Seeds & growth
Germination
The moment a seed sprouts and begins to grow, triggered by the right mix of moisture, warmth and (for some seeds) light.
Germination is the very start of a plant's life: the point at which a dry, dormant seed wakes up, swells with water and pushes out a tiny root and shoot. Get the conditions right and most seeds do this all by themselves. Get them wrong and the seed simply sits there, or rots before it can grow.
What a seed needs
Three things matter most. Moisture softens the seed coat and switches the seed back on, so the compost should be damp but never waterlogged. Warmth sets the pace: every seed has a temperature it likes, and below that it stays asleep. Air matters too, because the growing seed needs oxygen at its roots, which is why sodden, airless compost so often fails. A few seeds also have a light preference. Lettuce and many fine seeds germinate best with light, so barely cover them; others prefer darkness. As a rule of thumb, sow to a depth of about twice the seed's own width.
Typical UK temperatures
Hardy crops such as broad beans, peas, carrots and salad leaves will germinate in cool soil from around 7–10°C, which is why they can go outside from spring. Warmth-lovers are fussier: tomatoes, courgettes, pumpkins and chillies want roughly 18–21°C and will sulk or rot in a cold, wet seedbed. In our climate that usually means starting them indoors on a windowsill or in a heated propagator, then hardening them off before they go out once the frosts have passed.
Why some seeds are slow
Speed varies enormously. Courgettes and cucumbers often show in a few days; tomatoes take about a week. Carrots and parsley are famously slow and patchy, sometimes taking two to three weeks, so resist the urge to dig them up and check. Slow germinators reward patience, even, gentle moisture and a fine, crumbly tilth on the surface.
Common reasons for failure
Most disappointments come down to a handful of causes: compost that is too cold or too wet, seed sown too deep so the shoot exhausts itself before reaching the light, the surface drying out at the critical moment, or simply old seed. This last point is about viability, a seed's ability to germinate at all. Seeds gradually lose vigour with age, and some, like parsnips, are short-lived and best bought fresh each year. Always store packets somewhere cool, dark and dry, and check the use-by date.
If a sowing fails, you have lost only a week or two. Sow again, and once your seedlings are up and growing, successional sowing keeps a steady supply coming through the season.
In a UK garden
In the UK's cool springs, a sunny windowsill or a heated propagator often germinates warmth-loving seeds far more reliably than cold March or April soil outdoors.
Example
Tomato seeds sown 1cm deep in damp compost at 18–21°C on a kitchen windowsill push up their first pale loops in about a week.