Seedling Sowing Guide – When and How to Start Vegetables Indoors

Why Start Seeds Indoors?

Not all vegetables need an indoor head start. Carrots, beetroot, peas, and spinach are best sown directly outdoors – they don't tolerate transplanting well or they germinate quickly and gain nothing from an early start.

Indoor sowing pays off for plants that:

• Need a long growing season. A tomato needs 16–20 weeks from sowing to first fruit. Sown outdoors in May, it won't go in the ground until June and won't ripen before autumn.
• Are frost-sensitive. Peppers, aubergines, sweet potatoes, cucumbers – none of these tolerate even a light frost. Without an early indoor start, the season is too short for a good harvest in a temperate climate.
• Have tiny seeds that need careful handling. Celery, lovage, tomatoes – their small seeds germinate poorly in cold, wet spring soil outdoors.

Sowing Dates – Table for Central European Climate

The dates below are guidelines for Central Europe, where the last frost typically falls between early and mid-May. In warmer regions (south, west) you can start 1–2 weeks earlier; in cooler ones (mountains, north-east) shift sowing 1–2 weeks later.

How to Set Up for Sowing

The right growing medium is the foundation of healthy seedlings. Regular garden soil is too dense and heavy for delicate young plants. You need:

• Seed-starting compost – a specially formulated fine-textured mix. Don't use multipurpose compost, which contains too much fertiliser and can scorch tender roots.
• Pots or cell trays – small 6–9 cm pots for plants sown individually; trays for plants that will be pricked out later.
• A propagator or cling film – to cover containers after sowing and maintain moisture and warmth during germination.

How to Sow – Step by Step

1. Fill containers with seed-starting compost, lightly firm – it should be level, not compacted.
2. Moisten the compost before sowing – water it or stand the pot in water so the compost absorbs moisture evenly.
3. Sow seeds at the right depth – a good rule of thumb: sowing depth = 2–3× the diameter of the seed. Tiny seeds (celery, tomato) should be sown very shallowly or just on the surface.
4. Cover with a thin layer of compost or vermiculite. Vermiculite retains moisture and doesn't form a crust like compost can.
5. Cover with film or a propagator lid and check moisture daily – the compost should never dry out.
6. Place in a warm spot. Germination temperature for most vegetables: 18–25°C.
7. As soon as seedlings appear – remove the cover immediately and move to a bright position.

Pricking Out – When and How

Pricking out means transplanting seedlings from a tray into individual pots. Do it once the seedlings have their first true leaves (the second pair of leaves – the first pair are cotyledons, which look round and different from the plant's real leaves).

How to prick out:

• Water the tray a few hours beforehand – the compost should be moist, not wet
• Lift the seedling by holding a leaf, never the stem
• Make a hole in the destination pot and insert the seedling – burying the stem a little deeper than it was growing (tomatoes are especially tolerant of this)
• Gently firm the compost around the roots and water in

Hardening Off Before Transplanting

Seedlings raised in a warm house are not ready for outdoor conditions. They need to be hardened off – gradually acclimatised to wind, cold nights and the intensity of outdoor light.

Start 10–14 days before your planned transplanting date:

1. First few days: take seedlings outside for 1–2 hours in a sheltered, semi-shaded spot on a warm day
2. Gradually increase outdoor time and expose to more sun and wind
3. In the second week: leave outside all day, bringing in only at night if cold is forecast
4. Before transplanting: check the forecast – don't plant if frost is approaching

Skipping hardening off is one of the most common mistakes – seedlings planted without it often sit still for days or wilt before they acclimatise to the new conditions.

When Is It Safe to Transplant?

The most important factor is frost risk. Never transplant frost-sensitive crops (tomatoes, peppers, cucumbers, courgettes) before the last frost has passed. In Central Europe that typically means:

• Low-lying valleys and inland areas: after 15 May (to be safe, after 20 May)
• Coastal and western regions: from 1–10 May
• Upland and eastern areas: not until end of May – early June

Cabbage, broccoli, lettuce and leek tolerate light frost and can go out in April, or even March under fleece.