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Grass Seed Germination Calculator — Estimate Germination Time
Estimate germination time based on soil temperature and seed type.
Average soil temperature at 5 cm depth, not air temperature. Take readings over a week — most grass needs a steady 8-10°C to germinate.
Ryegrass germinates fastest and tolerates the coolest soil (~8°C); fescue and bent grass are slower and bent grass needs ~15°C.
Below this temperature germination is slow (default 10°C)
Upper limit of ideal germination range (default 18°C)
Above this temperature germination may be unreliable (default 25°C)
How We Calculate This
This calculator estimates germination time from seed type and soil temperature, using typical UK ranges from horticultural seed suppliers (Grass Seed Online, Lawn UK).
At ideal soil temperature: perennial ryegrass germinates in roughly 5-14 days (it needs only ~7-8°C, so it is the fastest and most cold-tolerant), red fescue 10-21 days (needs ~10-11°C), and bent grass 11-21 days (needs ~15°C and is the slowest). Micro clover 7-14 days; wildflower mixes 14-30 days.
Times are adjusted for the soil temperature you enter. Below the species minimum, germination roughly doubles to triples and may fail; in the cool band (below your cold threshold, default 10°C) it slows by ~1.5-2×; through the ideal band (up to your optimal max, default 18°C) the standard range applies; above that, germination speeds up until soil gets too hot and dries out, when results turn variable. Germination is a biological range, not a fixed number — treat these as estimates and keep the seedbed consistently moist.
Frequently Asked Questions
Most grass seed needs a consistent soil temperature of 8-10°C minimum. Ryegrass germinates fastest at 15-20°C.
Use a soil thermometer pushed 5 cm deep. Measure at the same time each day for a week to get an average. Morning readings are most useful.
Common reasons: soil too cold, seed not in contact with soil, dried out, sown too deep (should be on surface), or old seed with poor viability.
Consistent moisture is essential but over-watering can wash seed away. Light, frequent watering (2-3 times daily in dry weather) is ideal.
Keep the soil consistently moist, ensure good seed-to-soil contact, and sow when soil temperature is above 8°C. Pre-soaking seed for 24 hours can help too.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.