Growing tomatoes from seed in the UK: how to use a propagator
Quick answer
For growing tomatoes from seed in the UK, sow 5mm deep in a propagator held at 20–25°C between late February and early April. Seeds usually germinate in 5–10 days. Open the vents once shoots appear, move seedlings into bright light at 16–20°C, prick out at the first true leaf, and plant out after the last frost.
Most gardeners who try growing tomatoes from seed in the UK hit one of two problems. Either the seeds sit in cold compost for a fortnight and nothing happens, or they come up fast and then flop over as pale, spindly stems that never really recover. Both come down to the same thing: getting temperature and light in step with each other.
A propagator solves the first half of that problem by giving seeds steady warmth and humidity while they germinate. The second half is knowing when to take that warmth away. This guide covers both, with the temperatures, sowing dates and timings that work under British conditions. If you're still choosing a set-up, Vitako’s propagator range covers everything from simple lidded trays to heated units.
In this guide
Why tomato seeds do better in a propagator
Tomatoes are a warm-climate crop. Their seeds germinate best with the compost at 20–25°C, and below about 15°C the process slows to a crawl. A seed that takes a week at 22°C can take three weeks or more on a cold sill, and seeds sitting in wet, cool compost for that long are prone to rotting before they ever emerge.
A propagator is a seed tray with a clear lid. The lid traps moisture, so the compost surface doesn't dry out and crust over, and it holds warmth around the seeds. A heated propagator adds a low-wattage element in the base that warms the compost from below, which is where the seed actually feels the temperature. Air temperature matters far less at this stage than compost temperature.
The catch is that the conditions that make seeds germinate quickly are the same ones that make seedlings stretch. Once shoots appear, the job changes from warming to growing on in strong light, and the propagator lid has to come off.
Heated vs unheated propagators compared
There are four realistic ways to germinate tomato seed at home. The difference that matters most is whether you control the compost temperature or just hope the room is warm enough.
| Set-up | How it works | Temperature control | Running cost | Best for |
|---|---|---|---|---|
| Windowsill, no cover | Pots or trays on an indoor sill | None; follows the room | Nil | Sowings from late March onwards in a warm house |
| Unheated propagator | Seed tray with a vented clear lid | Holds humidity and a little warmth, adds no heat | Nil | Late March and April sowings; keeping compost evenly moist |
| Heated propagator | Lidded tray with a heating element in the base | Warms compost above room temperature; basic models have no thermostat | Low | February and early March sowings in a cool room or greenhouse |
| Heat mat under trays | Flat heated mat that sits under your own trays or a propagator | Adjustable only when paired with a thermostat | Low | Larger sowings, or adding heat to trays you already own |
Basic heated propagators raise the compost temperature by a fixed amount rather than holding it at a set point. In a cold greenhouse in February that is exactly what you want. On a sunny windowsill in April it can push the compost well past 30°C, where tomato germination starts to fall away, so check the stated temperature rise on the product before relying on it in a warm spot.
Starting in February?
Early sowings are where a heated base earns its place. Vitako stocks heated and unheated propagators in a range of tray sizes, so you can match the unit to how many plants you want.
Browse propagators →Which set-up suits you, and when to skip it
- Heated greenhouse growers: a heated propagator lets you sow in late February and have sturdy plants ready for the greenhouse border by April.
- Unheated greenhouse growers: sow in March with bottom heat indoors, then move plants out once night temperatures inside the greenhouse stay above 10°C.
- Growers raising lots of plants: a propagation heat mat under several trays is often more flexible than multiple small propagators, and pairing it with a thermostat stops the compost overheating.
- Outdoor and patio growers: your plants can't go out until after the last frost, so there is little to gain from sowing before late March.
Here is the honest trade-off. A heated propagator is the wrong buy if you don't have somewhere bright to put the seedlings once they come up. Heat gets tomatoes to germinate in February, but February daylight is short and weak, and seedlings raised early on a dim sill often end up weaker than ones sown in April. If your brightest spot is a north-facing window, or you only want a few plants for outdoor pots, an unheated lid and a later sowing will give you better plants for less money.
