Germination Time Calculator

How germination works — and why it varies

Germination is the moment a seed wakes up: it takes in water, switches on its stored food reserves, and pushes a root down and a shoot up. Three things control how fast that happens — moisture, temperature and oxygen — and of the three, temperature is the one gardeners most often get wrong. Every crop has a range of soil temperatures in which it germinates well, and outside that range germination slows sharply or stops altogether. This calculator shows you that effect: the same tomato seed that sprouts in a week in warm soil can take two weeks or more in cool soil, and a bean sown into cold, wet ground may rot before it ever sprouts.

Soil temperature, not air temperature

The temperature that matters is the soil's, not the air's, and the two can differ by a surprising amount. In spring, soil warms much more slowly than air: a sunny 70°F day does not mean 70°F soil, especially in heavy or wet ground, and soil in shade can lag weeks behind soil in sun. A cheap soil thermometer — pushed a few inches into the ground in the morning — is one of the best small purchases a gardener can make, and it takes the guesswork out of sowing dates entirely. As a rough guide, cool-season crops such as peas, lettuce and spinach germinate from about 40°F upwards and do best around 60–70°F, while warm-season crops such as beans, cucumbers and squash want at least 60°F and do best at 70–85°F. Above about 85–90°F, even warm-season seeds can struggle, and lettuce seed famously refuses to germinate in hot soil at all.

Moisture, depth and age

Temperature is not the whole story. A seed must stay damp from sowing until it sprouts — a single dry day can kill germination outright, which is why a light mulch or a board laid over a row helps in dry weather. Depth matters too: the old rule is to sow a seed about twice as deep as it is wide, because a seed sown too deep runs out of energy before it reaches the light, while one sown too shallow dries out. Some seeds need light to germinate and should barely be covered at all — lettuce is the best-known example. And seed age matters: fresh seed germinates faster and more evenly than old seed, so if your seeds are several years old, expect the slow end of the range — or test them first.

When to worry — and when to wait

The most common germination mistake is giving up too early. Parsnips can take four weeks; parsley and celery are famously slow; even carrots take two to three weeks in cool soil. Before you re-sow, check the calculator's cool-soil row — you may simply be waiting on temperature. If nothing has appeared by the end of the warm-soil window, something else is wrong: the seed may have dried out, been eaten, or rotted in cold wet soil. Dig up a few seeds and look — a swollen seed that never sprouted usually means cold or rot, while no seed at all usually means birds, mice or ants. When you do re-sow, change one thing: warmer soil, better moisture, or fresher seed. Sowing the same seed the same way in the same conditions will give you the same result.

A few crops deserve special mention because their germination is unlike the rest. Parsnips and parsley are slow even in warm soil, and old seed of either may barely germinate at all, so always use fresh seed for those two. Lettuce is the opposite problem: it germinates fast in cool soil but refuses in heat, so summer sowings often fail for temperature reasons that have nothing to do with seed quality — sowing in the evening, watering well, and shading the row can make the difference. Large seeds such as beans, peas and squash are the most forgiving of all: they carry big food reserves, push through the soil strongly, and germinate reliably if the soil is warm and damp. Small seeds — carrots, lettuce, celery — are the fussiest, because they have little stored energy and dry out fast; a fine, firm seedbed and steady moisture matter more for them than for anything else.

Related tools: the Seed-Starting Calculator works out when to sow indoors, the Seed Viability Checker tests whether old seed is still worth sowing, and the Harvest Date Calculator counts forwards from sowing to harvest. For sowing dates by zone, see the planting calendar hub.