What is the best soil temperature for seed germination?

Most UK garden vegetables germinate fastest at 18-21C soil temperature. Heat-loving crops like tomatoes and peppers need 21-27C. Cool-season crops like lettuce and peas germinate well at 10-18C and stall above 25C. Match the temperature to the crop, not to a single number.

How do I measure soil temperature?

Push a soil thermometer 5cm into damp compost or seed bed and leave for 2-3 minutes before reading. Take the reading at 9am for a representative overnight low. A £4 soil thermometer is the cheapest useful piece of propagation kit you can buy.

What is the minimum temperature for tomato seeds to germinate?

Tomato seeds need a minimum soil temperature of 16C to germinate, but germination is slow and erratic below 18C. The optimum is 21-27C, where 80-90% of viable seed germinates within 7-10 days. Below 16C most seed will rot in the compost before germinating.

Why are my seeds not germinating?

The most common cause of failed UK germination is soil temperature too low. Air may show 18C while compost sits at 12C, especially overnight. Other causes: compost too wet (seeds rot), too dry (seeds don't swell), or seed too old (over 3 years for most species).

Do I need a heated propagator?

For most UK vegetables sown March-April, a south-facing windowsill or greenhouse bench reaches 18-21C in daytime and is enough. For heat-lovers (tomato, pepper, chilli, aubergine, basil) and for sowing before March, a heated propagator (£40-£90) is necessary to hit the 21-27C optimum.

When can I sow direct into the ground in the UK?

Wait until soil temperature reaches 8C for hardy crops (peas, broad beans, spinach) and 12C for tender crops (French beans, sweetcorn, courgettes). In most of England this means mid-April for hardy crops and mid-May for tender crops. North of Yorkshire add 1-2 weeks.

Seed Germination Temperatures UK: Sower's Table

Soil temperature is the single biggest factor in UK seed germination success. Air temperature is a misleading proxy - compost in a propagator can sit 5-7C below daytime air temperature, especially overnight. A £4 soil thermometer reveals what your seeds are actually experiencing.

This guide gives the optimum, minimum and maximum germination temperatures for 40+ common UK garden crops, plus a practical UK sowing calendar by month and method.

For the kit and conditions that get your propagator into the right range, see our how to sow seeds indoors guide. For after-sowing techniques on tricky crops, our fluid sowing and pre-germination UK guide covers chitting and fluid drilling.

How temperature drives germination

Seeds germinate when three conditions are met: moisture, oxygen, and the right temperature range. Temperature controls the enzymatic activity inside the seed that mobilises stored starch and protein to fuel the emerging shoot.

Below the minimum, enzymes stop working and the seed stays dormant. Within the optimum range, germination is fast and uniform. Above the maximum, the seed is damaged or stalls.

A practical example - tomato seed:

Soil temperature	Germination rate	Time to emergence
10C	0% (no germination)	-
13C	15% (mostly rots)	21+ days
16C	50%	14 days
18C	80%	10 days
21C	95% (optimum)	7 days
24C	95% (optimum)	6 days
27C	90%	6 days
30C	70%	7 days
33C	30% (damage starts)	irregular

A 5C drop from optimum roughly doubles germination time. A 5C drop below minimum stops germination completely.

The reference table

Soil temperatures for the most-grown UK garden crops. Optimum is the fastest, highest-percentage germination band. Minimum is the lowest temperature at which germination still happens (slowly, partially).

Heat-loving vegetables (use heated propagator)

Crop	Minimum	Optimum	Maximum
Tomato	16C	21-27C	30C
Pepper (sweet)	18C	24-30C	33C
Pepper (chilli)	18C	24-30C	35C
Aubergine	21C	24-30C	32C
Cucumber	16C	21-29C	35C
Courgette	16C	21-29C	35C
Pumpkin/squash	16C	21-29C	35C
Sweetcorn	16C	21-26C	35C
Melon	18C	24-29C	35C
Basil	18C	21-26C	30C

UK heated propagator on a kitchen windowsill with a digital thermostat displaying 24C, containing seed trays of tomato, pepper and basil seedlings germinating uniformly Heated propagator at 24C - the sweet spot for tomato, pepper and basil. Without bottom heat, UK March sowings of these crops germinate poorly or not at all.

