Choose a common vegetable seed to see its optimal germination temperature, average days to emergence, typical germination rate, and a practical sowing tip, all under controlled germination conditions.
These figures describe germination under controlled conditions with fresh, viable seed, similar to a laboratory germination test. Actual results in open ground vary with soil temperature, moisture, seed age, and variety, and are often lower and slower than these reference values. This tool does not tell you when to sow in the season, only the conditions the seed needs to germinate.
Germination conditions versus a sowing calendar
This tool is not a sowing calendar. It does not tell you which week of the year to sow a given vegetable in your garden. Instead, it describes the conditions a seed needs to germinate at all: the soil temperature range at which the embryo reactivates, the number of days it typically takes to break through the soil surface, and the share of seeds in a batch expected to germinate under those conditions. A gardener can sow at the right calendar date and still see poor emergence if the soil is colder or wetter than the seed needs. For the calendar side of planning, see the Sowing Calendar.
Reference germination data by seed (RHS guidance, controlled conditions)
| Seed | Optimal temperature | Days to emergence | Germination rate | Sowing depth |
|---|---|---|---|---|
| Tomato | 20 to 25 °C | 6 to 12 days | 85% | 0.5 cm |
| Carrot | 15 to 20 °C | 14 to 21 days | 65% | 1 cm |
| Lettuce | 15 to 20 °C | 4 to 10 days | 80% | 0.5 cm |
| Runner bean | 18 to 25 °C | 6 to 10 days | 80% | 3 cm |
| Pea | 12 to 18 °C | 7 to 14 days | 85% | 3 cm |
| Parsnip | 15 to 20 °C | 18 to 28 days | 60% | 1 cm |
Limits of this reference guide
These figures come from controlled germination testing on fresh, viable seed, close to the conditions seed suppliers use to check batch quality before it goes on sale. Open-ground results in a UK garden are almost always slower and less complete, because soil temperature fluctuates with weather, moisture is uneven, and seed age reduces vigour. A seed packet more than two or three years old, stored in a warm shed rather than a cool dry place, can germinate well below the rate shown here even in ideal temperature conditions. Treat these numbers as an upper reference, not a guarantee.
Practical tip: presoaking, depth, and cold frames
Large or hard-coated seeds such as beans, peas, and parsnips benefit from soaking in water for a few hours before sowing, which softens the seed coat and speeds up the first stage of germination. Sowing depth matters as much as temperature: a seed sown too deep may run out of stored energy before reaching daylight, while a seed left too shallow can dry out before it roots. As a general rule from the RHS, sow at a depth of roughly twice the seed's own diameter, and use a cold frame or windowsill propagator to hold heat-loving seeds like tomatoes and peppers at a steady temperature during a cool British spring.