When to sow beans: the soil temperature, not the calendar

Every seed packet says the same thing: sow beans “from mid-May”. A packet does not know what your spring is doing, how high your garden sits, or that the last week was five degrees below the average. The seed does not read the packet either. It reads the soil.

16 °C, measured at 6 cm

A common bean (Phaseolus vulgaris) germinates reliably from 16 °C of soil temperature, measured at sowing depth — around 6 cm for a bean. Below that number, germination does not merely slow down: the seed takes on water, sits in cold ground without the energy to push, and rots before it emerges. A row sown a fortnight early is not a fortnight ahead. It is a row sown twice.

Above 16 °C, every extra degree buys speed. At 16 °C a bean takes about eleven days to show; at 21 °C it takes five or six. That gap matters more than it looks, because a seedling that spends eleven days underground spends eleven days available to slugs, soil fungi and a cold snap.

Why the air temperature misleads

An air thermometer tells you about today. Soil tells you about the last week. Twenty centimetres of earth is a slow, heavy thing: it warms behind the air in spring, cools behind it in autumn, and smooths out every warm afternoon that has not lasted.

That lag is exactly why soil is the better signal. A 22 °C afternoon in April says nothing — the soil underneath may still be at 11 °C. Three mild nights in a row move the soil far more than one hot day, because the soil is losing less overnight than it gains by day.

The practical version: use a seven-day average of the soil at 6 cm, not a single morning’s reading. One warm day is weather. Five is a window.

What the forecast adds

Knowing the soil is at 16 °C today is half the decision. The other half is what happens next. A bean sown into 16 °C soil that then spends a week under a cold front, or meets a late frost as it emerges, is a bean you have lost anyway.

So the rule has two halves:

the soil has held above 16 °C for five consecutive days, and
no frost appears in the 16-day forecast for that spot.

Both, or the window is not open. This is the rule Loam applies, and it is why the app sometimes says wait on a warm day in April — the number it is waiting on is not the one on the thermometer in the shade.

Where the soil warms first

Two gardens five kilometres apart can be a fortnight apart. What moves the date:

Altitude. Roughly half a degree of soil per 100 m of elevation, and more frost nights on top of that.
Aspect. A slope facing the sun warms days earlier than a flat bed; a north-facing slope runs behind.
Soil type. Sand warms fast and cools fast. Clay takes a week longer in spring and holds its warmth longer in autumn.
Cover. Black plastic or a cloche laid a fortnight before sowing buys two to four degrees at 6 cm, which is often the whole difference.

That is also why an almanac date for a region is the wrong unit. The relevant unit is much smaller than a region and much bigger than your own bed.

The short version

Beans go in when the soil at 6 cm has held above 16 °C for five days.
Check a seven-day average, not one reading.
Confirm no frost in the next 16 days before you sow.
Below 16 °C, waiting a week costs nothing. Sowing costs the row.

Germination thresholds here come from the Harrington germination tables, published by US university extension services, cross-checked against RHS sowing guidance.