Category: Beginner Gardening

There is nothing quite like the taste of a tomato still warm from the sun, picked from a plant you grew yourself. It is one of gardening’s simplest pleasures — and growing tomatoes from seed is far easier than many beginners expect.

Yes, you could buy ready-grown plants from the garden centre in May. But starting from seed opens up a world of varieties you will never find on a nursery shelf, saves you a surprising amount of money, and gives you that deeply satisfying feeling of having nurtured something from the very beginning.

Consider the economics. A packet of tomato seeds costs around £2 and contains anywhere from 20 to 100 seeds. Even with modest germination rates, that is potentially dozens of plants. Compare that to £2-3 per plant at a garden centre, and the maths speaks for itself. Then there is the variety: seed catalogues offer over a hundred tomato cultivars — cherry, plum, beefsteak, yellow, purple, striped — while your local garden centre might stock five or six standard varieties.

You do not need a greenhouse. You do not need fancy equipment. A warm windowsill, a bit of patience, and this guide are enough to get you from seed packet to first harvest. Thousands of British gardeners do this every spring. By the end of this article, you will know exactly how to join them.

What You’ll Need

Before you start, gather your supplies. None of this is expensive, and much of it can be found around the house or bought cheaply at Wilko, B&Q, or your local garden centre.

The Essentials

• Tomato seeds: Choose varieties suited to your space (see recommendations below). Buy from reputable UK suppliers like Suttons, Thompson & Morgan, or SimplySeed.
• Seed compost: Not multi-purpose compost — seed compost is finer and free-draining. Look for Westland John Innes Seed, Jack’s Magic, or Melcourt Sylvagrow. Expect to pay £4-6 for a 20L bag.
• Containers: Proper seed trays with small modules work best, but clean yogurt pots, toilet roll tubes, or recycled plastic containers with drainage holes punched in the bottom do the job perfectly well.
• Labels and a waterproof marker: Essential — you will not remember which variety is which once they are covered in compost.
• A spray bottle: For gentle watering that does not wash seeds away.
• A warm spot: More on this below, but tomatoes need consistent warmth to germinate.

The Nice-to-Haves

• Heated propagator: Makes germination much more reliable. A basic Stewart or Garland model costs £15-25. A premium Vitopod with variable temperature is £60-80 but will last years.
• Clear plastic bag or propagator lid: To create humidity while seeds germinate. A loose freezer bag works fine.
• Grow lights: Only if your windowsill faces north or is heavily shaded. A basic LED grow light from Amazon (£15-25) makes a huge difference.

Budget vs. Deluxe Setup

When to Sow Tomato Seeds in the UK

Timing is everything with tomatoes. Sow too early and your seedlings will become leggy and stressed long before it is safe to plant them outside. Sow too late and you will be harvesting green tomatoes in October.

The Royal Horticultural Society advises sowing from late February to mid-March if you are growing in a greenhouse, or from late March to early April for outdoor crops. This is not arbitrary — it is based on light levels, temperature, and the number of weeks tomatoes need to reach fruiting stage.

The Timing Breakdown

• Greenhouse or polytunnel growing: Late February to mid-March. The protected environment means you can plant out earlier, so you can start earlier.
• Outdoor growing: Late March to early April. Tomatoes cannot go outside until all risk of frost has passed — typically late May in the south, early June further north.

The Monty Don Approach

Gardening writer Monty Don recommends sowing in two batches: one in February and another in March or even April. This staggers your harvest and acts as insurance — if an unexpected cold snap damages your first batch, you have backups ready.

Regional Variations

These dates work for most of England and Wales. If you are in Scotland, northern England, or at higher altitude, add two to three weeks. If you are in the south-east with a sheltered garden, you might get away with the earlier end of the range.

The Cold Spring Contingency

British springs are unpredictable. If February and March are bitterly cold, wait. Tomato seedlings grow fast when conditions are right. Starting them too early in a cold house leads to weak, leggy plants that never really recover. Patience pays off.

