A seed comes up from the warmth of the soil, not from a date on the calendar
A seed comes up from the warmth of the soil, not from a date on the calendar. Each kind of seed has a soil-temperature range it germinates in; sown into ground colder than that range it comes up slowly and patchily or rots before it can emerge, and ground hotter than the range stalls it too.
Promising A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Germination is driven by temperature-dependent enzymes that mobilise the seed's stored reserves, so in soil below the species' range those enzymes work too slowly for the seedling to emerge before soil fungi rot the water-swollen seed, and in soil above the range that same enzyme machinery is suppressed. That is why the soil's own temperature is the reliable signal, and the day of the year is not.
What to do about it
Read the soil to know when to sow. A soil thermometer pushed to sowing depth for a minute gives the number. Sow each crop once the soil sits in its germination range (warm-season crops wait for warm ground even after the frost has passed, and cool-season crops go into cool soil on purpose), or start seed in a warm flat indoors and set the young plants out. Each plant's own range is on its page.
How much it matters: No date or degree figure is given here, because soil warms on its own schedule at each site and every crop's range differs. The rule advises checking the soil, whatever the calendar says; it never blocks, because the app has no soil-temperature feed and a gardener with warm ground or a warm flat is right to sow.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Days to appearance of seedlings at various soil temperatures
In our rapidly changing climate, a calendar date is not the best predictor for an extra early onset of spring weather in March or an extremely late, cold and wet April
Read next
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Sources updated
- Wording made clearer
- A person read the sources and confirmed it
- Added to the rules
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Milpa Gardens. “A seed comes up from the warmth of the soil, not from a date on the calendar. Each kind of seed has a soil-temperature range it germinates in; sown into ground colder than that range it comes up slowly and patchily or rots before it can emerge, and ground hotter than the range stalls it too.” Evidence grade: Promising. https://milpa.garden/rules/seed-comes-up-from-warmth-soil-not/
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.