Onion cultivar group must match site latitude.
Well established · fatal if ignored → block
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 north crosses its low threshold early and bulbs prematurely on small plants, so bulbs stay small. A long-day cultivar grown too far south never reaches its high threshold and fails to bulb. Changeover is around the 35th-38th parallel.
What to do about it
Choose long-day onions in the north and short-day in the south; the planner picks the group from your plot's latitude.
The evidence
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
NC State Extension, 'Bulb Onions', content.ces.ncsu.edu/bulb-onions (read 2026-07-11): '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.'
Univ. of Illinois Extension (The Humble Gardener), 'Growing Onions in Central Illinois', extension.illinois.edu/blogs/humble-gardener/2018-03-19-growing-onions-central-illinois (read 2026-07-11): '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' (in the south).
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- the remedy speaks to a gardener, not to the schema (approved 2026-08-13)
- promote to verified — onion cultivar group must match site latitude
- cite onion daylength sources; the two failure modes were swapped
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/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.