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How To Sow Tomato Seeds Indoors

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Tomatoes take a long time to grow, particularly in the UK. For this reason, it is often a good idea to start them early in the season. Because of the weather in late winter/early spring, this needs to be done indoors. Learn how I sow my tomato seeds indoors below.

When To Start

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…

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. You can start them properly – using all the equipment listed below, or use a windowsill.

A little warning about using windowsills.

Windowsills often don’t provide enough light in early spring, as there isn’t enough daylight in the day for tomatoes to grow properly. This often leads to your tomatoes growing long and ‘leggy’. A grow light is always the best way. They are not expensive to buy or run, so I strongly advise you to get one – you can learn more here.

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

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

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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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
  • Dimensions: 10"x 20.75"(25*52cm); Watt: 17.5W; Voltage: 240V; Power Cord's Length: 180cm.
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02/21/2024 12:09 am GMT

How To Start Using A Propagator

I like to use coir compost to start my indoor seeds. (the stuff with added fertiliser though!) There are two main reasons for this: first, I don’t have to store a bag of compost in the house, and second, with it being sterile, I don’t need to worry about fungus gnats!

I love these new propagators I got this year; they have a flexible silicone bottom, like an ice cube tray. When you need to transplant your seedlings, you push this up, and the whole plug comes out very easily without harming the roots.

I Love These!
6 Pack Propagators with 12 reusable pop-out cells
£12.99 (£2.16 / count)
  • Durable and Reusable - our seed trays for seedlings reusable built to last.
  • Hhen you need to transplant, just gently pop out, the whole seedling and root system can be transplanted out completely without affecting the root system.
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02/21/2024 12:45 am GMT
Great For Indoor Plants
Coco & Coir All Purpose Compost
£15.99 (£3.20 / kg)
We earn a commission from any items purchased through this link at no charge to yourself. This helps fund what we do here!
02/21/2024 12:09 am GMT
The Seed Trays
The Seed Trays
Use A Propagator
Filled with moist compost

I will then pop a single seed into each module. I like to leave the seeds sitting of the surface for now, just so I know I have one seed in each module.

One Seed Per Module

Once I have sown all of my seeds, I will go round and gently press them into the soil. The aim is to get them just below the soil surface; you don’t want to push them deep.

Push them into the soil

Once that is done, all you need to do is pop the propagator lid on and wait for them to germinate.

Pop the propagator lid on
Pop the propagator lid on

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 if possible. If not then static lights work almost as well.


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.

How To Start Using The Double Cup Method

The double cup method is a way of sowing tomato seeds using two drinking cups, as you can see in the photo above. This is a pepper plant but the method remains the same.

You can learn more about this here.

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 p

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