Lettuce and spinach that ripen in hot, long days run to seed.
As recorded: A lettuce or spinach sowing that comes ready in a month whose normal daytime high reaches the crop's bolting temperature (about 24 C, 75 F), while days are longer than nights, is likely to bolt before it can be picked.
Promising A person has checked it against its sources. If you skip this, you lose real yield. The planner warns you.
How it works
Long days and several days in a row above about 24 °C (75 °F) switch lettuce and spinach from making leaves to making a flower stalk, which turns the leaves bitter and ends the harvest.
What to do about it
Sow so the crop is picked before the hot months begin, or wait and sow for fall: lettuce about three months before your average first fall frost, spinach about two. Spinach sown for fall grows under shortening days and does not bolt.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Multiple days of high temperatures (greater than 75° F) can cause lettuce plants to flower.
Spinach plants can start to flower after multiple days of high temperatures (greater than 75° F).
Where this rule stops
It reads each month's normal daytime high, so it cannot see a heat wave in a mild month, and the monthly average stands in for 'several days in a row'. 'Long days' means days longer than nights (UMN: 'During May, the days are much longer than the nights'). A hot month after the autumn equinox does not count: UMN's fall spinach 'will not bolt'.
Lettuce and spinach only; each has its own sourced bolting temperature (UMN's '>75 °F' on each crop's page). Cilantro's 24 °C is an estimate and is not covered. Spinach's figure stays flagged estimated in its species record.
Plants it applies to
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Details updated
- A person read the sources and confirmed it
- Added to the rules
Cite this page
Milpa Gardens. "Lettuce and spinach that ripen in hot, long days run to seed." Evidence grade: Promising. https://milpa.garden/rules/lettuce-spinach-sowing-that-comes-ready-month/
A garden planner built on graded evidence. 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.
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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.