How to Plant Seeds Step by Step (Complete Beginner Guide)

How to plant seeds step by step is a complete beginner guide, from choosing seed to sowing depth, light and watering.

Plant seed at about two to three times its own diameter in fine, evenly moist, sterile mix, at the temperature that species prefers, and most of it will come up. Failures almost always trace back to four things: seed that was old, medium that was too cold or too dry, seed buried too deep, or seedlings that rotted at the soil line.

This guide is for anyone sowing in trays, pots or garden beds, and it covers depth, spacing, germination temperatures, a viability test, sowing indoors versus outdoors, and troubleshooting. It’s part of the Plant Growing Guides guides, and the natural next steps are transplanting seedlings and watering the right way.

Plant seeds at a glance

Test your seeds before you plant them

Seed packets are dated for a reason. Germination falls each year in storage, and heat and humidity speed the decline. Onion, leek and parsnip seed is often poor after one year. Carrot, lettuce, beans and peas usually keep two to three years, and tomato and brassica seed can stay good for four or five if kept cool and dry.

A simple test saves a wasted season. Dampen a paper towel, lay 10 seeds on it, fold it, slip it into a loosely closed bag and keep it warm. Check at the time the packet gives for germination. Seven sprouted seeds means 70 per cent, which is fine. Four means you should sow roughly twice as thickly, and below three it’s cheaper to buy fresh seed.

Germination temperatures that matter when you plant seeds

Packets say “sow in spring”, but each species has a temperature band. Too cold and seed sits and rots. Too hot and some crops go dormant. Lettuce is the classic case: above about 25 to 28°C many varieties refuse to germinate, so in summer sow it in a cool room or after chilling the damp seed in the fridge for a day.

Root-zone temperature matters more than air temperature. A seedling heat mat under the tray can bring peppers up in a week instead of three, but it will also dry the surface faster, so check moisture twice a day.

Plant seeds indoors in trays: the method

1. Fill the container. Use a fine seed-starting mix (see Choosing Pots and Potting Mix) in clean trays or pots with drainage holes. Fill to the rim, tap, and press down lightly so there are no air pockets.
2. Pre-wet the mix. Water from below until the surface darkens, then let it drain. Watering after sowing can wash small seed into clumps.
3. Sow. Place two or three seeds per cell, or space seed 2 to 3 cm apart in a seed tray. Cover to the correct depth with dry mix or vermiculite.
4. Label now. Write the variety and date on a plant label. Seedlings of different tomatoes look identical.
5. Keep it moist and warm. A clear lid or loose plastic bag holds humidity. Remove it as soon as the first seedlings show, and open it daily for airflow before that.
6. Give light at once. Seedlings that emerge in dim light stretch within a day or two. Use a bright south-facing window or a grow light for 14 to 16 hours, kept a few centimetres from the leaves.
7. Thin to one per cell. Snip the weaker seedlings at soil level with scissors rather than pulling, which tears the roots of the one you’re keeping.

Plant seeds straight into the garden

Direct sowing suits crops that dislike root disturbance: carrot, beet, radish, turnip, beans, peas, sweet corn, sunflower and most poppies. Rake the bed to a fine crumb in the top 3 to 5 cm, draw a shallow furrow with a stick, sow, cover and water gently with a rose on the can.

Carrots and parsley can take three weeks to emerge, and the surface of the bed must not crust or dry in that time. Lay a board or sheet of damp hessian over the row and lift it daily from day 7. Remove it the moment you see green threads. A crust is also the reason small seed often fails in clay soil, so a thin layer of fine compost or vermiculite over the row helps.

Light-needing seed, dark-needing seed and seed that needs a cold spell

• Needs light: lettuce, petunia, snapdragon, celery and many herbs. Press the seed onto the surface and don’t cover it.
• Prefers darkness: phlox, calendula and many beans germinate best covered. Follow the packet.
• Hard seed coat: sweet pea, morning glory and lupin germinate faster if you nick the coat with a file or soak for 8 to 12 hours. Don’t soak longer, as seed can drown.
• Needs cold (stratification): many hardy perennials and wildflowers, such as milkweed and some echinacea, need a moist cold spell. Mix seed with damp sand or mix, seal it in a bag and refrigerate at roughly 1 to 5°C for 4 to 12 weeks, or sow outdoors in autumn and let winter do it.

The soaking tip is only half true. It helps large, hard-coated seed in warm conditions. It hurts wrinkled peas and any seed sown into cold soil, because wet seed with no warmth is an invitation to rot.

Troubleshooting seeds that fail to sprout

When to plant seeds: working back from your last frost

For indoor sowing, count backwards from your average last frost date. Peppers and aubergines need 8 to 10 weeks, tomatoes 6 to 8, brassicas 4 to 6, and cucumbers and squash only 3 to 4, because they sulk when kept in small pots too long. Lettuce can be sown 3 to 5 weeks before planting out. In hot, dry regions the logic flips and the cool season is the main window, so sow brassicas and lettuce in early autumn. In humid tropical gardens, sow in the drier months where possible and keep trays off the ground. The seasonal planting calendar works through this by season.

What happens inside your seed tray

Cold soil temperature slows germination, so a heat mat protects your tomato and pepper seed from rot. A propagator holds humidity around lettuce and basil seed, but you’ll need to vent it daily to prevent damping-off. Fine compost needs perlite or vermiculite to drain well, because dense mix blocks air and encourages fungus gnat larvae.

A grow light keeps tomato seedlings short and stops the stretching you see on a dim windowsill. Thinning reduces competition, so each carrot and radish gets room to swell. Hardening off protects cabbage and kale from wind and sun, because a cloche or fleece lets the leaves toughen gradually. Frost kills tender courgette and cucumber seedlings, so you shouldn’t move them outdoors until the last frost has passed.

How to plant seeds: the step by step cause and effect

Tomato and pepper seed need warmth of 21°C, while lettuce and spinach germinate best in cool soil. Compost feeds seedlings and improves loam, and perlite or sand drains water and prevents damping-off. Sow seed at twice its width, because deep sowing causes weak growth and shallow sowing dries the seed. Bean and pea need 3 to 5 cm depth, and carrot and radish need only 1 cm. Mulch protects bed seed and reduces crusting, but you should keep it thin. Tomato and basil need light bulbs of 16 hours, and a bright window suits lettuce. Slug eats seedlings, and snail attacks bean, so gravel and traps protect them. Cosmos, zinnia and marigold tolerate direct sowing, and sunflower and nasturtium suit children. Harden off for 7 to 10 days, so you’ll avoid shock. Your extension service or the rhs lists sowing dates. The transplanting guide covers the next step.

Mistakes to avoid

• Using garden soil in trays, which compacts, drains poorly and carries disease
• Burying fine seed, or leaving large seed on the surface
• Letting the medium swing between soaked and dry
• Leaving the lid on after seedlings appear
• Sowing the whole packet at once, then not thinning
• Skipping labels

If your windowsill is dim, a grow light keeps seedlings short and sturdy, as explained in Grow Lights for Seedlings.

For a short list of tools worth buying first, see Best Gardening Tools for Beginners.

What to read next

Once seedlings have two sets of true leaves, follow How to Transplant Seedlings. For tomatoes in particular, see How to Grow Tomatoes, and for crops that forgive beginners, the easiest vegetables to grow. Back to the Plant Growing Guides.