Warm-season vegetables (greenhouse or warm windowsill)

Crop	Minimum	Optimum	Maximum
French bean	13C	18-24C	30C
Runner bean	13C	18-21C	30C
Beetroot	10C	16-24C	30C
Carrot	7C	16-24C	29C
Parsnip	5C	13-21C	27C
Onion (seed)	7C	15-21C	30C
Leek	7C	15-21C	30C
Sweet potato	18C	21-29C	35C

Cool-season vegetables (cold frame or cool greenhouse)

Crop	Minimum	Optimum	Maximum
Lettuce	4C	13-18C	25C
Spinach	4C	7-18C	25C
Pea	4C	13-21C	24C
Broad bean	4C	13-18C	24C
Cabbage	5C	13-21C	30C
Broccoli	5C	13-21C	30C
Cauliflower	5C	13-21C	27C
Brussels sprout	5C	13-21C	30C
Kale	5C	13-21C	30C
Pak choi	4C	13-21C	27C
Rocket	4C	13-21C	25C
Radish	4C	10-21C	24C

Hardy salads (will germinate at near-freezing)

Crop	Minimum	Optimum	Maximum
Mizuna	2C	10-18C	25C
Lamb’s lettuce	2C	7-18C	25C
Winter purslane	2C	7-18C	25C
Land cress	2C	10-18C	25C

Common UK flowers

Crop	Minimum	Optimum	Maximum
Sweet pea	7C	13-21C	24C
Cosmos	16C	18-24C	30C
Marigold (French)	16C	18-24C	30C
Nasturtium	13C	18-24C	30C
Sunflower	13C	18-29C	33C
Calendula	10C	16-21C	27C
Snapdragon	10C	18-21C	24C
Foxglove	13C	18-21C	24C

Common UK herbs

Crop	Minimum	Optimum	Maximum
Basil	18C	21-26C	30C
Parsley	7C	18-24C	30C
Coriander	7C	13-21C	27C
Dill	7C	13-21C	27C
Chives	7C	13-21C	27C
Thyme	13C	18-21C	24C
Sage	13C	18-21C	27C
Oregano	13C	18-21C	27C

How to actually measure soil temperature

A soil thermometer (£4-£12) is the cheapest useful piece of propagation kit. Two types:

Probe thermometer: narrow metal probe 15-20cm long. Push 5cm into the compost, wait 2-3 minutes, read.

Digital probe: same form factor but reads faster (10-15 seconds). Useful for measuring multiple trays.

Soil thermometer pushed into damp compost of a UK seed tray showing a digital temperature display reading 19.4C next to germinating pea seedlings A digital soil thermometer 5cm into damp compost - the only reliable way to know what your seeds are experiencing. About £8 at most UK garden centres.

When to take the reading

Take readings at 9am for an overnight low, or at 3pm for the daily peak. The 9am reading matters most - cold overnight compost is the most common cause of failed germination.

Where to take the reading

Centre of the compost, 5cm deep (typical seed depth for medium seeds).
For shallow-sown seeds (lettuce, basil), measure 1cm deep.
For deeper-sown seeds (peas, beans), measure 3-5cm deep.

A propagator set at 21C on the thermostat is usually 18-19C in the compost - the heat element warms the base, the seed sits 2-3cm up. Always check the actual compost.

The propagator setting cheat sheet

If your propagator has a thermostat dial without a digital display, set it for the target compost temperature plus 3-4C to account for compost-to-element gradient:

Want compost at	Set thermostat to
18C	21-22C
21C	24-25C
24C	27-28C

The exact offset depends on the propagator design - a 38W Garland on an unheated greenhouse bench needs +6C offset on a frosty night. A Vitopod on a heated bench needs only +2C.

UK sowing calendar by temperature

When can you sow each crop in different UK settings? This depends on getting soil temperature into the right range.