Month-by-Month Sowing Guide

Step-by-Step: Sowing Your Seeds

Now for the practical part. These steps work whether you are sowing one variety or ten, whether you have a heated propagator or just a warm windowsill.

Step 1: Prepare Your Containers

If you are reusing plastic pots or trays, wash them thoroughly in warm, soapy water. This removes old soil, salts, and any lurking fungi that could cause damping off disease. Rinse well and let them dry.

Ensure there are drainage holes in the bottom. Waterlogged compost is the fastest way to kill seeds.

Step 2: Fill With Seed Compost

Fill your containers with seed compost, tapping gently to settle it. Do not compress it heavily — seedlings need air around their roots as much as they need water. Level the surface with your hand or a flat tool.

Step 3: Sow the Seeds

For modules or small pots: place two to three seeds per module. This gives you insurance — if one fails, you have backups. You will thin to the strongest seedling later.

Cover the seeds with about 6mm of compost — roughly the thickness of a pound coin. Tomato seeds need darkness to germinate, so do not leave them exposed.

Step 4: Label Immediately

Write the variety name and the date sown on a label. Do this now, before you forget. Use a waterproof marker — pencil fades and ordinary pen runs when wet.

Step 5: Water Gently

Use a spray bottle to moisten the compost thoroughly. You want it damp like a wrung-out sponge, not soggy. Avoid pouring water from a can — it washes seeds out of place.

Step 6: Cover for Humidity

Cover the tray with a clear plastic propagator lid, or slip the whole thing into a loose freezer bag. This traps moisture and creates the humid microclimate seeds love. Do not seal it completely — a tiny bit of airflow helps prevent mould.

Step 7: Place in a Warm Spot

Tomato seeds need a temperature of 18-21°C to germinate reliably. This is the critical factor — more important than light at this stage.

Options include:

• A heated propagator set to 20°C
• An airing cupboard (check daily — move to light immediately when seedlings appear)
• A warm windowsill above a radiator
• On top of a fridge or freezer (slightly warmer than room temperature)

Avoid anywhere that gets cold at night — a draughty windowsill can drop below 10°C after dark, which will delay or prevent germination.

Germination: What to Expect

Given the right temperature, tomato seeds are usually reliable germinators. Here is what to expect.

The Timeline

At 18-21°C, expect seedlings to emerge in 7 to 14 days. In optimal conditions — around 24°C with a heated propagator — this can be as fast as 5 to 7 days. At the lower end, around 15°C, germination can stretch to 3 or even 4 weeks.

The first sign is usually a tiny green loop pushing through the compost surface. This is the stem bending to pull the seed leaves (cotyledons) up and out. Within a day or two, these leaves will unfold and turn green.

Immediate Action Required

As soon as the first seedling appears — immediately — remove the plastic cover and move the tray to a bright spot. Seedlings that germinate in darkness become leggy and weak within hours.

Light is now more important than warmth. A south-facing windowsill is ideal. East or west-facing works too, though growth will be slightly slower. If you only have a north-facing window, invest in a basic LED grow light or the seedlings will stretch desperately toward the light and become spindly.

Troubleshooting: Seeds Not Germinating?

If nothing has appeared after three weeks, check:

• Temperature: Is the spot consistently warm? A thermometer helps — do not guess.
• Moisture: Has the compost dried out? Seeds need consistent dampness.
• Seed age: Tomato seeds keep for 3-5 years if stored cool and dry. Older than that, germination drops off.
• Depth: Did you bury them too deep? 6mm is plenty.

Caring for Seedlings

Once your seedlings are up, they need consistent care for the next four to six weeks until they are ready for their next move.

Light

Tomato seedlings need as much light as possible. A south-facing windowsill is perfect. Turn the tray a quarter turn each day to prevent the seedlings leaning toward the light.

If seedlings start stretching — growing tall and thin with gaps between leaves — they are not getting enough light. Move them to a brighter spot immediately, or add a grow light positioned 10-15cm above the foliage.