Temperatures, depths and timings
These are the figures to work to from sowing through to planting out. Timings assume the compost is held at the germination temperature; allow longer if it runs cooler.
| Stage | Target | Typical timing | What to watch |
|---|---|---|---|
| Sowing depth | About 5mm, covered with fine compost or vermiculite | — | Too deep delays emergence |
| Germination | Compost at 20–25°C, lid on | 5–10 days | Open vents as soon as the first shoots appear |
| Growing on | 16–20°C by day, not below 10°C at night, maximum light | From emergence | Stretching means too warm or too dark |
| Pricking out | First true leaf showing | Around 2–3 weeks after sowing | Handle by the leaf, never the stem |
| Potting on | Individual 9cm pots, stem buried to the seed leaves | Straight after pricking out | Keep compost moist, not wet |
| Planting out | 15–20cm tall, first flower truss forming | 6–8 weeks after sowing | After the last frost, typically mid-May in most of the UK |
For final containers, give each plant at least 10 litres of compost, ideally 15–20 litres. Smaller pots dry out quickly in summer, and irregular watering is one of the main causes of split fruit and blossom end rot. Space cordon plants in the greenhouse border or garden about 45–60cm apart.
How many seeds to sow depends on how many plants you want, but sow roughly half as many again as you need. Not every seed will germinate, and it lets you keep the strongest seedlings and compost the rest.
How to grow tomatoes from seed in a propagator, step by step
- Fill the tray. Use a fine, free-draining seed compost rather than multipurpose, which is often too lumpy and rich for germinating seed. Firm it lightly and water from below until the surface darkens, then let it drain.
- Sow thinly. Space seeds about 2cm apart in a tray, or sow two per module or small pot. Cover with about 5mm of sieved compost or vermiculite and label each variety.
- Warm and cover. Put the lid on, open the vents only slightly, and switch on the heat if you’re using it. Keep the tray out of direct midday sun, which can overheat a closed lid.
- Check daily. Wipe condensation off the lid if it drips, and mist or bottom-water only if the surface starts to pale and dry.
- Vent at emergence. As soon as shoots appear, open the vents fully. Over the next two or three days, take the lid off and move the seedlings off the heat into the brightest position you have, turning the tray daily so stems grow straight.
- Prick out and pot on. When the first true leaf appears, lift each seedling by a seed leaf and plant it into its own 9cm pot, burying the stem up to the seed leaves. Tomatoes root along buried stem, so this also corrects any stretching.
- Grow on. Keep plants at 16–20°C by day and above 10°C at night. If they’re moving into a greenhouse in April, a greenhouse heater with a thermostat protects them through cold nights.
- Harden off and plant out. For outdoor crops, stand plants outside by day for 7–10 days before planting, bringing them in at night. Plant into beds, grow bags or large planters after the last frost, and keep horticultural fleece to hand for any late cold snap.
Don't start feeding until the first flower truss has set fruit. Seed and potting compost carry enough nutrients for young plants, and early feeding mainly produces soft, leafy growth. Once fruit sets, switch to a high-potash tomato feed and use it at the rate given on the label.
Getting the greenhouse ready for this year’s crop?
Propagation, heating, staging and ventilation, all in one place.
Shop greenhouse equipment →Frequently asked questions
When should I sow tomato seeds in the UK?
Sow tomatoes six to eight weeks before you can plant them out. For a heated greenhouse that means late February to early March; for an unheated greenhouse, mid to late March; and for plants going outdoors, late March to mid-April, so they are ready once the last frost has passed in mid-May.
What temperature do tomato seeds need to germinate?
Tomato seeds germinate best with the compost at 20–25°C, usually sprouting within 5–10 days. Below about 15°C germination slows sharply and seeds may rot before they emerge, which is why a heated propagator or heat mat helps with early sowings. Once seedlings are up, drop the temperature to 16–20°C.
Why are my tomato seedlings leggy?
Leggy tomato seedlings are almost always a sign of too much warmth and too little light. Warmth drives fast stem growth, and weak light makes the plant stretch towards it. Move seedlings off the heat once they emerge, give them the brightest spot you have, and bury the stretched stem when you pot on.
Should I leave the propagator lid on after germination?
No. Open the vents as soon as the first seedlings break the surface, then remove the lid entirely over the next few days. Keeping the lid closed traps humidity around the young stems, which encourages mould and damping off, and the extra warmth makes seedlings stretch rather than grow sturdy.
Can I grow tomatoes from seed without a heated propagator?
Yes, if you sow a little later. From late March onwards a warm, bright windowsill indoors is usually warm enough, especially with an unheated propagator lid to hold in moisture. Heat mainly matters for February and early March sowings, when UK windowsills can drop well below germination temperature overnight.
Tomatoes reward a bit of care at the start: steady warmth to get them up, strong light once they’re growing, and patience before planting out. Get those three right and the rest of the season is far more forgiving. Whether you’re sowing a single tray on the windowsill or filling a greenhouse, Vitako has the propagators, heat mats and growing essentials to give your seedlings the best start.