Seed Germination Temperature: A Reference Table (and Why Scotland Needs Its Own Sowing Calendar)

A seed sitting in cold soil isn't simply waiting. It takes on water whether or not the temperature is right, because that's a physical process, and the enzymes that turn stored starch into the sugars feeding the first root and shoot are temperature-dependent. Below a crop's minimum, the seed has absorbed water it can't yet use. It sits swollen and stalled in damp soil, which is exactly what soil-borne fungi and bacteria exploit, causing seed rot and damping-offA fungal disease that kills seedlings at soil level, usually caused by overwatering, poor airflow, or reused unsterilised compost or trays. Full definition → (a fungal infection that kills seeds before or just after they emerge). So sowing into cold soil doesn't just mean slower germinationA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition →, it changes what happens to the seed while it waits. That's why a soil-temperature figure matters more than a calendar date, and why a UK-average seed-packet date and a realistic Scottish sowing date can be weeks apart.

Germination temperature reference table

These are the published soil or germination temperatures for each crop. A dash or "not stated" means the figure isn't published for that crop, so treat it as unknown, not as "no risk".

As a general baseline across vegetables, most seeds won't germinate below a soil temperature of about 7°C (45°F). That's the floor, not a target, and several crops need considerably more warmth to get going reliably.

The table splits into two tiers. Beetroot, radish, pea, parsnip and sweetcorn (outdoors) need soil in the 7-12°C range, not far above what Scotland typically reaches in early-to-mid spring. Tomato, courgette, chilli and sweetcorn raised indoors want 18-21°C, warmer than most Scottish outdoor soil until late spring or early summer, hence the standard advice to start those under cover.

Minimum versus optimal isn't just "warmer is faster"

Parsnip is the clearest example: seedlings can take from 14 to 30 days to appear, depending on conditions, more than double the time at the slow end. A seed sown at its minimum can sit doing very little for weeks, while the same seed sown once the soil is comfortably above that minimum gets moving in days. Sowing as soon as it's technically possible and sowing at the crop's actual optimum are two different decisions, with a real time cost attached to the first.

Air temperature and soil temperature aren't the same measurement

A weather forecast reports air temperature, measured about 1.2 metres above ground in shade, which can swing by 10°C or more in a day. Soil has far greater thermal mass, so it heats and cools much more slowly, and the lag is most pronounced in spring. The Met Office puts it this way for May: ground frost is more common than air frost, because the ground has not yet accumulated as much stored warmth as it does later in summer, so similar air temperatures can lead to colder ground conditions than they would in early autumn.

A mild, sunny week in April is therefore evidence the air is ready, not proof the soil is. It's the soil number that governs whether a seed rots or grows.

How to measure soil temperature

A soil thermometer is cheap and the only reliable way to know where you stand.

• Depth: push the probe into undisturbed soil at about the depth you'll sow at, commonly 2.5-5cm (1-2in) for most vegetable seeds. A 10cm depth gives steadier readings.
• Timing: take the reading in the morning, before the sun has warmed the surface, so you're not measuring a temporary daytime spike.
• Consistency: don't act on a single reading. Take readings over several consecutive days to confirm the soil has settled above the threshold rather than briefly touched it.

Warming soil faster, rather than just waiting

Where soil is close to a crop's minimum but not quite there, warm it by putting clochesA small, portable clear cover placed over an individual plant or short row to trap warmth and keep off wind, rain and pests. Full definition →, clear plastic sheeting or even bottomless plastic bottles over the soil a few weeks before sowing. Keep the cover in place for a few weeks after sowing too, or until the weather warms, and remove it gradually rather than all at once, since a sudden drop back to unprotected outdoor temperatures can set young growth back hard.

An unheated greenhouse or polytunnel bed works on the same principle at a larger scale, so soil inside typically reaches germination thresholds ahead of an open bed, which is worth using for the high-tier crops (tomato, courgette, sweetcorn, chilli) if you're sowing or planting them out directly, since a few degrees' head start matters most in a Scottish outdoor bed. An early or quick-maturing variety is a further way to work with a shorter warm season. Sowing indoors ahead of the soil being ready only solves half the problem: seedlings raised on a windowsill or in a propagator still need hardening off before they go into open ground, and shouldn't be planted out before the soil is warm enough, however established they look.

What too-cold soil does to a seed

"Slower germination" understates it. The extended vulnerable period described above is why cold-soil sowings tend to come up patchy rather than uniformly late: some seeds rot outright while others scrape through, and the gappy rows are the visible outcome of that uneven survival.

Heavy, poorly drained soil, common on Scottish clay, is likely to compound the risk: it holds more water for longer than free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → soil and is slower to warm. A raised bed warms and drains faster than flat, undrained clay, and is worth using for the higher-threshold crops if your ground is heavy. See improving clay soil.

Scotland's spring lag: what the numbers show

No Scottish soil-temperature series is available here, so this comparison uses Met Office 1991-2020 air-temperature averages as a proxy. Treat it as directional, not a substitute for a soil reading in your own garden.

Daytime highs are close between the two (Edinburgh's April maximum, 12.2°C, is essentially Camborne's 12.1°C), but nights are where the gap sits: Edinburgh runs about 1.7 to 2.5°C colder overnight than Camborne in each spring month. Soil temperature reflects day and night together, not just the afternoon peak, so a mild-day, cold-night spot like Edinburgh doesn't build up soil warmth as fast as a location that's mild around the clock. Edinburgh's April overnight low (4.7°C) is below Camborne's March low (5.4°C), so Edinburgh's April nights are colder than Cornwall's March nights.

That is air-temperature data from one station in the milder Central Belt, not a measured soil gap, and Scotland's spread runs from the moderated west coast to the colder Highlands and upland interior. It is a reasonable, not proven, indication that the Central Belt reaches common germination thresholds later than a UK-average seed packet assumes. For the rest of the country, Met Office frost data for named stations shows the same coastal-versus-inland and lowland-versus-upland pattern, and gives a starting point for when to begin checking soil in your own area:

These dates come from average last-frost timing, not measured soil temperature, so treat them as a prompt to bring out the thermometer. The full station-by-station breakdown, including the Hebrides, Shetland and Borders, is on the regional frost-dates page.

The practical takeaway for Scottish sowing dates

• For crops with a high minimum (tomato, chilli, courgette, sweetcorn), sowing under cover indoors and planting out once outdoor soil has demonstrably warmed, checked with a thermometer rather than guessed from a mild day, is the safer default in Scotland.
• For crops with a lower minimum (pea, beetroot, radish, parsnip), outdoor sowing is realistic earlier, but treat the seed packet's "early spring" date as a starting point to check against actual soil readings.

Frost risk and soil-temperature risk are separate checks, and both are worth doing before an outdoor sowing.

Common questions

Sources