Side-by-side comparison of a heated propagator, an unheated cold frame, a south-facing windowsill and a greenhouse bench in a UK garden, each with seed trays at different stages of germination Four UK propagation environments compared - heated propagator (24C), windowsill (19C), greenhouse bench (15C), cold frame (8C). Match the seed to the temperature, not the calendar.

Heated propagator (21-27C)

February-March: tomato, pepper, chilli, aubergine, basil
April: cucumber, courgette, pumpkin, sweetcorn, melon

Unheated greenhouse or warm windowsill (15-21C)

March-April: cabbage, broccoli, cauliflower, leeks, onions
April-May: French beans, runner beans, sweet peas, parsley

Cold frame (8-15C)

Late February-March: peas, broad beans, hardy lettuce
March-April: cabbage, broccoli, kale, rocket, radish

Outdoor seed bed (8C+ soil)

Mid-April onwards: parsnip, carrot, beetroot, radish
Late April onwards: peas, broad beans, lettuce
Mid-May onwards: French beans, runner beans, sweetcorn

The key check: stick the soil thermometer in the bed before sowing direct. A sheltered south-facing bed reaches 8C 2-3 weeks before an exposed north-facing one. Your soil tells you when, not the calendar.

Common temperature failures

Failure 1: Sowing too early on a windowsill. The compost in February-March windowsills often sits 4-6C below room temperature because cold air pools at the glass. Move the tray 30cm away from the window or use a propagator.

UK windowsill in February showing seed trays where the front edge tray (closest to the glass) has poor germination and patchy seedlings while the rear tray (further from glass) shows uniform healthy seedling emergence The windowsill cold-trap - February seed trays at the front edge sit 5-6C below room temperature where compost touches cold glass. Move trays 30cm back from the pane.

Failure 2: Greenhouse benches at night. A clear-night greenhouse can drop to outside air temperature within 2 hours of sunset. Compost temperature follows. Use a fleece overnight or a heated bench.

Failure 3: Direct sowing into cold soil. April soil is often still 6-8C in shaded UK gardens. Pea and broad bean seeds rot rather than germinate. Wait for soil to hit 8C minimum (check with thermometer) or use modules indoors.

Failure 4: Overheated propagator for cool-season crops. A propagator at 24C will fail to germinate lettuce reliably - the seeds go into thermo-dormancy above 25C. Use a separate unheated tray for cool-season crops or set the propagator to 16C.

Failure 5: Heat-lovers without bottom heat. Aubergine and pepper seed will sit in 18C compost for 4 weeks without germinating, then mostly rot. Bottom heat at 24C reduces this to 10-14 days with 80%+ success.

UK gardener's hand holding a sealed seed packet of dwarf French bean Tendergreen next to a small notebook with handwritten germination test results showing 8 of 10 seeds germinated at 18C compost temperature Home viability test - 10 seeds on damp paper at 18C for 7 days. Fresh seed gives 8-10 out of 10; year-three seed often drops to 4-6.

A 5-second daily check

The single highest-value habit during the UK sowing season: stick the soil thermometer into every active propagator first thing each morning. Adjust thermostats based on actual compost reading, not air temperature. Move trays in or out of the propagator as needed.

This 5-second routine adds about 30 days of useful sowing time over a UK spring compared to working from air temperature or guesswork.

Field note: The RHS seed sowing guide covers timing and technique in detail. The Vitavia and Garland propagator datasheets list compost-to-element offsets for their specific models if you need exact figures.

Seed viability and temperature

Old seed and cold compost is the worst combination. Fresh seed in optimum compost gives 90-95% germination. Three-year-old seed in cold compost gives 20-30%. Always:

Check seed date on the packet (most viable for 1-3 years).
Sow at optimum temperature for fastest, highest-percentage germination.
Discard old seed rather than waste compost and time on it.

Now you’ve got the temperature right

For matching seed quality and after-care to your propagator setup, our fluid sowing and pre-germination UK and multi-sowing vegetables UK guide cover the techniques that turn good germination into strong seedlings.