How to Start Seeds Indoors
Starting seeds indoors gives tender vegetables and flowers a useful head start before British spring weather settles. A bright windowsill, greenhouse or heated propagator can work well when paired with clean module trays and peat-free compost. Sow only crops that benefit from an early start, and time each sowing to your local planting-out conditions.
Choose crops that are worth starting under cover. Tomatoes, peppers, aubergines, courgettes, French beans, sweetcorn and many annual flowers gain time indoors, while carrots, parsnips and most root crops are usually better sown where they will grow. Read the seed packet before sowing, noting its recommended months and eventual planting-out date. In mild southern gardens, sowing can often begin earlier than in northern England, Wales, Northern Ireland or Scotland, where light levels and spring warmth arrive later. Avoid starting everything in January. Seedlings raised too early become leggy or root-bound before frost risk has passed. Plan a few sowings in stages, so you have sturdy plants rather than a crowded windowsill.
Gather clean module trays or small pots with drainage, labels, a watering can with a fine rose and fresh peat-free seed compost or fine peat-free multipurpose compost. Reused containers should be washed thoroughly to reduce disease problems. Fill modules loosely, then tap them on the bench so compost settles without becoming compacted. Water the compost before sowing, allowing excess water to drain away. This prevents tiny seeds being displaced by a heavy first watering. Write labels before you sow, including crop and date, because seedlings can look surprisingly alike. Use separate modules for larger seeds where practical; this reduces root disturbance and makes planting out easier. Keep spare compost nearby for covering seed evenly.
Sow at the depth stated on the packet, usually only a light covering for small seed and deeper for larger seed. Drop one or two seeds into each module rather than scattering a dense pinch. If both germinate, retain the stronger seedling and carefully snip out the other at compost level with clean scissors. Do not pull it, as this can disturb roots. Cover seed lightly, firm gently and mist or water with a fine rose if the surface has dried. Place trays somewhere bright and warm enough for the crop, but not against a cold pane of glass. A clear cover can retain humidity during germination; remove it as soon as shoots emerge to improve airflow.
Light matters as much as warmth once seedlings appear. Put trays on the brightest available windowsill, in a greenhouse or in a polytunnel, turning them regularly if light comes mainly from one direction. Keep tender seedlings away from cold draughts at night. Water when the compost surface begins to dry, aiming water at the compost rather than soaking leaves. Peat-free compost can dry unevenly, so lift a module to judge its weight before watering. Constantly wet compost encourages weak roots and disease, while repeated drying checks growth. Give seedlings space as leaves expand; crowded trays stretch towards light and have poor airflow. Open greenhouse vents on mild days, but close them before a cold night.
Pot on seedlings when roots are visible at drainage holes or when they have developed several true leaves and are filling their module. Hold a plant by a leaf, never its delicate stem, and move it into a slightly larger pot of peat-free multipurpose compost. Set it at the same depth unless the crop is tomato, which can be planted a little deeper to support its stem. Water gently and return plants to bright conditions. Do not pot on simply because a calendar suggests it; a loose root ball can fall apart if moved too soon. Equally, do not delay until roots circle tightly. Gradual potting on gives plants room without leaving wet, unused compost around small roots.
Prepare for planting out well before the garden is ready. Check the crop's recommended temperature and wait until the risk of late spring frost has generally passed for tender vegetables. Harden plants off over one to two weeks by putting them outside in a sheltered, lightly shaded place for increasing periods, bringing them in if nights are cold. Water before moving trays, since wind dries small pots quickly. Plant on a calm, overcast day if possible, then water each plant in thoroughly. Keep fleece or cloches ready for an unexpected cold spell. Continue sowing suitable quick crops outdoors, so your indoor work complements rather than replaces direct sowing.
Frequently asked questions
Why are my indoor seedlings long and floppy?
They are usually receiving too little light, too much warmth, or both. Move them to a brighter position, turn trays regularly and avoid keeping emerged seedlings on a warm indoor radiator shelf. Give each plant space and sow later if necessary. Stocky seedlings started slightly later often establish better outside.
Do I need a heated propagator?
No. A bright windowsill or greenhouse is enough for many seeds once spring light improves. A heated propagator is useful for warmth-loving crops such as tomatoes and peppers, but remove seedlings from strong bottom heat once they emerge. Warm, dark conditions create weak growth rather than stronger plants.
Can I use compost from my garden for seed sowing?
It is usually better to use fresh, fine peat-free seed compost or peat-free multipurpose compost for sowing. Garden compost may be coarse, contain weed seeds and drain unpredictably in small modules. Save your homemade compost for mulching beds, enriching larger pots and improving the soil around established plants.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 18 Sep 2026.