–47°).
- Start bulbing at 14–16 hours of daylight.
- Typically planted in late winter or early spring.
- Includes sweet, specialty and long-keeping onions.
- Shop long-day onions…
Intermediate-Day Onions
- Grow best at latitudes of 32–42°.
- Start bulbing at 12–14 hours of daylight.
- Typically planted in spring in the middle latitudes, early spring farther north, and fall where winters are mild.
- Shop intermediate-day onions…
Short-Day Onions
- Grow best at latitudes of 25–35°.
- Start bulbing at 10–12 hours of daylight.
- Need mild winters (Zone 7 or warmer).
- Typically planted in fall to mature in late spring.
- Usually sweet or mild.
- Shop short-day onions…
