Start Tomatoes Indoors

When to Start Tomatoes Indoors in the UK

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Knowing the appropriate time to sow your tomato seeds is crucial for a successful harvest. By starting your seeds indoors, you can extend the growing season and give your plants a head start before transplanting them outdoors.

In the UK, it is advisable to sow tomato seeds indoors approximately six weeks before your estimated last frost date. This approach allows your tomato seedlings to develop strong root systems and healthy foliage prior to being moved outdoors. Remember that tomatoes are warmth-loving plants, so ensuring a frost-free environment when transplanting them outside is vital for their survival and growth.

For me, in northern England, up in the Pennines, this is early May, but the date can vary by quite a few weeks, depending on where you are exactly. This means sowing time for me should be mid-March – I often tend to get excited and start a bit earlier than this, though…

Seed Starting Dates

We typically sow tomatoes indoors in February or March, but April isn’t too late. This early start allows your plants enough time to grow strong and healthy before transplanting them outdoors.

It all comes down to how much space you have to grow your tomatoes. They will grow quickly, particularly if provided with artificial light.

The question is, do you have enough space to accommodate larger plants? If you do, then sow as early as possible; if not, then wait until 4-6 weeks before your last frost date.

What Do You Need?

Sowing tomatoes indoors is more complicated than just growing them outdoors or sowing in a greenhouse. Here is the equipment you might need:

Seed Trays and Containers

I like to use a propagator to get my Tomatoes started, but in truth, it isn’t needed when sowing indoors; any pot, tray or even milk bottle will work!

Pop The Propagator Lid On And Grow
The propagator I often use

If your seedlings are “leggy” (long and weak) then transplanting them deeply and giving them more light can help.

Grow Lights

In the UK, the sunlight during early spring may not be sufficient for growing tomatoes indoors. Artificial grow lights can provide the necessary light to encourage strong growth:

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03/01/2024 02:58 pm GMT

Heating Mats

Tomato seeds need warmth to germinate, and during the early spring months, even a windowsill might not be warm enough. Heating mats can help maintain an optimal temperature for seed germination:

  • Waterproof heating mats: These are designed to distribute heat evenly, promoting consistent germination rates for your seeds. Place your seed trays on the mat during the germination process.
  • Thermostat controls (optional): Some heating mats come with thermostats that allow you to control the temperature, ensuring optimal conditions for your tomato seeds.
The Mats I Use!
Hyindoor Heated Seed Mat
£15.99 £13.59
  • Dimensions: 10"x 20.75"(25*52cm); Watt: 17.5W; Voltage: 240V; Power Cord's Length: 180cm.
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03/01/2024 02:27 pm GMT

Steps to Start Tomatoes Indoors

Sowing Seeds

Fill a container with well-moistened, sterile seed-starting mix and create shallow furrows about 1/4 inch deep.

Place the seeds in the furrows, cover them lightly with the mix, and gently pat down the surface to ensure good seed-to-soil contact. For optimal germination, maintain a consistent temperature of around 18-20°C.

Providing Proper Lighting

Tomato seedlings require sufficient light in order to grow strong and healthy. In the UK, natural winter light levels may be too low, so consider using LED grow lights to supplement the light needed for the seedlings’ growth.

Place the LED lights about 6-12 inches above the seedlings and adjust them as the plants grow.


Water the seedlings gently, ensuring that the soil stays moist but not waterlogged. Avoid overwatering, as this can lead to root rot and other problems.

A good method to water tomato seedlings is by using a spray bottle, as it allows you to apply water evenly and in the right quantity without displacing the seeds.

Transplanting Seedlings

About a month after planting the seeds, when the seedlings have developed two to three sets of true leaves, it’s time to transplant them into individual containers or pots.

diagram of tomato leaves
True Leaves

Gently separate and remove the seedlings from the original container, taking care not to damage their delicate roots. Plant the seedlings in pots filled with a high-quality potting mix and continue to provide ample light, warmth, and water as needed.

Hardening Off Tomato Seedlings

Before transplanting your tomato seedlings outdoors, it is important to help them adjust gradually to their new environment. This process, known as hardening off, involves slowly exposing your seedlings to outdoor conditions, particularly colder temperatures and direct sunlight.

To begin hardening off your tomato plants, start about 2-4 weeks before the last frost in your area. Initially, take your plants outside for 1-2 hours per day in a spot that is sheltered from direct sunlight and wind. After a few days, increase their “outside time” up to 3-4 hours.

As your plants become more accustomed to the outdoor environment, gradually increase the time they spend outside and expose them to sunlight and windier conditions. This process should last two weeks, after which your tomato seedlings should be ready for transplanting into their final position.

Remember to continuously monitor the health of your tomato seedlings throughout the hardening-off process. If you notice any signs of stress or damage, adjust the conditions to ensure a successful outdoor transition.

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  1. I have only bought plants so far but will try seed next year. I’m assuming that 1 main leader is a good idea and that growing from seed makes it easier to prevent the plant putting out shoots in all directions?

    1. One leader is what I normally do. You will be able to get them off to a good start growing from seed but you need to be really active with your pruning to keep the shoots tidy. Tomatoes just love to grow bushy!

  2. Already started some in a conservatory, They are growing quite strong and need a watchful eye on them. I have Fed them now and again as I wanted to keep them strong. I probably have started to early but are anxious to get going as I started too late last year. Last year was a really bad year, for most of my veg. Hopefully I have read up on many ways people say how to do, and will try a lot more this Year.

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