A warm-season bean needs warm soil to germinate, not just a passed frost date — sown into cold, frost-free soil the seed rots before it sprouts.
Promising · suboptimal if ignored → advise
How it works
A bean seed takes up water within hours of sowing. In cold soil that fast, cold-water uptake injures the seed's rehydrating cell membranes, and the damaged membranes leak sugars that feed soil fungi (Pythium, Rhizoctonia), so the seed rots in the ground instead of germinating. Peas and fava tolerate cold soil and are exempt. The last-frost date is a floor for the seedling, not the seed: the soil can be frost-free and still too cold for a bean, so this bar sits above the frost floor another rule already sets for tender crops.
What to do about it
Wait for the soil to warm, which in a cold spring is a week or more after the last frost, not on it. A cheap soil thermometer pushed to seed depth in the morning is the honest check; sow when it reads reliably warm. A cold-soil sowing that rots is simply re-sown into warm soil, at the cost of the lost time.
How much it matters: No degree figure is stated — the corpus carries no sourced soil-temperature minimum for these species and inventing one would be a fabricated number. The direction is well established: cold-soil bean sowings rot and grow slowly, warm-soil sowings emerge fast and even. It advises rather than gates, because the soil-warmth bar is regional and the app cannot read a bed's soil.
The evidence
Evidence: not yet confirmed against its 1 source.
what this rests on - one pointer
UMN Extension, 'Growing beans', extension.umn.edu/vegetables/growing-beans (fetched 2026-08-30): 'Plant beans once the soil has warmed. In much of Minnesota, this is not until late May or even early June'; 'Bean seed planted in cold soil may rot rather than germinate, and plant growth will be very slow in cooler weather.'
Where this rule stops
A STANDING sowing advisory scoped to warm-season legumes (applies_to family legumes + frost_tolerance tender: common, lima and runner bean, cowpea, soybean). It COMPLEMENTS the tender-crop frost floor rather than duplicating it — that rule gates on the frost date, this names the higher soil-warmth bar the frost date misses. Like the inoculant rule it does not gate placement or dock a score: the app has no soil-temperature signal, so it advises on the species page and can be cited by the bean guides. It is the corpus's first soil-temperature sowing rule.
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.
- corpus: another rule here — warm-season beans need warm soil to germinate, not just a passed frost date
Cite this page
Milpa Gardens. "A warm-season bean needs warm soil to germinate, not just a passed frost date — sown into cold, frost-free soil the seed rots before it sprouts." Evidence grade: Promising. https://milpa.garden/rules/warm-season-bean-needs-warm-soil-germinate/
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.