Water

Keep the compost consistently moist but never waterlogged. The best method is bottom watering: sit the tray in a shallow dish of water for 10-15 minutes, letting the compost draw moisture up from below. This prevents disturbing delicate seedlings and reduces fungal problems on the soil surface.

Temperature

After germination, seedlings are happy at 15-20°C. They will tolerate slightly cooler nights, but avoid anything below 10°C. Keep them away from cold draughts — that gap under the window, the hallway, or near an external door.

Preventing Leggy Seedlings

Leggy seedlings are the most common beginner problem. They stretch toward light, becoming tall, thin, and weak. The stem cannot support the weight and may collapse.

Prevention: More light is the only real cure. A slightly cooler night temperature (15°C instead of 20°C) also helps keep growth compact.

The good news: Tomatoes can recover from mild legginess. When you pot them on, bury the stem up to the first leaves — tomatoes are unusual in that they will sprout roots from the buried stem, creating a stronger plant.

Potting On: When and How

Your seedlings will outgrow their first home within three to four weeks. Knowing when and how to move them on is crucial.

When to Pot On

Look for one of these signs:

• The first true leaves have appeared (these look like miniature tomato leaves, different from the smooth, oval seed leaves)
• Roots are visible through the drainage holes
• The compost dries out within a day of watering

What You’ll Need

• 9cm pots (one per seedling you want to keep)
• Multi-purpose compost (not seed compost — they need more nutrients now)
• A dibber or pencil for making holes

The Technique

Fill your 9cm pots with multi-purpose compost, firming gently. Make a hole in the centre deep enough to accommodate the seedling’s root ball.

Water the seedling tray first — this helps the roots release from the compost. Then, handle the seedling by its leaves, never the stem. The stem is fragile and easily bruised. If you damage a leaf, the plant grows another. If you damage the stem, the plant dies.

Lift the seedling and lower it into the new pot. Here is the key trick: bury the stem as deep as the plant allows, right up to the lowest leaves. Tomatoes have the remarkable ability to sprout roots from their stems. A deeply planted seedling develops a strong, extensive root system.

Firm the compost gently around the seedling, water well, and return it to a warm, bright spot.

Thinning: Choosing Which to Keep

If you sowed multiple seeds per module, you now need to thin them. Choose the strongest seedling — the one with the thickest stem and most leaves — and snip the others at soil level with scissors. Do not pull them out; you risk disturbing the roots of the one you want to keep.

Hardening Off

Plants raised indoors are soft. The sudden shock of wind, direct sun, and temperature swings will damage or kill them. Hardening off is the gradual process of acclimatising them to outdoor conditions. Once hardened off, you can plant them out or grow them on in containers — see our guide to how to grow tomatoes in pots.

Start this process about seven to ten days before you intend to plant out.

The progression:

1. Day 1: Place seedlings outside in a sheltered, shaded spot for one hour.
2. Day 2: Two hours, slight breeze okay.
3. Day 3-4: Half a day, morning sun tolerated.
4. Day 5-6: Full day, increasing sun exposure.
5. Day 7+: Leave out overnight if temperatures stay above 10°C.

Watch the weather — bring plants in if strong winds, heavy rain, or frost is forecast. A cold frame is excellent for hardening off; simply prop the lid open during the day and close it at night.

Planting Out

The moment of truth: moving your plants to their final growing positions.

When to Plant

Wait until:

• All risk of frost has passed (late May in the south, early June in the north)
• Night temperatures are reliably above 10°C
• Plants are hardened off

Where to Plant

Tomatoes need:

• Sun: At least 6-8 hours of direct sunlight daily. More is better.
• Shelter: Protection from strong winds which damage stems and flowers.
• Warmth: Against a south-facing wall is ideal — the wall stores and radiates heat.

Spacing and Support

Space plants 45-60cm apart. Cordon varieties need sturdy supports — 2m canes driven firmly into the ground, or strong strings hung from an overhead frame. Bush varieties need less support but may need propping up when laden with fruit.

