By Ann, gardener and garden designer at Ann's Garden Heaven
Growing from seed is the cheapest, most satisfying way to fill your garden. A packet of tomato seeds costs about the same as a single plant from a garden centre – and gives you twenty. Starting seeds indoors also gives you a head start of several weeks while it's still too cold outside.
If you've never done it before, don't worry. Here's my simple, step-by-step method.
When to sow
- January–February: chillies, sweet peppers, aubergines and sweet peas – they need a long season.
- March–April: tomatoes, lettuce, herbs like basil and parsley, cosmos, marigolds and most bedding flowers.
- April–May: courgettes, squash, cucumbers, sweetcorn and sunflowers – these grow fast, so don't start them too early.
Always check the back of the seed packet – it tells you the best sowing time and depth.
What you'll need
- Seed trays or modules – modules (trays divided into cells) make transplanting easier.
- Seed compost – finer and lower in nutrients than multi-purpose compost, which is perfect for delicate roots.
- A propagator or a clear lid/plastic bag to keep moisture in.
- Plant labels and a pencil – trust me, you won't remember which tray is which.
- A watering can with a fine rose or a hand sprayer.
- A bright windowsill, ideally south- or west-facing.
Step-by-step: sowing seeds indoors
Step 1: Fill your tray
Fill trays or modules with seed compost, break up any lumps, then tap the tray on the table to settle it. Level it off about 1cm below the rim. Don't press it down hard – seedlings need air in the compost.
Step 2: Water before you sow
Stand the tray in a few centimetres of water for 10–15 minutes until the surface darkens, then let it drain. Watering first stops tiny seeds being washed away.
Step 3: Sow thinly
Large seeds (beans, courgettes, sunflowers): push one or two into each module, about 2cm deep. Medium seeds (tomatoes, peppers): sow 2–3 per module and cover lightly. Tiny seeds (lettuce, petunias): sprinkle sparingly over the surface. A good rule: cover seeds with compost to about twice their own depth.
Step 4: Cover and label
Write the plant name and date on a label. Cover with a propagator lid or slip the tray into a clear plastic bag to hold in moisture.
Step 5: Keep them warm
Most seeds germinate best at 18–21°C. A heated propagator is ideal for chillies and tomatoes; a warm windowsill or airing cupboard works for many others (but move them into the light as soon as shoots appear).
Step 6: Give them light – lots of it
The moment you see green shoots, move them to the brightest spot you have and remove the lid for a few hours each day. Turn the tray every day so seedlings don't lean towards the window. Pale, tall, floppy seedlings ("leggy") are a sign they need more light.
Step 7: Water little and often
Keep the compost just moist, never soggy. Water from below by standing trays in water for a few minutes, or mist gently. Overwatering is the number one cause of seedlings collapsing.
Step 8: Prick out and pot on
When seedlings have their first pair of "true" leaves (the ones that look like the adult plant), they're ready to move into their own small pots. Hold each one gently by a leaf, never the stem, and ease the roots out with a pencil or dibber. Plant slightly deeper than before and water in.
Step 9: Harden off
Plants raised indoors need to get used to outdoor life slowly. For 1–2 weeks, put them outside during the day and bring them in at night. Then leave them out at night under fleece or in a cold frame. Plant tender crops like tomatoes and courgettes out only after the last frost – usually mid-May in the south, late May or early June in the north. Our Garden Weather page will tell you when frosts have finished in your area.
Common problems
- Nothing came up: too cold, too wet, or old seed. Check the date on the packet.
- Seedlings fell over at the base: "damping off" – a fungus caused by overwatering and poor air flow. Use clean trays and fresh compost, and don't overwater.
- Tall and spindly: not enough light. Move them somewhere brighter and sow slightly later next time.
- Mould on the surface: remove the lid more often to let air in.
Shop this guide
- Seed Sowing – seed trays, propagators, labels and dibbers
- Garden Watering – watering cans with fine roses and mist sprayers
There's real magic in watching the first shoots push through the compost. Start with something easy like sunflowers or tomatoes, and before long you'll be filling every windowsill in the house.
– Ann
Photos from Wikimedia Commons, shared under Creative Commons and similar free licences. Follow each link for the photographer and licence.
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