Onion cultivar group must match site latitude.
Well established A person has checked it against its sources. If you skip this, the planting fails. The planner won’t allow it.
How it works
Bulbing fires when daylength crosses a cultivar's critical threshold (short ~10-12h, intermediate ~12-14h, long ~14-16h). A short-day cultivar grown too far from the equator crosses its low threshold early and bulbs prematurely on small plants, so bulbs stay small. A long-day cultivar grown too close to it never reaches its high threshold and fails to bulb. Changeover is around the 35th-38th parallel, north or south.
What to do about it
Choose long-day onions far from the equator and short-day near it; the planner picks the group from your plot's latitude.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
… onions will not form bulbs until daylength reaches the cultivar's critical threshold. Short-day onions bulb at approximately 10-12 hours of daylength ... Intermediate-day onions bulb at approximately 12-14 hours ... Long-day onions bulb at 14-16 hours of daylength.
Long day varieties are for northern growing areas (above the 35th latitude); short-day varieties are for southern. If short-day varieties are grown in our zone, they won't do well. They will not form bulbs of any decent size. And since the long day onions need more daylight, they will not do well
Plants it applies to · 1
Plant teams that rest on it
Read next
- Frost dates vs hardiness zones vs day lengthThree different numbers describe your climate, and they do three different jobs. Your zone, your frost dates, and your latitude each answer their own question. Most garden tools blur them into "your zone," which is a mistake. Here's which is which.
- Growing onions by day length and garlic from a fall plantingOnions (Allium cepa) and garlic (Allium sativum) are close cousins on opposite calendars. Onions go in early in spring, garlic in autumn.
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- How it works updated
- Sources updated
- Details updated
- Advice on what to do added
- A person read the sources and confirmed it
- Added to the rules
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Cite this page
Milpa Gardens. “Onion cultivar group must match site latitude.” Evidence grade: Well established. https://milpa.garden/rules/onion-cultivar-group-match-site-latitude/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.