Growing Options

• In the ground: Dig in plenty of garden compost. Plant deeply, firming soil well.
• Grow bags: Two plants per standard bag. Shake the bag to loosen compost before planting.
• Large pots: Minimum 30cm diameter, 40cm better. Use good quality peat-free compost.
• Greenhouse borders: The classic approach. Dig in well-rotted manure in winter.

For detailed advice on growing vegetables in containers, see our dedicated guide.

Feeding Your Tomato Plants

Tomatoes are hungry plants. While seedlings get all they need from seed compost and then multi-purpose compost, mature plants producing fruit need regular feeding to perform their best.

When to Start Feeding

Begin feeding when you see the first flowers opening on your plants — or immediately after planting out, whichever comes first. Feeding too early encourages leafy growth at the expense of fruit. Feeding too late means smaller crops.

What to Use

Use a liquid tomato fertiliser specifically formulated for fruiting plants. These are high in potassium (the K in N-P-K ratios) which promotes flower and fruit production. Good options available in the UK include:

• Tomorite — the classic choice, widely available at B&Q, The Range, garden centres
• Levington Tomorite — concentrated formula, economical for regular use
• Organic options: Liquid seaweed feed (Maxicrop), comfrey tea, or homemade nettle feed

Avoid general-purpose plant foods — they are too high in nitrogen and will produce leafy plants with few tomatoes.

How Often to Feed

Once a week throughout the fruiting season, from first flowers until the plants finish cropping. In grow bags, which hold limited nutrients, you might feed twice weekly during peak harvest (July-August).

How to Apply

Follow the dilution instructions on the bottle — usually around one capful per watering can. Apply to the compost around the roots, not over the foliage. Water the plants first if the compost is dry, then apply the feed. This prevents root damage from concentrated fertiliser on dry soil.

A Warning on Overfeeding

More is not better. Overfed plants produce dark green leaves but fewer flowers. Stick to the weekly schedule and the manufacturer’s recommended dilution. If leaves become very dark and curled, cut back on feeding.

Best Tomato Varieties for Beginners

With hundreds of varieties available, choosing can feel overwhelming. These five are proven winners for UK beginners — reliable, productive, and widely available.

Cordon vs Bush: Which Should You Choose?

Cordon (indeterminate) varieties grow tall and need regular support. They produce side shoots that must be pinched out, and they fruit over a long period. They take more work but give higher yields over the season.

Bush (determinate) varieties stay compact, need little or no staking, and produce a concentrated crop. They are lower maintenance but finish fruiting earlier.

For your first year, consider one of each: a cordon like Gardener’s Delight for the greenhouse or a sunny border, and a bush like Tumbling Tom for a patio container or hanging basket.

Common Problems and Solutions

Even experienced gardeners lose seedlings occasionally. Here is how to recognise and fix the most common issues.

Damping Off (Seedlings Collapse)

Symptoms: Seedlings fall over at the soil line, often with a pinched, discoloured stem. Happens suddenly, usually within days of germination.

Cause: Fungal disease favoured by cold, wet conditions and poor airflow.

Prevention: Use clean containers and fresh compost. Do not overwater. Remove covers as soon as seedlings appear. Ensure good air circulation.

Treatment: None. Affected seedlings die. Start again with fresh compost and better hygiene.

Leggy Seedlings

Symptoms: Tall, thin stems with large gaps between leaves. Seedlings lean toward the light source.

Cause: Insufficient light. Over-warm nights can also contribute.

Fix: Move to a much brighter spot immediately. When potting on, bury the stem deeply — tomatoes will root from the buried portion. Consider a grow light for future sowings.

Poor Germination

Possible causes:

• Temperature too low (below 15°C)
• Compost dried out during germination
• Seeds too old (viability drops after 3-5 years)
• Seeds planted too deep

Fix: Check your propagator temperature with a thermometer. Keep compost consistently damp. Buy fresh seed if yours is old.

