Complete Tomato Sowing Guide

Sowing tomatoes correctly is the foundation of a healthy, productive crop. Get the timing, compost, light, and early care right, and tomatoes are one of the easiest plants to grow from seed. This complete guide walks you through every step of tomato sowing, from choosing seeds to strong seedlings ready for planting out.

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• Tomato Seed Collection (Garden & Greenhouse Varieties)
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• Tomato Plants (Ready-Grown)
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• Tomato Feed & Fertiliser
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• Tomato Grow Bags / Containers
Ideal for patios, balconies, or greenhouse growing — provides good drainage and space for roots to develop big, productive plants.
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When to Sow Tomatoes (UK Timing)

Timing is critical to avoid weak, leggy plants.

Best sowing window for the UK:

• Mid-March to early April – ideal for most gardeners
• Late February – only if you have strong grow lights and warmth
• After April – still fine, but harvest may be slightly later

Sowing too early without enough light is the most common mistake.

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Choosing Tomato Seeds

Choose varieties that suit how and where you’ll grow them.

Good choices for beginners:

• Cherry tomatoes
• Bush (determinate) tomatoes
• Patio or dwarf varieties

Consider before sowing:

• Outdoor vs greenhouse varieties
• Space available
• Length of growing season

Open-pollinated and heritage varieties are also ideal if you plan to save seed later.

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Compost for Sowing Tomatoes

Use the right compost for strong root development.

Best compost:

• Fine seed compost
• Peat-free if possible
• Free-draining but moisture-retentive

Avoid rich composts at this stage — tomatoes don’t need high nutrients when germinating.

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Containers for Sowing Tomato Seeds

You can sow tomatoes in:

• Seed trays
• Small pots
• Modular trays

All containers must have drainage holes.

Shallow trays are fine initially, as roots develop later.

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How to Sow Tomato Seeds (Step by Step)

1. Fill containers with seed compost
2. Lightly firm the surface
3. Water the compost before sowing
4. Sow seeds thinly on the surface
5. Cover with 0.5–1cm of compost
6. Label clearly with variety and date
7. Place somewhere warm

Tomato seeds do not need light to germinate.

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Temperature for Germination

Tomatoes germinate best with warmth.

Ideal temperature:

• 18–22°C

Below this, germination slows or fails. A warm windowsill or heated propagator works well.

Seeds usually germinate in 7–14 days.

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Light After Germination

As soon as seedlings emerge, light becomes critical.

Light requirements:

• Bright windowsill or grow lights
• 12–14 hours of strong light daily
• Rotate trays to prevent leaning

Poor light causes tall, weak seedlings.

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Watering Tomato Seedlings

Watering mistakes cause most early problems.

Best practice:

• Keep compost moist, not wet
• Water from below where possible
• Never let seedlings sit in water

Overwatering leads to weak growth and disease.

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Pricking Out Tomato Seedlings

When seedlings develop true leaves (not the first seed leaves), they’re ready to move.

How to prick out:

• Lift gently using a label or spoon
• Handle by the leaves, not the stem
• Pot into individual containers
• Plant deeper than before

Tomatoes grow roots along buried stems, making plants stronger.

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Potting On Tomatoes

As roots fill pots:

• Move plants into larger containers
• Use multi-purpose compost
• Plant deeper each time

Tomatoes tolerate multiple potting stages very well.

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Feeding Young Tomato Plants

Do not feed seedlings early.

• Seed compost contains enough nutrients
• Feeding too soon causes weak, leafy growth

Begin feeding only once plants are established and flowering later on.

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Preventing Leggy Tomato Seedlings

Leggy seedlings are caused by:

• Too little light
• Too much warmth
• Sowing too early

Fixes:

• Improve light immediately
• Replant seedlings deeper
• Reduce unnecessary heat

Leggy tomatoes can almost always be rescued.

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Hardening Off (Before Planting Out)

Before moving tomatoes outside:

• Harden off for 7–10 days
• Gradually increase outdoor exposure
• Protect from cold nights and wind

Never rush this stage.

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When Tomato Seedlings Are Ready to Plant Out

Tomatoes are ready when:

• Frost risk has passed
• Night temperatures stay above 10–12°C
• Plants are sturdy and well-rooted

For most of the UK, this is late May.

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Common Tomato Sowing Mistakes

• Sowing too early
• Poor light after germination
• Overwatering
• Using overly rich compost
• Not potting on soon enough

Avoiding these guarantees far better results.

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Tomato Sowing Timeline Summary

• Sow: Mid-March to early April
• Germinate: 7–14 days
• Prick out: 2–3 weeks after sowing
• Pot on: As roots fill pots
• Plant out: Late May (UK)

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Final Thoughts

Tomato sowing is simple once you understand the basics: right timing, warmth for germination, strong light after emergence, and careful watering. Tomatoes are forgiving plants, and even mistakes can often be corrected by replanting deeper and improving conditions.

Start well, and you’ll set your tomatoes up for strong growth, fewer problems, and excellent harvests all season long.