Growing Plants From Seed: Getting Germination Right

Growing plants from seed is the most fundamental propagation technique and the cheapest way to produce large numbers of plants. The challenge is that seeds have specific requirements — the right temperature range, the right moisture level, the right light conditions — and failing to meet any one of them results in erratic or absent germination. Understanding what each seed actually needs, rather than following a one-size-fits-all approach, transforms results dramatically.

Choosing the Right Compost and Container

Seed compost is finer-textured and lower in nutrients than multipurpose compost — nutrients at high concentrations damage emerging roots. Fill trays or individual cells to within 1 cm of the rim, firm lightly, and water before sowing, not after. Watering after sowing washes small seeds to the edges and buries others too deep. Use clean containers — old compost left in trays harbours the fungal pathogens that cause damping off, the most common killer of seedlings.

Sowing Depth and Temperature

The general rule is to sow seed to a depth of twice its own diameter. Tiny seeds such as lobelia or petunia need light to germinate — sow on the surface and do not cover. Others, like sweet peas, need darkness and a deeper covering. Check the packet. Temperature is the other critical variable: most half-hardy annuals and vegetables germinate best between 18–24 °C. Cool-season crops like lettuce and spinach prefer 10–15 °C and often fail in warm conditions. A thermometer in your propagator is a worthwhile investment.

Moisture Management During Germination

The compost must stay consistently moist but never waterlogged — the emerging root tip is extremely sensitive to both drought and oxygen deprivation. The best approach is to water trays from below: stand them in a shallow tray of water until the surface darkens, then remove. Cover the tray with a propagator lid or glass to reduce evaporation and check daily. As soon as germination begins, remove any cover to prevent the seedlings rotting at the base in the resulting humid, still air.

Pricking Out and Growing On

Prick out seedlings as soon as they are large enough to handle — ideally when the first true leaves appear above the seed leaves. Hold each seedling by a leaf, never by the fragile stem. Use a dibber to lever the roots free without tearing them and drop the seedling into a hole in fresh compost, firming gently around the roots. Pot into individual cells or small pots, water in, and place in good light. Begin feeding with a half-strength liquid fertiliser after two weeks.