Yellowing Leaves

On seedlings: Usually overwatering or cold roots. Let the compost dry slightly between waterings and ensure temperatures stay above 10°C.

On mature plants: Lower leaves yellowing is normal as plants age. Widespread yellowing suggests nutrient deficiency — start feeding with tomato fertiliser when the first fruits set.

Pests

The most common greenhouse pests are whitefly and aphids. On outdoor plants, slugs can be problematic when plants are young. For comprehensive pest and disease advice, see our guide to common tomato problems in the UK.

Month-by-Month Tomato Calendar

Frequently Asked Questions

Can I use seeds from a supermarket tomato?

Technically yes, but it is not recommended. Most supermarket tomatoes are F1 hybrids, which means their seeds will not produce identical plants — you might get something quite different and usually inferior. There is also a small disease risk. Proper seed packets cost £2-3 and guarantee variety, vigour, and disease-free stock.

How many seeds should I sow if I want three plants?

Sow at least six to nine seeds. Not every seed germinates, and not every seedling thrives. Sow two to three seeds per module, then select the strongest seedling from each when potting on. This gives you insurance against failures.

Do I need a greenhouse to grow tomatoes from seed?

Absolutely not. A sunny windowsill works perfectly for starting seeds. While a greenhouse helps tomatoes crop earlier and more heavily, many excellent varieties grow perfectly well outdoors in a sunny, sheltered spot. Start them on your kitchen windowsill, harden them off, then plant outside in late May.

Can I sow tomato seeds directly outside?

In the UK, direct sowing is not recommended. Our growing season is too short and unpredictable. Seeds sown direct in May might not produce ripe fruit before autumn frosts. Starting indoors gives you a crucial six to eight week head start.

Why are my seedlings tall and thin?

They are not getting enough light. Tomato seedlings stretch toward light sources when illumination is insufficient. Move them to a brighter windowsill immediately — ideally south-facing. If that is not possible, invest in a basic LED grow light positioned 10-15cm above the foliage.

When should I start feeding my tomato plants?

Not until the first flowers appear, or when you plant them out into their final positions — whichever comes first. Seedlings have all the nutrients they need in the seed itself and then in fresh compost. Early feeding can actually harm young plants.

How do I know if my seeds are too old?

Tomato seeds remain viable for three to five years if stored in a cool, dry place. If you are unsure, do a germination test: place ten seeds between damp kitchen paper in a warm spot. If fewer than seven sprout, the seed is past its best.

What is the lowest temperature tomato seedlings can tolerate?

Tomato seedlings will survive brief drops to 5-10°C, but growth stops and they become stressed. Prolonged cold leads to stunted growth, purplish colouring, and vulnerability to disease. Aim to keep them consistently above 15°C for best results.

Conclusion

Growing tomatoes from seed is one of gardening’s most satisfying achievements. From a tiny speck of seed no larger than a grain of sand, you can nurture a plant that produces dozens of sun-warmed fruits over months of summer.

It is not complicated. Sow at the right time — late February to April depending on your setup. Keep seeds warm at 18-21°C until they germinate. Give seedlings plenty of light. Pot them on when they outgrow their first home. Harden them off gradually. Plant out after frost.

Even if you lose a few seedlings to damping off, or your first attempt produces oddly leggy plants, you are learning. Gardening is a craft built on observation and experience. Every mistake teaches you something for next year.

The key is to start. Check the calendar — if it is February, March, or early April, you are in the sowing window. Gather your supplies, choose a variety that appeals to you, and sow a few seeds. In a matter of weeks, you will have healthy green seedlings on your windowsill, and by midsummer you will be eating tomatoes that taste like nothing you can buy in a shop.

Ready to plan your growing season? Check our guide to what to plant this month for timely reminders throughout the year.

This article was written for UK gardeners based on Royal Horticultural Society guidance, BBC Gardeners’ World recommendations, and practical experience. All varieties and products mentioned are available in the UK. You might also like our guide to choosing the best compost for containers.